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By Webster Hodge

MALL generators and motors are required to Soperate, in some critical applications, at temperatures where cold-worked silver-bearing copper re-crystallizes. Copper containing up to 30 oz Ag per ton has been used for many years to build the highest grades of commutators and slip rings. With failure of commutators impending because silver-bearing copper cannot withstand the stresses imposed at these higher temperatures, Cr-Cu was tried as a substitute with varying degrees of success. Cr-Cu proved difficult to fabricate because a critical heat treatment was necessary to obtain sufficient ductility. Besides, its rupture strength was shown to be low at operating temperatures, its electrical conductivity lower than desired, and its cost high. Furthermore, it often contained hidden flaws. The Nippert Electric Products Co. set out to find a better material by undertaking a research project with the stated objective of providing a material suitable for commutators that would have 1) a softening temperature above 750°F (400°C), 2) a minimum electrical conductivity of 90 pct of that of pure copper, 3) a strength not less that that of cold-worked silver-bearing copper, and 4) a materials cost of not over 1.5 times the cost of copper containing 25 to 30 oz Ag per ton. No commercial alloy met these specifications. However, it was found that copper containing from 0.18 to 0.35 pct Zr could be produced quite easily in the laboratory, and promised to fulfill the major requirements noted above. Interest naturally centered in the lower part of this composition range, since such alloys gave somewhat better electrical conductivities, and were lower in

Jan 1, 1958

By Stanley F. French

ELECTRICAL power and distribution for the sulphide ore plant is an addition and extension of the existing power system which has served the oxide reduction plant since its inception in 1914. The power plant is located at Tocopilla on the west coast of Chile, some 90 miles from Chuquicamata. A brief description of this plant and the existing transmission system is necessary to an understanding of the problems involved in the provision for the sulphide plant load. Originally the Tocopilla power plant consisted of four Escher Wyss turbines driving Siemens Schuckert generators, each rated 10,000 kw. At various times, units of 10,000 and 20,000 kw were added, all of which operated at 215 psi and 600°F at throttle. In order to improve the economy of the station, a Topping unit was installed in 1938 together with a high-pressure steam generator of 360,000 lb per hr capacity. This unit is rated at 11,000 kw and was designed for 750 psi, 880°F. The exhaust from this turbine passes to the low pressure station header for operation of part of the low pressure turbines. Total kw generated by the high pressure boiler, topping turbine and associated low pressure turbines totals 33,000 kw and the total station capacity is 113,000 kw. This installation proved one of the most successful of its kind, operating almost continuously. The original plant was of German design and consequently European characteristics were incorporated into the system, the frequency being 50 cycles and voltages of 500, 5000 and 100,000 were used for low, generator, and transmission voltages,

Jan 1, 1952

By P. B. Bucky

THE purpose of this paper is to present a scientific method for deter-mining the proper span and shape of roof for safe and economical mining; at present, these two vital factors wherever pillars are left to support the roof are determined by local practice or general experience. To develop the method described here, experimental laboratory work was done. The arch adopted was a segment of a circle, a flat roof being considered an arch with an infinite radius. Hitherto, arches have been used underground without the background of stress analysis that generally precedes arch construction on the surface. In many cases results have not been as expected, because of the lack of knowledge of the factors affecting its strength and behavior. A theoretical and experimental study of arches should furnish informa-tion that will make it possible to design safe and economical surface structures as well as underground openings. For example, on the subject of mine drifts and tunnels, the present idea is that arching their backs increases their strength; this paper will indicate that this is not always true, and if it can be shown that under certain conditions arching becomes unnecessary an economic saving will result. In room-and-pillar work a question may arise as to whether arching the roof and gobbing the excavated material will result in timber saving, and a decrease in the dan¬gers from falls of roof. It is to be distinctly understood that each mine must be treated as an individual problem, and that answers to these ques-tions and many others can now be found after experimental laboratory data have been obtained.

Jan 1, 1934

By Wallace E. Johnson, Leslie R. Hill

As part of a continuing study of intense energy effects on the earth, such as explosive cratering for Plowshare, large scale hydro- dynamic computer codes which numerically integrate the two-dimensional mass, momenta, and energy conservation equations have been adapted for use on geological materials. Experimentally determined nonlinear dynamic equations of state and constitutive equations governing material strength are incorporated into a two-material Eulerian code, DORF. With the use of this code, impact studies have been made of projectiles with arbitrary shapes in the r,z cylindrical plane normally incident on semi-infinite half-space targets. The impact of projectiles on rocks has been studied empirically and experimentally; however, this paper employs a theoretical approach. Particular examples selected involve the impact of aluminum and iron projectiles on oil shale at velocities of 0.5 and 1.0 cm/µsec. These studies offer a quick and economic means to edit quantities unavailable in an experiment and to independently study the effect of varying parameters such as material shape, density, and strength, and impact velocity. Results reveal early time agreement with experimental high-speed photography as seen in the bowl-shaped crater and spray. Initial impact pressures at the projectile-target contact are in close accord with the one-dimensional Rankine-Hugoniot relations for the impact of dissimilar materials. These equations require local continuity of pressure and particle velocity across the interface and were found to hold locally, quite independent of projectile shape. Essentially all projectile energy is transferred quickly to the oil shale target (tens of microseconds). Over 75% of the original kinetic energy

Jan 1, 1971

By W. R. Toeplitz

Probably a more descriptive title would be "Much Ado About Nothing." By "nothing," of course, I refer to the porosity which is the main feature of this type of bearing. Porosity has a decided effect on the manufacturing techniques, the structure and physical properties, the oil content, and on the installation and service of oil-retaining porous bronze bearings. A simple curve can be drawn by plotting specific gravity along the abscissa and percentage of porosity by volume on the ordinate. A bronze composition, analyzing 89 Per cent copper, 9.75 Per cent tin, and 1.25 per cent graphite, has an absolute solid specific gravity of 8.5. At this specific gravity, of course, occurs the point of zero porosity. Conversely, at loo per cent porosity, the specific gravity is zero. A similar curve can be drawn for bearings impregnated with oil; the only change being that the specific gravity at loo per cent oil will be 0.89 if we assume that that is the specific gravity of the lubricant. The curve is useful in determining the oil content by volume for any specific gravity. The normal range of oil content for porous bronze bearings of this composition is from 35 to 16 per cent, which is in the specific gravity range of from 5.8 to 7.3. In addition to the amount of porosity, the type of porosity is important. The first consideration here is that the porosity must be intercommunicating. This can be determined by a number of simple tests. The first method, which is very accurate, is to extract the oil in the bearing by cycling

Jan 1, 1945

By H. M. Chance

Stretching from New York southwestwardly to Georgia is a great range of hills and mountains consisting of pre-Palæozoic schists, slates, .and gneissic and granitoid rocks, known locally by many different names; in Pennsylvania as the South mountains, the Berkshire or Durham and Reading hills, and in Virginia and farther south as the Blue ridge. To the northwest of this range lies the " Great Valley," underlain by Lower Silurian limestones, and known as the Lehigh valley, Cumberland valley, Shenandoah valley, valley of Eastern Tennessee and Georgia and by other local names, but all these are parts of one valley, geologically and geographically continuous from New Jersey to Georgia. Lying upon and along the northern flank of this range of mountains, and dipping northwestwardly under the limestone of the valley, is the Potsdam formation (Primal or No. 1 of the Pennsylvania Geological Survey), which consists of sandstone or conglomerate overlain by slates. The latter are sometimes arenaceous and occasionally are replaced in part by sandstone. This formation varies from a few feet to several hundred feet in thickness. At the outcrop the slates are completely decomposed, oxidized or disintegrated, forming a belt of clay several hundred feet wide occupying the interval between the outcroppings of the sandstone and limestone. This clay-belt includes a more or less continuous series of iron-ore (brown hematite) deposite, which have been, and still are, extensively worked. The belt is more or less continuously milleralized with iron throughout its length, and hundreds of large deposits of ore have been developed and worked, producing many millions of tons of ore.

Jan 1, 1909

By Edward H. Robie

NEVER before have the annual company reports in the mineral industry field exhibited the typo-graphical art so abundantly as does the current crop. Time was when most company reports made a drab appearance and dry reading, mostly confined to financial data and text that gave little information to the public or technical men. Now, the dollar statistics assume less importance. Considerable information is given on what is going on at the company's plants, new equipment being installed, new methods, properties under development, ore reserves and grades, and similar information of interest to professional men working in the field. We have one suggestion for further improvement: The names of the officers and directors of the company are always given, but only a few companies give the names of the principal operating officials. We think, in the case of a mine for example, that the name of the general manager, his assistant, the mine and mill superintendents, and any other top operating officials who have been largely responsible for the success of operations, are worthy of mention. The reports are now widely read and this recognition is due them. Conspicuous among the annual reports that we happen to have seen, in point of typographical attractiveness, are that of the Texas Co., celebrating its fiftieth anniversary with a gold-plated cover and gold printing inside; and that of Noranda Mines Ltd., with a red and black cover that would attract attention anywhere. Comment on economics, taxes, and labor give an excellent picture of what our industry is facing. Consider the following, from the report of the Union Oil Co. of California:

Jan 1, 1952

By J. Lyndon Rosenblad

The strength of a jointed-rock mass is not well-understood. It is believed, however, that the behavior of a rock mass is governed by both the intact rock properties and the properties of the discontinuities. Rock research in the past has been concentrated on the determination of intact rock properties. Several studies are presently underway which should provide more insight into the properties of the discontinuities. Few research programs, however, have been undertaken with the intent of studying the composite rock mass behavior where either the intact rock strength or the properties of the discontinuities may control failure. There are three approaches available for studying the behavior of a rock mass, namely, field studies, mathematical models (finite element), and geomechanical models. A geomechanical model has been developed in this study where the strength and failure modes of a model rock mass are studied as a function of intact block geometry, joint orientation, joint properties, and confining pressure. The objective of this study is to gain some insight into the parameters that govern the behavior of a rock mass, which parameters are most important, and when these parameters control. This chapter describes only one phase of the study, the development of equipment for use with the rock-blocks model. The models being tested are approximately 22 x 22 x 6-in. The size varies slightly depending upon the joint orientation used. The equipment developed includes model material molds, reaction frames, loading apparatus, measuring and recording instrumentation, and other accessory equipment pertinent to the operation of a successful study. The usual problems in equipment development, such as uniformity of load application, loading capabilities, and reaction capacities are all

Jan 1, 1970

In the examination of an undeveloped prospect a decision must be arrived at from an inspection of the outcrops and the exposures in a few shallow pits. Prospects that are offered for sale rarely expose any important quantity of good ore, work having usually been stopped when the immediate exploration no longer yielded favorable results. It should be borne in mind, furthermore, that a great majority of prospects have been examined many times, and if of conspicuous promise, would have been acquired-for development. In most cases the problem for the engineer, therefore, is to discover the traces of a valuable mineralization that has disappeared through solution, or the conditions that indicate the possibility, or probability, of underlying secondary enrichments. The available data being meager, every feature should be the subject of careful study-the condition of the outcrop, the structural relationships, the associations of minerals, the alterations of wall rocks, the chances for underlying enrichments, and so forth, as well as the actual assay value of the material exposed. It is frequently advisable to spend the time and money necessary to have trenches dug at various significant points along a promising outcrop; such exposures, even if the depth attained is slight, often disclose conditions that are not apparent at the actual surface, as for example, the existence of soft minerals and the distribution of the various minerals through the mass of the deposit. Trenches also permit samples to be taken from points that were not accessible during former examinations. Strong, persistent outcrops of uniform width may be taken to indicate the probable size and character of the underlying vein,

Jan 1, 1932

By Charles E. Glass

The stabil ity of engineered embankments subjected to earthquakes has received considerable attention in the last 10 years, but most of the effort has been directed toward stability of earth dams. Based upon past performance, earthquake- induced failures on non-impounding mine waste structures will likely correspond to small, local slumps or debris slides, and their stability should be considered as an integral part of mine economic planning. As such, stability considerations should include earthquake hazard, characteristics of earthquake ground motion, dynamic slope stability, and the impact that waste-dump failures have on mine operations. Numerous techniques have been proposed for analyzing the stability of embankments subjected to earthquakes. Before choosing an analysis technique, it is worth noting two important differences between the dynamic response of waste embankments and natural slopes. First, waste embankments respond as structures having unique modes of vibration, which causes embankment motions to differ from nearby ground motion. Second, because of embankment response, accelerations can be different at embankment crests than elsewhere in the structure and occur at frequencies that do not correspond to input ground motion. Because of these factors, embankment response should be calculated prior to slope stability assessment. In this paper several analysis techniques designed to calculate slope stability are evaluated, and conventional pseudostatic techniques are shown to be inappropriate for dynamic considerations. Techniques to calculate dynamic stability should include embankment response, all characteristics of earthquake motion, and roughness along failure planes as input, and should provide a displacement time-history of the slide mass as output. Two techniques satisfy this criterion, and in this paper the LADRS technique is recommended for its versatility and simplicity.

Jan 1, 1985

By H. B. C. Nitze

A description of some of the magnetic ore-deposits in this region should be of interest to the mining and metallurgical public, inasmuch as very little has been said or written concerning them. I refer to the magnetite deposits of Franklin and Henry counties, Va., and of Stokes county, N. C. These deposits remained comparatively unexplored until recently, when the construction of the Roanoke & Southern R. R. through this territory has called attention to them, by furnishing the means of transportation for the ores. It is true that the ores have long been known, at least locally, and were smelted in catalan forges and old-fashioned, cold-blast charcoal-furnaces long before and during the late war. At the present time, however, the old workings are caved in, filled up and over-grown to such an extent that, as a rule, but little can be learned from them. No prospecting of any consequence has been done, and as my time was too limited and the territory to be examined too extensive to permit additional openings of a more satisfactory nature to be made, the incormpleteness of some of the data I have to offer (notably, those referring to the absolute width of the veins) will be pardoned. It will hardly be necessary to say that this ore-bearing formation belongs to the Archaean, or Eozoic age, the usually occurring rocks being schist, shale, gneiss, granite, sandstone and quartzite. The stratigraphy is, as a rule, quite regular (excepting in the Danbury section, Stokes county, N. C.), the general strike being northeast and southwest, and the dip nearly vertical. The country is gently roll-

Jan 1, 1892

By Robert L. Zook, Bernard J. Olup, James J. Pierre

Coal refuse slurry impoundments are dams constructed of coarse coal refuse to impound fine refuse (slurry) and water (25 to 30% solids). Both products are waste from coal preparation plants. A number of these structures in West Virginia, Pennsylvania, and Kentucky were investigated to determine embankment stability. During the investigation a variety of embankment designs were encountered. The investigation determined the soil and geological characteristics of the embankments and the sites. This was accomplished by a test drilling and monitor installation (observation wells and piezometers) program, geologic study, and large-scale mapping which allowed subsurface profiles of the embankments to be constructed. The laboratory testing program concentrated on textural classification of the coal refuse, permeability, and determining strength parameters to be utilized in the stability analyses. For the stability analyses, the modified Fellenius slip circle, simplified Bishop slip circle, and analysis for translational failure were utilized. In all stability analyses hydrostatic pressure and seismic acceleration probability were considered. This investigation indicated that 62% of the embankments analyzed were in a stable condition. The coal seam, the type of roof rock, and the gradation of the refuse influence the degree of compaction obtainable and the permeability of refuse. The shear strength of coal refuse appears to be controlled by its composition, moisture content, degree of compaction, amount and grain size of fines present. This investigation demonstrated these embankments should not be treated as pure state-of-the-art dams consideration must be given to the particular properties of coal refuse and the total refuse disposal system. Refuse dam slurry impoundments generally lack the site selection phase that is standard practice with siting most other types of dams.

Jan 1, 1976

By G. E. Anderson

THE Asientos-Tepezala district is in the north of the State of Aguascalientes, about 30 miles north of the city of Aguascalientes, the capital. The district is reached by a standard-gage railway on the west side through the junction, Rincon de Romos, which is on the main line of the National Railways of Mexico from El Paso to Aguascalientes. The terminal is Tepezala. On the east side a narrow gage connects Asientos with the junction, San Gil, on the main line, Asientos to San Luis Potosi. Tepezala on the west side of the Asientos hills is about 2 miles from Asientos on the east side. The area between the two mining centers is separated by a low subued divide which forms the central part of the hills. The district is in a group of isolated hills surrounded by characteristic bolson plains. The hills rise above the surrounding plains about 6500 to 10,000 ft.; the highest peak is Alta Mira. The two mining districts lie somewhat upon the flanks of the hills at about the same elevation, 7200 ft. The district is an old one and has been known since early in the 17th Century when the mines were worked by the Franciscan Monks. Their mining operations which were limited to the oxidized ore above the water level, consisted chiefly in extracting silver from the Santa Francisca vein. To judge from the old workings and the remains of surface plant, the operations were on a large scale. By modern mining operations since about 1900, the various mines of the district have been one of the principal sources of ore for the smelter located at Aguascalientes.

Jan 2, 1926

By C. R. Austin

In a recent paper1 the authors presented information on a refinement of the Rohn type of creep test with data on pure iron that exemplified the behavior of the apparatus. The present paper extends that work to nickel and cobalt and to the examination of the comparative high-temperature properties of two widely used commercial alloys. Careful attention has been given to the effect of plastic deformation in decreasing creep rate, using the elementary metals iron, nickel and cobalt for purposes of analysis. In any endeavor to develop new materials for high-temperature service one soon encounters a real problem as to the sort of test to be employed in evaluating the commercial possibilities of the alloys. At one time it was considered satisfactory to determine the tensile strengths at elevated temperatures and to use these values as a criterion for the commercial application of the alloys. More recently the proportional limit has been used as a means of classification. Data obtained from a form of bend test have also shown promise as providing a "merit index" of high-temperature serviceability.= Koerber and Pomp3 state that since the influence of testing time is small, the high-temperature yield point of steel furnishes sufficiently reliable and concordant values up to about 350" C. (and a maximum of 450" C. for alloy steels) to serve as a suitable basis for comparison evaluations of materials and for judging permissible stresses. Koerber and Pomp consider, however, that above these temperatures creep limit must be substituted for the high-temperature yield point as the all-important characteristic of the material. Whatever may be their definition of

Jan 1, 1934

By Edward Richardson

As DEVELOPED up to the end of the Great War, an airplane was essentially a mechanism of wood and fabric, joined and held together by metal fittings and, fastening. The engine and accessories, wire for wheel spokes and other tension members, and the occasional use of metal for interplane struts and the shafts of surface controls required some metal, but essentially it was a. wood and fabric structure. Attempts had been made to use metal for other purposes, such as struts in the drag-trussing of wing panels and for short interplane struts, and Junkers had produced an airplane of iron in 1915. By 1917 he was using steel tubes and duralu-min sheets, securing noteworthy advantages aerodynamically and from the standpoint of weigh PRINCIPLES OF AIRPLANE CONSTRUCTION The loaded weight of airplanes ranges from 500 lb. for the light plane of which we heard so much a short time ago, to about 40,000 lb. The great majority of planes range from 2000 to 10,000 lb. Of this weight from 15. to 30 per cent. is for the dividend-earning load. This percentage is so low that every per cent. saved on the rest of the weight increases the pay load from 3 to 6 per cent. The economic value of every pound that can be saved in designing or constructing a plane is sufficiently great to warrant spending money on refined methods of structural analysis, materials of superior strength properties for a given weight, methods of manufacture which may make possible the use of refinements in structural design, and the like, to an extent unknown heretofore.

Jan 1, 1928

By Guy Ruggles

IN this paper the authors aim to chronicle the experience and salient points brought out in changing flotation reagents at a concentrator which had probably been using a minimum amount of oil at a minimum cost per ton, to a more complicated system requiring a partial chemical control. The original oils were coal tar and pine oil. The new reagents are line, xanthate and pine oil. It was found that there were no insur-mountable difficulties and eventually the cost of the new reagents per ton of ore proved to be very little more than that of the old. LIME PLANT When the question of handling lime came up there were two propositions open: (1) to handle the lime dry; (2) to handle it wet in the form of an emulsion. Granting that a dry lime feeder would function properly, delivering lime uniformly, the chief objection was the dust that would be raised when these feeders were being charged or operated, as the only possible location was on a gallery beneath which men were working. With this objection the handling of lime as an emulsion seemed to he the better scheme. The lime plant which was first installed was for experimental work on one section. This was gradually changed and developed until it was of sufficient size to accommodate 20 sections. The various changes will not be discussed and the following description deals only with the fully developed plant. Crude as it is, and perhaps somewhat expensive to operate, the lime-handling plant has been quite successful. Much has been learned which will pave the way for a better installation in the future.

Jan 1, 1927

By R. B. Russell

SINCE Cahn's¹ paper on plastic deformation in orthorhombic (a) uranium an increasing amount of work is to be expected on orientation of single crystals and preferred orientation in this metal. It would seem desirable to have a table of angles between crystallographic planes (hkl) and (HKL). Table I shows these angles as distances from (loo), (0l0), and (00l), and in Fig. 1 as a standard (001) stereographic projection. Only the locations of poles whose diffracting planes produce Cu Ka reflections up to 0 = 70.5" (i.e. (154) ) are given." The relative reflection of these planes are represented by circle diameters. It is hoped that the projection may also be used in making inverse pole figures, originally suggested by Harris.' Values of 4 were calculated from the expression hH kK 1L cos = hH/a²+kK/b²-lL/c² where the lattice parameters a, b, c, and the spac-ings d were taken from the work of Jacob and Warren." (They report a = 2.852 kX (2.8578 A), b = 5.865 kX (5.8768 A), c = 4.945 kX (4.9550 A).) The parameters reported by them may be subject to revision,4,5 accordingly the interplanar angles are listed to only the nearest tenth of a degree. References ¹R. W. Cahn: Acta Metallurgica (1953) 1, p. 49. ¹G. B. Harris: Philosophical Magazine (1952) 43, p. 113. 3C. W. Jacob and B. E. Warren: Journal ACS (1937) 59, p. 2588. 4J. S. Lukesh: Acta Crystallographica (1949) 2, p. 420. ³ J. Thewlis: Nature (1951) 168, p. 198.

Jan 1, 1954

By Paul H. Lu, Fred D. Wright

Numerous laboratory tests and field measurements conducted by different persons have indicated that most mine rocks behave viscoelastically rather than elastically. Hence, for an accurate and realistic design of underground structure, the time factor should be taken into account, and the theory of viscoelasticity can be used. By considering the time factor, the time-safe-strength may be used for economical design instead of liberal safety factors with design based on the elasticity theory. The deflection and moment distributions on an elastic roof slab resting on elastic pillars have already been studied.l In this chapter, the stress- strain analysis for a viscoelastic mine roof resting on pillars of the same material or on supports of different material is first considered theoretically. Viscoelastic solutions for deflections, moments, and shears are de- rived from the elastic plate theory on the basis of the elastic-viscoelastic correspondence principle. The Laplace transformation technique is employed as the operational method in the mathematical treatment. The roof slab materials are represented by four different rheological models and four different loading histories are considered. The method of finite difference equations is employed for solving the partial differential equations, using FORTRAN programs to calculate roof deflections, moments, and shears. Numerical examples are given for one of the four models under two of the four loading histories with regular and staggered pillar patterns. The results are discussed in terms of the variance with time of deflections, moments, and shears as well as their variation in space due to the dimensions and configuration of the structure system. A viscoelastic slab resting on viscoelastic pillars or on elastic supports

Jan 1, 1972

By Clyde Orr

INTRODUCTION Cracks, crevices, holes, and fissures -- here collectively termed pores -- occur in all manner of substances in sizes from those evident to the unaided eye to those well below electron microscopic levels of detection. Naturally occurring materials having varying degrees of porosity are represented by wood, rock, soil, and mineral formations. Man-made materials that are porous include ceramics, sintered metals, catalysts, adsorbents, filters, and various compacted objects. The extent of their porosity greatly influences the hardness, rigidity, integrity, and general usefulness of such materials, and assessing both their pore volume and size is becoming increasingly necessary. Means by which this pore information can be obtained is the subject here. No attempt will be made to include all techniques and variations of them that have been suggested, but, hopefully, those of most general applicability will be described in sufficient detail to enable a potential user to judge their merit for a particular need. The results of pore measurements are made useful by relating them through other tests to performance criteria such as weatherability, water penetration, frost damage, oil recovery, strength, or thermal or electrical conductivity. These tests, of course, are outside the bounds for inclusion here. A pore is variously defined in dictionaries as "a minute opening", "a small interstice admitting passage of liquid", and "an interspace between particles". Clearly any object through which it is possible to find a continuous path from one side to the other either for a liquid or a gas must be regarded as porous. Passageways are not necessary for porosity to exist, however. There are so-called blind

Jan 1, 1980

By C Gunther

In the examination of an undeveloped prospect a decision must be arrived at from an inspection of the outcrops and the exposures in a few shallow pits. Prospects that are offered for sale rarely expose any important quantity of good ore, work having usually been stopped when the immediate exploration no longer yielded favorable results. It should be borne in mind, furthermore, that a great majority of prospects have been examined many times, and if of conspicuous promise, would have been acquired for development. In most cases the problem for the engineer, therefore, is to discover the traces of a valuable mineralization that has disappeared through solution, or the conditions that indicate the possibility, or probability, of underlying secondary enrichments. The available data being meager, every feature should be the subject of careful study-the condition of the outcrop, the structural relationships, the associations of minerals, the alterations of wall rocks, the chances for underlying enrichments, and so forth, as well as the actual assay value of the material exposed. It is frequently advisable to spend the time and money necessary to have trenches dug at various significant points along a promising outcrop; such exposures, even if the depth attained is slight, often disclose conditions that are not apparent at the actual surface, as for example, the existence of soft minerals and the distribution of the various minerals through the mass of the deposit. Trenches also permit samples to be taken from points that were not accessible during former examinations. Strong, persistent outcrops of uniform width may be taken to indicate the probable size and character of the underlying vein,

Jan 1, 1932