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By E. J. Roberts

F. C. Bond (Allis-Chalmers Mfg. Corp., Milwaukee) —This paper considers comminution as a first order process, with the reduction rate depending directly upon the amount of oversize material present. The data show that other factors should be taken into account, and it is possible that in time these may be evaluated as simultaneous or consecutive reactions: Development of the theory of comminution has been retarded for many years by the assumption that surface area measurements constitute the sine qua non of the work done in crushing and grinding, and it is encouraging to note the belated growth of other ideas. In the Abstract the term "net power" should be changed to "net energy." Throughout the paper the term "hp per ton" should be changed to "hp hrs per ton", or "hp hr t." The term "Probability Theory" in the title does not seem appropriate, since it is not clear how the probability theory is used in developing the ideas in the paper. There seems to be a contradiction between the large calculated advantages of closed circuit operation and the statement following that the closed circuit test results showed no significant change in grinding behavior, when compared with the batch grind curves. Tables I and II show that between 75 pct and 50 pct solids the energy input required decreases with increasing moisture content and may indicate the advisability of grinding at higher dilutions in certain cases. The calculation of the hp-hr per ton factor indicates an input in the laboratory mill of only 7.32 gross hp per ton of balls; this casts some doubt upon the accuracy of the factor used, since the power input in commercial mills at 80 pct critical speed is customarily much higher. The tests show that within fairly wide limits the amount of ore in the laboratory mill may be varied and a product of constant fineness obtained, provided that the grinding time is varied in the same proportion. This has often been assumed, and confirmation by actual testing is of value. The Cavg corrections for differences between the plant and laboratory size distributions do not seem very satisfactory, since in many cases the plant/laboratory ratio is farther from unity after correction than before. The following equation has been derived from the data in Table VI: Relative Energy (log new ball diam in in. + 0.410) Input = --------------—--------------- from which the relative energy inputs for balls of different sizes can be calculated and compared. The relative energy input is unity for balls of 2.715 in. diam. The equation indicates that the work accomplished by a ton of grinding balls per unit of energy input is roughly proportional to the square root of the total ball surface area; provided, of course, that the balls are sufficiently large to break the material. The data in support of this statement are admittedly meager, but are fairly consistent when plotted. The relative grindability values listed in Table VI for 200 mesh multiplied by 4/5 apparently correspond approximately to the A-C grindability at 200 mesh.' It would seem that for open circuit tests comparable accuracy could be obtained much more simply by the old method' of plotting the test grind, extending the mesh grinds to the left of zero time if necessary, and determining from the plot the equivalent time required to grind from the plant feed size to the plant product size, using the average of several mesh sizes. The en- ergy input value of one time interval could be determined by tests on materials of known grinding resistance, and this multiplied by the interval required should give the desired energy input value. The relative grindabilities would be the relative time intervals required for a specified feed and product size. When the plotted mesh size lines of a homogeneous material are extended to the left beyond zero time they meet at one point at zero pct passing. The horizontal distance of this point from zero time indicates the equivalent energy input required to prepare the mill feed. The author's results show that the closed circuit grinding tests give about the same K values as open circuit tests, from which he concludes that open circuit tests are satisfactory in many cases. The value of the closed circuit test is its ability accurately to predict energy requirements in closed circuit grinding for both homogeneous and heterogeneous materials. If the material is homogeneous, the open circuit test gives satisfactory results; but if the material contains appreciable fractions of hard and soft grinding ore, the open circuit tests will not be accurate because of the accumulation of hard grinding material in the circulating load. Since in most cases it is not possible to determine a priori whether the material contains hard and soft fractions, the closed circuit tests are preferable and more reliable. B. S. Crocker (Lake Shore Mines, Ontario)—Dr. Roberts probability theory of grinding is very similar to our log pct reduced vs. log tonnage method of plotting and evaluating grinding tests at Lake Shore. However, although we both seem to start at the same point we finish with different end results. Shortly after publishing our grinding paper (referred to by Dr. Roberts) in 1939, we did pursue the subject of the "constant pct reduction in the pct +28 micron material for each constant interval of time. We ran innumerable tonnage tests on the plant ball mills, rod mills, tube mills with 11/4 and 3/4 balls, and lastly pebble mills, with tonnage variations from 180 tons per day to 950 tons per day. We found that when we plotted the log of the tonnage against the log of the pct reduced of any reliable mesh, we had a straight line up until 90 pct of the mesh is reduced. We have also tested this in our 12-in. laboratory mill with the same results. We have used this method of evaluating grinds for the past 8 years and developed the recent four stage pebble plant on this basis. By pct reduced we mean the percentage of any given mesh that is reduced in one pass through a mill at a given tonnage (or time). For example, if the feed to a rod mill is 90 pct +35 mesh and the discharge at 500 tons per day is 54 pct +35, the pct reduced is 90 — 54/90 = 40 pct. If the feed had been 80 pct +35 the discharge would have been 48 pct +35 or pct re- duced 80-48/80 = 40 pct as long as the tonnage re- mained constant at 500 tons per day. Thus we can easily correct for normal variation of mill feeds. This log — log relationship derived from the tonnage tests of all our operating mills has proved of tremendous help in checking laboratory work and in designing alternate layouts or new plants. The difference between the log — log and the semi-log plot is only shown up when the extremes in tonnages are plotted. When the relationship between the pct reduced and the tonnage was first investigated, we used semilog

Jan 1, 1952

By T. M. Morris

R. T. Hukki (Finland Institute of Technology, Helsinki, Finland)—Two schools of thought are developing concerning the order of the rate of flotation. The paper by Morris is intended to show experimental evidence in favor of the first order rate. Arbiter, on the other hand, has presented an earlier paper" to demonstrate the validity of the second order rate equation. Unfortunately his method of plotting is open to criticism, as pointed out by Morris." A more sensitive method of plotting has been described by the writer,= whereby the data presented by Arbiter demonstrate the superiority of the second order plot. The general rate equation may be written in the following forms for the integrated first and second order equations: (A) t = -1/k In Co-R/Co (first order) [4] 1 (B) 1/t = - kCo + kCo2 1/R (second order) As a first approximation the value of Co may be taken as a unity, 100 pct or 1. Whether or not this is the case in all tests performed by Morris is impossible to judge, owing to the fact that the flotation times are extremely short, e.g. 150 sec in tests A. It would, indeed, be very interesting to know what the total recovery R would be after 1 hr of flotation, for example, instead of after 150 sec. Certainly there are circumstances when C, approaches unity within a very small margin of error. Morris has used the same approximation in his example D "since the feed to this cleaner

Jan 1, 1954

By William Y. Holland, Roger H. Cook

Dexter H. Reynolds (Chapman and Wood, Mining Engineers and Consulting Geologists, Albuquerque, N. M.)—A number of questions are raised by conclusions and inferences made in the above-mentioned paper. The more troublesome of these concern use of the various pozzolans to combat the deleterious effects of the alkali-aggregate reaction. The most alkali-reactive materials listed are opal and rocks containing opaline silica. The pozzolans mentioned specifically for use as amelioratives are opaline shales and cherts. These are stated to retard the expansion caused by the alkali-aggregate reaction. Another well-recognized pozzolan is diatomaceous earth, which consists principally of opaline silica. A pozzolan presumably owes its effectiveness to its high reactivity with the alkaline liquid phase of the concrete mix. It appears reasonable to expect that finely divided opaline silica added as a pozzolan would be more susceptible to reaction with the alkalies present than would larger particles of the same material. The authors report that work with high and low alkali cements indicates that in the presence of alkali-reactive materials, deleterious expansion depends upon the alkali content of the cement. The total effect, therefore, should be more or less independent of the amount of reactive aggregate present, and still more independent of its state of subdivision. The deleterious effects should, if anything, be aggravated by the addition of a finely divided, highly reactive pozzolan. Further, if the alkali-aggregate reaction is of great importance in the long-term soundness of concrete structures, the addition of a pozzolan to a concrete made with aggregate free from known deleterious materials would be a questionable procedure. The benefits reportedly accruing from such use of pozzolans are greater ultimate strength for a given cement content, increased resistance to deterioration by exposure to sulphate solutions and other mineral waters, and greater resistance to damage by wetting and drying and freezing and thawing. In view of the deleterious effects of highly reactive materials are these benefits ephemeral? The same considerations apply to another alkali-reactive material, chalcedony, which appears to consist of ultrafine-grained quartz, with opal absent in detectable amounts. Quartz flour is notably reactive chemically and physiologically (cf. Ref. 11 of Holland and Cook's paper), a fact borne out by its effectiveness as a pozzolan, which presumably might be expected to offset the deleterious effects of the presence of chalcedony in the aggregate. A second question of some importance concerns the reportedly highly deleterious reactivity of acidic and intermediate volcanic glasses, such as rhyolite, perlite, and pumice. Air entrainment is listed as one of the ameliorative measures to combat the deleterious effects of the alkali-aggregate reaction. The alkalic-silica gel formed by the reaction may expand into air bubbles and thus not cause appreciable expansion of the concrete mass. It would appear then that pumice and perlite, particularly perlites of the pumiceous types and other types after expansion, would also tend to counteract the expansion, since these materials consist largely of voids and air bubbles. Certainly this would be expected of structural concrete in which pumice or perlite is used as total aggregate. Finely ground pumice, perlite, and volcanic ash have been demonstrated to be active pozzolans (cf. Pumice as Aggregate for Lightweight Structural Concrete by Wagner, Gay, and Reynolds, Univ. of New Mexico Publications in Engineering No. 5, Albuquerque, 1950). In fact, the term pozzolan was first associated with finely divided pumice or volcanic ash. Such materials were used with hydrated lime as the sole cementitious agent in constructing public buildings, roads, and aqueducts by the ancient Romans. The deleterious alkali reactivity of the volcanic glass, itself containing several percent of the alkalies, apparently did not contribute to the remarkable state of preservation of those ancient structures, as exemplified by the Appian Way and the Pantheon Dome. Still a third question involves .the reactivity of constituents of concrete when exposed to various salt solutions. Resistance to. deleterious expansion and cracking as a result of contact with mineral waters and its relationship to the mineral content of the aggregate are not mentioned by the authors. Yet the phenomena pictured in Fig. 1, and especially in Fig. 2, appear very much like those caused by exposure to mineral waters. The deterioration of concretes exposed to sulphate waters is generally considered related to the chemical constituency of the cement itself, particularly to the relative amount of tricalcium alum-inate contained. Could not many of the ill effects presently blamed on alkali-aggregate reaction really have been caused by contact with sulphate or other salt-containing mineral waters? Or perhaps their use as mixing waters? May not the deleterious expansion be as much a function of the chemical makeup of the cement as it is of the mineral constituency of the aggregate? Would it not be just as important to use alkali-free mixing water as it is to use a low-alkali cement? It appears obvious that resistance of cements and concretes to sulphate and other salt solutions cannot be left out of account in discussion of deterioration of concrete structures with time. This factor may be of equal or even greater importance than the alkali-aggregate reaction, particularly for concrete subjected to wetting and drying cycles, such as airstrip paving, water-retaining dams, and highway structures. Another very important factor is called to attention on page 1022 of the article in Mining Engineering, October 1953, in that failure of concrete structures may result from poor construction practices and use of too high water-cement ratios. Both of these can contribute remarkably to decreased resistance to attack by sulphate waters, and presumably could have an equally remarkable effect upon extent of damage resulting from the alkali-aggregate reaction. From the above remarks it appears that while alkali-aggregate reaction may be an important factor in decreasing the useful. life of a concrete structure, it is not the only factor involved, and it may not be even a controlling factor. Likewise, many of the phenomena apparently associated with the alkali-aggregate reaction may have resulted from cond'itions which had little relationship to the alkali-reactivity of a constituent of the aggregate. Certainly if alkali-aggregate reactivity is a major factor in bringing about early failure, one cannot help feeling anxiety concerning the future of the many concrete structures in this country and abroad in which pumice and perlite were used as total or partial aggregates. This anxiety can only be dispelled by calling to mind that among the best-preserved relics coming down to us from ancient times are structures made with mortars containing highly alkali-reactive aggregates.

Jan 1, 1955

By Bernard York, John J. Reed

J. P. Zannaras (U.S. Tungsten Corp., Congress, Ariz.) —In the investigation described in the paper by Bernard York and John J. Reed, the 8x8x8-in. square block was subjected to compressive forces from three sides, top, side, and rear. Let us refer to the stresses set up by these forces as Sz due to top pressure, Sy due to side pressure, and Sx due to rear pressure. At the section across the 2-in. hole (when failure occurred at 317 psi) the compressive stress Sy was equal to 317 x 8/6 = —422 psi, the top compressive stress S; is of the same intensity, —422 psi, and the rear compressive stress S, equals —317 psi. The true stress is T = Sy — S — S Taking as 0.2,* Taking the case when the opening was ½ in. and failure occurred at 517 psi, 8 Sy - 517 x -----= —537 psi 7.5 8 Sz = 517 x-----= —537 psi 7.5 Sx = —517 psi The true stress T = Sy — Sz — S, T - —537 + 0.2 x 537 + 0.2 x 517 = —327 psi. It can be shown that the above fundamental formula holds good not only for the above two examples but for all sizes of openings used, because when this formula is expressed in terms of the external pressure and size of the opening it agrees perfectly with the experimental results obtained by the authors and given in Fig. 12 of their paper. General formula: T = Sy — s: — S....

Jan 1, 1955

By W. A. Olds, H. F. Yancey, M. R. Greer

John Griffen (Consulting Engineer, Pittsburgh)— This paper presents much data offering tentative answers to many technical questions that have been raised by the employment of dense-medium processes and, particularly. by use of the drum-type vessel. It is to be hoped that the Bureau of Mines will publish complete detailed data for the 50 or more tests so that it may be studied and analyzed by everyone interested. Such study will take considerable time and effort. The writer is therefore obliged to confine his discussion to a few points he has noted as being open to question. The data in Table VI which shows the recovery efficiency for the 1 to ½-in. fraction as 100 pct raised a question concerning the accuracy of these calculated figures; washability data on the washed coal and refuse show that such a figure is impossible. When the ash content of the misplaced material in the washed coal is higher than that of the misplaced material in the refuse, recovery efficiency must be less than 100 pct. A etudy of the data In Tables 111, IV, V, and VI suggest that this error probably arose from slight errors in calculating the data in Table V and more probably from the small scale used in plotting the washability curves from the Table V data. Using the size consist data and the weight and ash percentages of individual gravity fractions of each size as given in Tables 111 and IV and the reported recovery of 80 pct of washed coal, a revised Table V was calculated, carrying weight and ash percentages to hundredths of a percent instead of rounding off at tenths. This washability data for each size of Belling-ham reconstituted feed was then plotted on large scale washability curves. From this data the new Table XIV was prepared, which agrees very closely with data shown in Table VI except for two important values, theoretical yields and recovery efficiencies. As will be noted the recovery efficiencies in Table XIV are significantly lower than those given in Table VI. A study was then made of the data showing distribution of the specific gravity fractions to washed coal and refuse. This study indicated that when this distribution was based on the fractions as tested at 0.1-sp gr intervals the number of points obtained were insufficient to establish the distribution curves with reasonable precision, which in turn would affect the accuracy of the values for the specific gravity of separation and the error areas. The data in Tables III and IV of the paper and curves prepared therefrom make it possible for the specific gravity fractions to be subdivided further. This was done using 0.05-sp gr fractions and the distribution data so obtained is given in Table XIV, with resulting values for the specific gravity of separation and the error areas. The writer believes that the derived values shown on Table XIV are more accurate than the correspond-

Jan 1, 1955

By K. F. G. Hosking

I read Professor Gaudin&apos;s paper with great interest and pleasure because for some time I have held that the chemical aspect of comminution is a subject of considerable importance to the mineral dresser and deserves to be thoroughly investigated. It does seem appropriate, however, to emphasize the fact that "fresh" edges and corners produced by the grinding of solids display enhanced reactivity has been recognized and utilized in the development of certain mineral identification techniques. Some of these techniques are worth noting, not only because they might facilitate research in the aspect of mineral dressing under discussion, but also because they emphasize the fact that many mineral species commonly regarded as being very inert can display a surprising reactivity when in the freshly ground state. In 1951 Isakov6 published a number of tests for the components of certain mineral species which depend essentially on grinding in a mortar a mixture of the material under investigation with a solid reagent. Thus when stibnite, 4(Sb2S3), is ground with sodium or potassium hydroxide. the antimony is revealed by a momentary development of a yellow color which changes in air to orange-red. Other antimony minerals need a preliminary treatment before the test can be carried out. This consists of grinding with aluminium sulfate, ferric sulfate or potassium bisulfate, and breathing upon the resultant mixture. I have employed a similar technique to determine the approximate magnesia content of certain limestones.&apos; The method depends essentially on the fact that when a sample of limestone is ground under controlled conditions with solutions of sodium hydroxide and Titan yellow the color of the final product is, within limits, a function of the amount of magnesia present. I have also shown that the components of a wide range of minerals can be identified by applying chemicals to their streaks on portions of vitrified, unglazed floor tiles, etc. Under such circumstances the diversity of the reactions which take place in the cold (because of the reactivity of fresh corners and edges) is surprising. Thus, for example, if a garnierite, (Ni,Mg)3Si2O5(OH)1, streak is treated first with a drop of 0.880 ammonia and then with a drop of a 1 pct alcoholic dimethyl-glyoxime it immediately becomes red, indicating the presence of nickel.&apos; Stevens and Carron9 have evolved a simple field test for distinguishing minerals by "abrasion pH." A soft nonabsorbent mineral is scratched in a drop of water on a streak plate until a milky suspension is formed. A piece of pH indicator paper is dipped into the suspension, after which it is removed and the maximum deviation from neutrality noted. When a hard mineral or one which absorbs water is being tested, fragments are first ground for 1 min with a few drops of water in a mortar to make a heavy suspension. The importance of the findings of such tests to mineral dressing may be judged by the abrasion pH values, Table 11, recorded by Stevens and Carron for certain species usually regarded as comparatively inert. The combined results Of the above researches clearly indicate that comminution is capable of altering the pH of a pulp and of causing the chemical nature of the surfaces of some of the components to be profoundly changed&apos; Depending On circumstances such surface alterations may have a beneficial or an adverse effect if these products are subsequently subjected to flotation. The tests also suggest that by grinding "inert" minerals with appropriate solid or liquid reagents "reactive" surfaces may be developed which might facilitate separations by flotation. It is an interesting and instructive problem to determine the reactions that are likely to take place when dry solid substances are subjected to comminution and to the unavoidable heat liberated during the process. To do this it is theoretically necessary to know the free energy values of the reactants and possible resultants, but unfortunately there is a dearth of such data! However, the heats of formation of many substances are known, and generally speaking, if in a reaction of the type AB + CD = AD + CB the sum of the heats of formation of AB and CD is less than that of AD and CB the reaction will probably proceed to the right. Thus, according to a note I have (the author of which I cannot name) if PbS (black) is warmed with CdSO, (white), PbSO., (white) and CdS (yellow) are formed, and that the reaction does, in fact, take place is indicated by the change in color of the mixture. The reaction is expected, as the sum of the heats of formation of PbS and CdSO, is less than that of PbSO, and CdS (as shown below). PbS + CdSO4 = PbSO4 + CdS 22.2 + 218.0 < 216.2 + 33.9 Finally, certain other aspects of the chemistry of comminution, which are neither mentioned by Professor Gaudin nor referred to by me are to be found in a paper by Welsh" and in the printed discussion thereof. A. M. Gaudin (author&apos;s reply)—The observations contributed by Dr. Hosking are indeed welcome, as they add to our experimental knowledge of a topic which is believed to be of the first importance. In connection with the experiments cited it should be kept in mind that oxidation, hydration, and carbonation at various rates should always be deemed to be possibilities when grinding is done in water or in air, even in "industrially dry" air. Special precautions might lead to sufficient minimizing of these reactions and to the assertion, instead, of deliberately-created reactions. The author wishes to thank Dr. Hosking for his contribution.

Jan 1, 1957

By F. C. Bond

B. H. Bergstrom (Allis-Chalmers Mfg. Co., Milwaukee)—In reading this article, it was difficult to follow through the example given as a starting point in the derivation of the theoretical equation which relates ball size to feed size. A discussion with F. C. Bond led to the following clarification: Bond postulates that "the breaking force exerted by a ball varies ... as the cube of its diameter . . ." and that the force in pounds per square inch required to break a particle varies ... as its diameter squared." He concludes: "It follows that when a l-in. ball breaks a 1-mm particle, a 2-in. ball will break a 4-mm particle, and a 3-in. ball a 9-mm particle." The above example can be true only for a different set of assumptions. Since the first assumption is apparently true, it is the second that must be revised. The second assumption should read that the breaking force required to break a particle varies as its diameter to the 3/2 power. This relationship was derived in the next paragraph of the article.

Jan 1, 1960

By E. B. Mayo

David LeCount Evans (Consulting Petroleum and Mining Geologist, Wichita, Kans.)-—Not only E. B. Mayo but also W. C. Lacy, who apparently urged the preparation of this analysis, is to be commended. Regional thinking of this type is needed to assure future success in the never-ending search for new mineralized and petroliferous districts. As is usually the case, here is a regional study that will be read by the mining geologist alone. It is ironic that several of the trends established in this study have suggested themselves in northern mid-continent, detailed, and regional studies. These, where established, have offered new keys to petroleum exploration and have provided a possible basis for unraveling a number of broad generalities. The oil geologists, active in Colorado, Kansas, and Oklahoma, would find much food for thought in Mr. Mayo&apos;s projections. To be more specific: 1) The parallelism between E. B. Mayo&apos;s Texas Lineament and the Amarillo Uplift, the Wichita Complex and the Arbuckle Complex of the Texas Panhandle and Southern Oklahoma is viewed with interest and appears especially significant when compared with the similar northwest trend of the Central Kansas Uplift, a major trend of production. 2) Considering the various northeast zones of Fig. 2, and with particular reference to Mayo&apos;s C-C, the Jemez Zone is on direct line with one of several northeast-southwest controls which the present writer has been using with some success in Kansas subsurface correlations. Considering zones of shearing, with no apparent vertical displacement, but suggesting strike-slip movement, because of the staggered effect on other features which cross such trends, Mayo&apos;s philosophy presents regional possibilities for lines of weakness, considered to this time of only local significance. 3) And, finally, in an area as distant from the Southwest as central Kansas, the north-south trends of the Fiarport-Ruggles anticline, the Voshel-Hol-low Nikkel-Burrton structures, the Dayton to Stut-gart trend, the north, slightly east trend of the Ne-maha structural complex, and others all seem to approach the north-south alignments, a through f, of Mayo&apos;s Fig. 3. Mayo&apos;s employment of structural intersections to pinpoint crustal weakness, to localize igneous activity and its accompanying mineralization is not, perhaps, a new concept, but it is a 1958 model, produced by tools improved from the ever-increasing accumulation of geological observations. The use of intersecting trends in petroleum geology is not a new idea, since much production in earlier days was encountered via the straight line projections of established trends to centers of intersection. A tragedy in this age of specialization is that iron curtains have been raised between groups, all seeking raw materials, all acolytes at the altar of structural geology, but all smugly content in and protected by the ivory towers of petroleum geology, engineering geology, mining geology, and geophysics. Mayo presents basic ideas which can stimulate mid-continent structural thinking and, in the case of cen- tral Kansas. he provides a key to replace the broad and overworked simple monoclinal, sinkhole-dotted, Karst topography credo, which is not finding its share of new oil in a state where the declining discovery ratio is disconcerting. The American Association of Petroleum Geologists would do well to add E. B. Mayo to its list of Distinguished Lecturers. Evans B. Mayo (author&apos;s reply)—In reply to David LeCount Evans&apos; comments, it is pleasing to learn that some of the elements discussed in my paper may interest petroleum geologists as well as mining geologists. This should not be surprising, however, because the lineaments make up the framework of the continent, and the oil-bearing sediments must reflect to varying degrees adjustments of basement blocks along their boundaries. A further possibility that petroleum geologists must have considered is that the slow escape of heat from buried lineaments and their intersections has aided the separation of oil from the sediments and started the migration into traps. Regarding the specific points listed by Evans, the following are suggested: 1) The branch of Texas Lineament marked 1&apos; (Fig. 3) is thought to extend eastward through the Capitan Mts., New Mexico, through the long Tertiary dikes east of Roswell, and beyond via the Matador and Electra ranges of the Red River Uplift, Texas. Its further continuation might be the eastern flank of the Ouachita Fold Belt. The Amarillo-Wichita-Arbuckle zone of uplifts appears to continue east-southeastward the Spanish Peaks belt (3-5, Fig. 3). The northwest-trending Central Kansas Uplift would not belong to the above set, except insofar as the Central Kansas Uplift is traversed by west-northwest folds, possible continuations of the Uinta belt (5-5, Fig. 3). 2) The possible continuations into Kansas of the Jemez zone are new to me and are most welcome suggestions. 3) Most of the nearly north-south Kansan structures mentioned by Evans are unfamiliar to me, but the Nemaha Uplift itself appears to be part of a very pronounced structure traceable from the Cerralvo Fault Zone, south of the Rio Grande, through the Bend Arch, Texas, and the Nemaha Uplift, into the Pre-Cambrian of Minnesota (?). This nearly meridional zone is crossed and broken by the Rio Grande Embayment and by the Red River-Wichita Syntaxis. Petroleum geologists realize the economic importance of these features. Perhaps it is inevitable that some papers of general interest be buried in the journals of specialized groups. Moreover, papers dealing with regional, or lineament, tectonics and its applications to exploration for economic mineral deposits are as yet few in the American literature. The opportunity to advance this field is open to all those who are not ultra-conservative and who have a lively curiosity, plenty of patience, and not too many business restrictions. In conclusion, much appreciation is extended to D. L. Evans for his comments.

Jan 1, 1960

By R. J. Charles

F. C. Bond: This is an outstanding paper on comminution theory and represents a considerable advance in mathematical formulation. It clears the way for a discussion that should ultimately decide whether the work index should be considered as a parameter according to the Third Theory or as a constant. In the empirical differential equation dE = —Cdx/x [1] either Corn must be treated as a variable to satisfy known crushing and grinding conditions. The author has chosen the exponent n as the variable, with C as a constant. According to the Third Theory, in the general case n is a constant equal to 3/2, and C is a variable related to the work index Wi. If x is placed equal to P (the size in microns which 80 pct of the product passes) and F is the size 80 pct of the feed passes, then C equals 5 W1, and dE = 5 -5 Widx/x = 10 Wi 10 Wi w = vP vF which is the basic equation of the Third Theory. It is apparent that this integration involves the error of treating the parameter Wi as a constant, just as the author&apos;s treatment involves the error of integrating his variable n as a constant. These errors are probably relatively unimportant, considering the small range of the variables. When more is known about the factors contained in the work index, Eq. 1 can be revised as a partial differential equation. The Third Theory exponent of 1/2, or n = 3/2 in the Charles&apos; differential equation (Eq. l), was not chosen primarily because it is the average between the Kick and Rittinger exponents; this may be merely fortuitous, since it is illogical to define truth as the average between two errors. It was first discovered from the study of a large mass of crushing and grinding data and confirmed by laboratory testing. The general correspondence of the Third Theory equation to actual crushing and grinding results over the entire size reduction range argues strongly that a fundamental phenomenon is involved. The a posteriori theoretical explanation of the constant Third Theory exponent is that the work done in breaking rock is proportional to the length of the crack tips formed. Compressive force applied to the rock results in deformation and strain energy. When this locally exceeds the breaking strength a crack tip forms, the surrounding strain energy flows to the crack tip and extends it to split the rock, with release of the energy as heat. Under rapid impact the crack tip may form before the strain has reached equilibrium, resulting in less energy expenditure and probably a coarser product. In any case, the elastic limit of a brittle material is practically the same as its breaking strength, and the work necessary to form the crack tip is the work required to break. The crack length of a ton of broken rock cannot be measured directly. However, the particle shapes of feed and product are similar, and the crack length is considered equal to the square root of one half of the new surface area formed. It is, therefore, proportional to 1/vD, where D is the equivalent product particle diameter. This is the basis of the Third Theory equation. Many secondary factors can be imposed upon the crack length breakage. These may cause the Third Theory work index to increase or decrease somewhat as the product size becomes finer. The work index is properly a parameter rather than a constant, making it an increasingly valuable practical criterion of the work input required in crushing and grinding.

Jan 1, 1959

By A. F. Henderson, C. F. Cornell, A. F. Dunyon, D. A. Dahlstrom

Ossi E. Palasvirta (Development Engineer, Oliver Iron Mining Diu., U. S. Steel Gorp.)—The authors are to be congratulated for their interesting article, which thoroughly illustrates the variables inherent in filtration of taconite concentrate. The work and the conclusions based thereon largely parallel the test work done by the writer at the Pilotac plant" and the experience gained with a commercial size agitating disk filter in the same plant. At Pilotac, however, a thorough study was also made of the effect of depolarizing (demagnetizing) the filter feed, and it is the purpose of this discussion to comment on the merits of depolarization of the magnetite concentrate prior to filtering. The work at Pilotac was done in three phases: 1) preliminary laboratory testing with a circular filter leaf of 0.047 sq ft, followed by 2) plant testing using a 4-ft diam, single-disk agitating filter that was purchased on the basis of the pilot tests on the 4-ft model. In the laboratory tests depolarization was achieved by slowly withdrawing&apos; batches of thickened concentrate from a coil producing an alternating field of about 300 oersteds. In plant tests the standard Pilotac procedure&apos; was employed, wherein the pulp falls freely through the depolarizing coil. The preliminary tests in the laboratory at first seemed to indicate that although depolarization of the filter feed decreases the cake moisture, it also tends to decrease the thickness of the cake, thus decreasing filtering rate. The tests with the 4-ft disk filter soon showed, however, that the compactness of the cake, attained during the form period because of depolarization, permitted a considerable decrease in drying time without any sacrifice in final moisture content. Thus, the filter could be operated at a much higher speed, and the overall capacity was higher than with magnetized feed. Because of the great compactness of the cake there was little shrinkage during the drying period, which prevented cracking and subsequent loss in vacuum. This in turn permitted operation with as thick a feed pulp as the diaphragm pumps could handle, eliminating the necessity of pulp density control. On the basis of these findings, the 6-ft agitating disk filter has been operated at 2 rpm, using feed pulps varying from 65 to 73 pct solids. Initially Saran 601 was used as medium, but it was later replaced with a relatively open, tight-twist nylon cloth. Filtering rates up to 750 lb per ft- er hr can be attained with feeds averaging about 70 pct- 270 mesh, and there is no trouble because of cracking. The cake moistures vary between 8.5 and 9.5 pct. To recapitulate, the merits of depolarizing the filter feed may be summed up as follows: 1) The well dispersed pulp shows less tendency to settle in the filter tank. 2) The homogeneous filter pool results in more even cake formation. 3) Because of absence of flocs, great compactness of cake is attained during the form period. 4) The cake does not tend to crack during the drying period. 5) A drier cake is produced. 6) A shorter drying period is necessary, permitting higher operating speed, which in turn results in increased capacity. 7) Density of the feed pulp can be kept as high as the equipment can handle. This increases capacity, since it is directly proportional to the percentage of solids in the pool. A few tests were also made to study the effect of chemical flocculants on filtration efficiency. Results showed that the effects of chemical and magnetic floc-culation were quite similar. Thus the use of a floccu-lant would impair rather than improve the filtering of magnetite concentrate. A. F. Henderson, C. F. Cornell, A. F. Dunyon and D. A. Dahlstrom (authors&apos; reply)—We want to thank O. E. Palasvirta for his comments, particularly in view of the encouraging results obtained with demagnetized taconite concentrate. In our studies an attempt was made to evaluate the effects of depolarizing the feed to the plant filters by passing the slurry through a coil, similar to the method described by Palasvirta. Unfortunately, in our experiments there were no startling improvements in performance level, neither cake rate increase nor cake moisture reduction. However, when slow filter cycle speeds were employed, the filter cake tended to crack and the vacuum level dropped, resulting in an increase in cake moisture content. When demagnetized feed was used during slow speeds, no cake cracking was evidenced and the vacuum level remained constant. Thus the depolarizing coil was found necessary only in cases of cracking. It should be noted that most of our test work concerned a feed of 85 to 90 pct —335 mesh and about 60 pct by weight solids concentration. This contrasts with 70 pct —270 mesh and 65 to 73 pct by weight solids as noted by Palasvirta. Reviewing both sets of results, it might be concluded that depolarizing may be successfully employed to alleviate cake cracking tendencies and may markedly improve cake rates and moistures on the coarser taconite concentrates. Further investigations may disclose the exact relationship of grind and pulp density to the depolarizing action.

Jan 1, 1959

By J. L. Gillson

Andre Hubaux: In the writer&apos;s opinion, more stress should be put on field and microscope observations, as J. L. Gillson does. His discovery of relics of big labra-dorite feldspars from the Marcy anorthosite in all the other rocks is proof enough that previous field observations have been inadequate. The writer&apos;s field experience with anorthositie massifs is limited to the Egersund-Sogndal district in southern Norway; however, this is a very large and well exposed area. It seems to have so many points in common with the Adirondack region that a detailed comparison should be beneficial. This comparison was not possible 20 years ago, owing to the scarcity of geological data, at least for Norway. Both regions are now far better known—the Norway district because of the work of Professors Bugge and Barth 33-35 and especially because of the extensive field and microscopic research of P. Michot,30-35 who introduced me in the field to the problems of that region. In a particular unit of the Egersund district, a sodi-fication of the plagioclase, very similar to what Gillson calls "andesinization," has taken place. 12&apos; The percentage of anorthite in the plagioclase has dropped from 42 to 43 pct in the original norite down to 27 pct, but never below that level. This metasomatical transformation is a part of a whole series of processes of mig-matitization which have produced, above the level of granitization, concentrations of Mg, Fe, and Ti and among them, patches and parallel bands of ilmenite and magnetite. Thus both regions present somewhat comparable features and the theories to explain them have several points in common. However, as there seem to be differences between some American conceptions and the views commonly accepted at the University of Liege, I would like to make some comments about J. L. Gill-son&apos;s article. The author speaks of the garnet as being an anti-stress mineral. However, garnet may be found in schists, where its helicitic texture, when present, demonstrates its formation during differential movement. Furthermore, the whole concept of stress and anti-

Jan 1, 1959

By B. S. Butler

A. D. Mutch (Falconbridge Nickel Mines Ltd., Ona-ping, Ontario)—This contribution is prompted by the fact that the writer has recently published an article&apos;!&apos; which has in part the same general conclusions as B. S. Butler&apos;s, namely, the form of the standard mineral paragenesis in hypogene deposits and the parallel transition from early high temperature acid to late low temperature alkaline or neutral solutions. Since the writer also recognizes that he is not fully qualified to discuss the formation of ore deposits as a chemical problem, this discussion will be limited to summarizing other observations and will suggest how Butler&apos;s Table II might be made more effective. The form of Butler&apos;s table is not unlike that widely used by European geologists&apos;" in which the various minerals are related to the types of ore fluids—peg-matitic, pneumatolic and hydrothermal. Thus the decrease in temperature and change in pH of the ore-bearing fluids as recognized by Butler may also be paralleled by perhaps even greater changes in composition, density, and character. W. H. Newhouse&apos;s article:" on the sequence of hypo-gene ore mineral deposition is still one of the most complete analyses of the problem. In it he makes four major observations: 1) The order of O, S, As, Sb is the same as their increasing atomic weights and the same as their melting points. 2) The order of the metallic elements is the same as the Schuermann series. 3) The general mineralogical order is paralleled by decreasing heats of formation. 4) The order is generally in decreasing hardness, higher metal content, and higher specific gravity. The order of 0, S, As, and Bi is also the same as their relative abundance. Another point worth mentioning about this series is that As, Sb, Bi, when abundant, often substitute for sulfur in the early pyrite-like minerals, whereas later in the sequence these elements appear to act more like metals. This behavior may be a further indication of the acid character of the early fluids. C. J. Sullivan"&apos; has attempted to show that the sequence of the metallic elements is related to the order of their melting points and has discussed the interrelationship of the different types of bonding of sulfo-phile and oxiphile elements in minerals. Items 3 and 4 might best be summarized by saying that bonding of the minerals in the series appears to be from ionic to covalent to metallic, the early minerals being light in weight, hard, and near-transparent; the intermediate minerals of intermediate weight, semi-transparent, and semi-conducting; and the end minerals heavy, soft, and malleable, opaque and good conductors. A rapid fall in temperature has been assumed to be the cause of ore deposition in many deposits. Perhaps this in turn is the effect of a rapid release of energy in the form of pressure release and change of volume, the first minerals to form being those with the highest heats of formation which absorb the released energy the most rapidly. Butler&apos;s Table II could probably be streamlined and made more effective if the listed compounds were assembled as mineralogical groups as in Dana. Thus the sulfides, arsenides, antimonides, bismuthides, and sulfa salts could be arranged in AX,, AX, A2X2, etc. groups. Since these are continuous substitution series, in which the fields of stability and the heats of formation of the various individual minerals can be fairly regarded as similar, this should be accomplished with no loss of clarity. The occurrence of the individual minerals and the precise sequence of the mineral deposition in any one deposit is probably more dependent on the concentration and relative abundance of the component elements. The assembling of geological data in tabular form such as Butler&apos;s Table II will continue to be a very effective method of studying geological phenomena. The importance of this particular problem has been pointed out by C. F. Park,% who concludes that understanding of mechanisms of the standard paragenesis and interrelated problems of mineral zoning in space and time is one of the major keys to classification of ore deposits.

Jan 1, 1959

By James M. Schopf

Gilbert H. Cady (Senior Geologist, Head of Coal Division Emeritus, Illinois State Geological Survey, Ur-bans)— Those coal mining and preparation engineers and operators who read this article will probably be most concerned with those parts which deal with the applied aspects of coal petrology in the fields of mining and technology. Actually the two categories overlap to such a degree that they are essentially one. The article appears to suggest that there are two points of veiw in the field of applied coal petrology. From one viewpoint the interest is in the attributes of the various macerals (Fig. 4)—vitrinite, exinite, and inertinite—-in terms of utilization (when the coal is burned, coked, gasified, hydrogenated, etc.). From the other point of view, assuming that the macerals do possess individuality in behavior, interest attaches to the possibility that coal of a particular type (maceral composition) can be prepared by discriminating methods of preparation and blending. Most of the 29 items listed for mining and technology can be assigned to one or the other of these two categories. Somewhat the same simplicity that resides in the items of the proximate chemical analysis is desirable for the petrologic analysis. This simplicity appears to be fairly well achieved by the group maceral device provided mineral matter is also included. At present, without fully adequate experimental substantiation, the three types of material represented by the group macerals vitrinite, exinite, and inertinite are regarded as sufficiently different in composition and properties so that the preponderance of any one type gives the coal distinctive properties provided it is not too high in rank. This appears to be increasingly true with coals of decreasing rank from medium volatile bituminous coals downward. The article reflects the great emphasis placed on the technique of descriptive coal petrology in America. Such description has resulted in a good understanding of the nature of the fundamental variability in the physical composition of bituminous coals. Such understanding has been attained very largely by the thin section technique used in coal bed profile analysis introduced by R. Thiessen after the manner suggested by the data assembled in Table 11. In contrast, the group maceral concept of coal analysis in terms of the categories indicated on the left hand side of Fig. 4 is particularly useful for broken coal analysis and applied coal petrology, in which evaluation is largely based on quantitative concepts. It is inadequately emphasized in this article that although the Thiessen procedure is suitable for the purpose of petrologic description in which texture and orientation are of critical importance, it is less well adapted to the technological uses of applied coal petrology. No doubt those persons whose interest may have been aroused in the possibilities of more refined coal preparation by this and other recent contributions to the literature of coal petrology will understand that certain technical aspects of coal petrology, particularly in the field of descriptive petrology, are still a matter of some controversy among the coal petrologists, but they will also appreciate that Schopf has made an important contribution to the field of coal petrology in America by pointing out that such petrology requires consideration of disoriented or broken coal samples and that results are expressed less in descriptive terms than in quantitative values with respect to the four types of material composing coal—-vitrinite, exinite, inertinite, and mineral matter. Unfortunately because so much American coal petrology has been descriptive of the coal in the bed there is a dearth of literature on applied coal petrology. Research and experimental data in this field are greatly needed.

Jan 1, 1959

By M. C. Fuerstenau, D. N. Collins, R. B. Bhappu, D. N. Moir

D. N. Moir (Senior Scientific Officer, Warren Spring Laboratory, D.S.I.R. Stevenage) and D. N. Collins (Scientific Officer, Warren Spring Laboratory, D.S.I.R., Stevenage, England) — During work on the petroleum sulphonate flotation of beryl in 1959 at Harwell, and subsequently at Warren Spring Laboratory, effects similar to those mentioned by the authors were noticed. Brief details of some of this work were given to the 6th International Mineral Processing Congress at Cannes.&apos; It is interesting to note that the authors experienced strong activation effects with Fe+++ ions on beryl when using petroleum sulphonate collectors. This phenomenon was of interest in our work when

Jan 1, 1963

By AIME AIME

THREE of the addresses presented at this interesting and important session are printed in full else- where in this issue. The fourth, Mr. Hewett&apos;s paper, on "Cycles In Metal Production" has been issued as Technical Publication 183, and an abstract of it was published on page 162 of last month&apos;s issue. The general discussion that followed these four important papers is given in full below. S. A. TAYLOR: I would like to ask Mr. Veasey whether the control of oil in pools includes royalties, and, if so, in case of a new pool how are you going to control, or rather how are you going to describe the limits of that pool?

Jan 1, 1929

Secretary&apos;s NOTE—on page 176,of this paper, in the fourth line of the first footnote, " 4° " should, be " 1" )&apos;; and on page 180, at the beginning of line 23, (&apos;lined boot" should be " unlined boot." Discussion. Frank Owen, London, Eng. (communication to the Secretary) : In his very interesting and valuable paper, Prof. Snow

Jan 1, 1900

I have sketches of the radiators, and on this sketch up in here (indicating) we show the line from the fan-exhaust steam from the fan, and the trip spray at the mouth of the mine, which is usually near the tipple, the set of radiators on each side of the entry, exhaust steam exhausting in by the radiators, the small atomizers which I mentioned, and the man sprinkling at the hose. There are times, when at different times we have a man sprinkling during the day time, when he is out of the way, and during the night, sprinkling on the motor roads. I will leave that print up here and let you look at it and if you have any questions you wish answered, Mr. McAllister and myself will be glad to answer them. (Applause.) PRESIDENT PRYDE: . I am going to ask Mr. Hair to leave the sketch on the wall. From the back part of the hall, it is hard to see what is there. The figures are very important. I think one member of this association will be glad to know the cost of water, Friend Littlejohn. MR. LITTLEJOHN: I don&apos;t know as I want to ask any questions. The only thing I&apos; would say about the cost there, it is very close to that cost that we tabulated for the Utah Fuel Company at the Utah mines here some time ago. We have not tabulated lately. It may be a little over that at the present time if you take the. labor costs and the material. costs. As I remember, some time ago we did get up some figures and it was, as near as I can remember, exactly two cents per, ton for the labor and material. We haven&apos;t got the radiators, etc., that the C. F. & I. have in their mines there, probably due to the fact that we have no shortage of water. We have plenty of that there, and our system is absolutely nothing more nor less than pipe lines, hose and nozzles and the man at the end of the nozzle. PRESIDENT PRYDE: I would like to ask Mr. Hair about the cost of the steam. MR. HAIR: That steam we use there in exhausting, of course, is from the fan, and we have a back pressure valve set right here (indicating), from three to four and sometimes five pounds, and the exhaust from the fan was a five or six inch exhaust, and our exhaust into the mine is nothing less than eight inches, and is practically low back pressure you might say, and all those large radiators, that I spoke of, are heated by live steam, and these probably take a little more coal, but as to just what that cost is I couldn&apos;t say. As far as the exhaust steam is concerned, you might as well use it in the mine as discharge it outside. MR. HARRINGTON: Mr. Hair, I would like to ask, is this installation on an intake haulage-main haulage course? MR. HAIR: Yes, sir. MR. HARRINGTON: In the winter time, when it is, say between five or ten degrees below zero, how far into that haulage road does the fogging go? MR. HAIR: Well, that goes in half a mile, sometimes in 3,000 feet. MR. HARRINGTON: Wouldn&apos;t that interfere pretty seriously with the motor? MR. HAIR: To some extent, but I wouldn&apos;t say seriously enough to interefere with the production of coal. We have one mine we are working in two and three-quarters miles. Our mines are all pretty deep, and we don&apos;t find it objectionable. I don&apos;t believe that it interferes in the production of coal. I don&apos;t believe it makes any more difference between a real cold day in winter time than it does in the summer time. MR. HARRINGTON: I was down at Dawson, and in their mines they have an installation very much like Mr. Hair describes, but we went in there one night and stayed all night, getting a certain amount of data, and the fog was intense for-a full half mile; so intense that you could hardly see your hand in front of your face-see your hand that far (indicating) dis-

Jan 1, 1924

CHAIR: I see a great many safety men in the audience and I hope you will all take part in using up the time that we have left. The point brought up by all was the necessity of education. It is very hard to know just how this is going to be done. We get very little encouragement to do this. At a recent meeting at Rock Springs, 100 in attendance, and most of them mine officials. Another condition which contributes to accidents is the labor turnover in many of our States. With many companies I am told from actual statistics the labor turnover is 125 per cent. Each field has different mining conditions so that men coming from one field are not always familiar with the mine conditions in the field they come to. These are conditions which have to be overcome. WM. LITTLEJOHN: Mr. Harrington brought up the subject of sprinkling. I will try to tell what we are doing at the Utah Fuel Company camps. Our dust at Castle Gate is the most inflammable gas that has yet been tested at the Pittsburgh station of the Bureau of Mines, so it is up to us to use every precaution to keep it down. I will try to tell what we are doing regarding sprinkling from tanks. We now have in operation two tanks in our locomotive roads, one in Castle Gate, one in Sunnyside. These tanks are something like 500-gallon tanks with pump on one end of truck which gets its power from the power wire and has a half circle perforated pipe on the other end of tank. This is simply hooked on to the locomotive. When the pump is started up it throws a fine spray of water all around which wets the roof, the sides and floor. We found this very successful from an economical as well as a working standpoint. A roadway that we probably would have had to pay a man at least two shifts for, covering the full distance, with the ordinary hose and spray, we can do in half a day and do it more thoroughly. We are also figuring on at the present time using the same system on rope haulage ways, which of course will have to be run by a reel from the feed lines, handling say four hundred feet each way before reconnecting. We haven&apos;t got it finished, but are working on it at the present time. MR. COLBURN: I would like to say a few words -about this question of education. Education was mentioned by all three sections of the Safety Committee. Education regarding safety might be divided into two branches. One branch is for the official at the mine to keep him acquainted with the latest developments in safety equipment and safety methods. The other part of education is for the workmen. The United States Bureau of Mines is co-operating in both directions. Safety Clubs are organized around the Bureau of Mines First Aid Training, but it is the idea to expand those clubs and teach the miner the advantage of such-things as the closed safety lamp, and also the regulations that have been made for his safety. In Illinois we can&apos;t operate with the closed lamp, because the miners in that section of the country are not educated to the advantages of the safety lamp. Another form of education has been taken up by the National Safety Council, consisting of voluntary organizations similar to the Institute, to which all mining men are invited to attend. Such conferences nave been held in the Lake Superior district, in Butte, and in Globe, Arizona, this year. These are to be extended throughout the coal mining regions. CHAIRMAN PRYDE: The Union Pacific Coal Company has also built two tank cars of 600 gallons capacity each. The tanks are connected with hose, so that water can flow from one tank to the other. A centrifugal pump is mounted on a truck, and connected to the front tank. This pump takes its power from the trolley wire. The discharge from the pump is perforated, so that it sprays the floor and sides of the entries. We expect to use this water train on main haulage ways where we have no sprinkling line.

Jan 1, 1923

MR. D. HARRINGTON (U. S. Bureau of Mines) : I happen to be a member of the Safety Committee, and the men on the Committee, I believe are the ones who will find the most fault with the report; but I am absolutely not in accord with the last recommendation. (The use of the "Self-Rescuer.") I do not think it safe. I do not think you are always sure you are going to have a sufficient amount of oxygen; in fact I know there are occasions in mines when there is not enough oxygen to make a man safe in a gas mask or self rescuer. At one mine fire, workers using open oil and carbide lights experienced quick reversal of air which extinguished the lights and one of the workers having carbide light was overcome. It took about 45 minutes hard work with Sylvester method of artificial respiration to restore him to consciousness. Fortunately these men were working close to fresh air but had they been far from fresh air base and relying upon self-rescuers or gas masks they would undoubtedly have perished. I was at a mine fire in Wyoming. A bunch of men were working and the air turned on them, and if they hadn&apos;t oxygen breathing mine rescuing apparatus I think there would have been at least a half dozen men killed they would have been killed before you could say, "Jack Robinson." PRESIDENT PRYDE: I would like to ask if the self-rescuers are nut approved by the Bureau of Mines? MR. HARRINGTON: They are not; absolutely not.* Furthermore, the Bureau of Mines&apos; Engineers have opportunities to go around in mines, and in mine cars as much as anybody else in the country, and I have in mind but one who is in the slightest degree in favor of allowing self-rescuers to go in a mine. MR. BEVAN: Mr. Chairman, I would like to ask the Safety Committee if it would be of any use to the fire boss traveling through the mine? PRESIDENT PRYDE: Mr. Chairman, I think we should finish up the discussion on the self-rescuer before we take up any other discussions.

Jan 1, 1924

By AIME AIME

Maurer&apos;s review summarizes quite thoroughly the various theoretical developments and experimental findings that contribute to the knowledge of rock mechanics in drilling. This discussion supplements Maurer&apos;s paper by including generalities that either have been incompletely treated in his review, or formulated from new considerations by the discussant and from hitherto unpublished experimental data. These generalities are associated with the observation that all mechanical methods of rock drilling essentially comprise a sequence of indexed, concentrated loadings on the bottom of the hole being drilled.

Jan 1, 1967