Search Documents

Jan 1, 1936

By Charles W. Frey

COMPRESSED air is one of the most useful tools that the mine operator has at his disposal. It is clean, nontoxic, easily handled, and can be distributed anywhere that a man can drag a length of rubber hose. Without it a great deal of drilling would still be a laborious hand process, and although the John Henry of legendary fame could beat a mechanical drill, John Henrys are few and far between, and in the long run the air drill is the victor. As a general rule, horizontal compressors are lubricated by mechanical lubricators, which furnish oil to the cylinder walls, whereas vertical units are splash lubricated. In most horizontal machines oil is introduced at the top center of the piston, from which point it can run down and lubricate the rest of the piston. Some large units introduce oil at more than one point on the cylinder wall, the necessity for this and the number of points of introduction depending upon the size of the cylinder.

Jan 1, 1938

By H. M. Varner

Under sponsorship of the US Bureau of Mines and the Department of Energy, a new type of rotary rock drill was developed by The Bendix Corp. Originally developed for the production of holes for the installation of longer than seam height roof bolts, the flexible drill is a useful tool for many drilling applications. It allows hands-off or remote production of holes for increased drilling speed and greater operator safety in any strata suitable for rotary drilling. This paper discusses various applications and experience for the flexible drill system in coal, metal/nonmetal mining, and for civil engineering uses. Manual, remote control, and automatic roof bolter modules are also discussed.

Jan 1, 1984

By Robert N. McIndoo, M. G. Woodle

10.5-1. Introduction. In the not-too-distant past, maintenance and the facilities for maintenance were considered a necessary evil to be given only secondary consideration in the development and planning of a surface mine operation. With the increasing sophistication of mining equipment and machinery, this concept has been replaced; and today's mine managers recognize that planning for adequate maintenance facilities is as important to a successful operation as a sound mining plan. This chapter will present the basic maintenance facilities and the fundamental considerations necessary to meet the varied requirements of surface mining. 10.5-2. Determination of Shop Facilities. The size, shape, and extent of maintenance facilities are contingent on the general mining plan, the location relative to outside facilities, and to the available capital. The type of construction employed is dependent on climatic conditions and preference. HAULAGE SYSTEMS. The type of haulage equipment used will dictate the kind of shops needed. Truck haulage requires entirely different facilities than rail haulage. Some mines will employ both systems, and can include hoists, skips, or conveyor belts in combination with locomotives or trucks. The requirements of each type will be discussed. MAINTENANCE ORGANIZATION. The pre-planning decision as to centralized maintenance vs. area maintenance, or a combination of both, will greatly affect the maintenance facilities required. A single pit with relatively short travel from shops to pit will probably have a centralized maintenance system. Where more than one mine is involved, or where there are diversified sections or functions in a mine (mine equipment, mill, conveyor, or

Jan 1, 1968

By Laurence Pellier

Electron microscopical studies were made of the effect of sensitization at 1200oF on a Type-.104 stainless steel with high carbon and low nitrogen and oxygen contents, after solution annealing and after cold working. The loci of the carbide precipitates in the early stages of sensitization were followed in dry stripped plastic replicas. Some observations were made by direct electron transmission on thinned films. Intragranular dislocations showed a high degree of stability through the annealing necessary to produce sensitization. I HE present work was undertaken to determine the effect of sensitization, at 1200°F, for 1/2 hr, on the electron microstructure of a Type-304 stainless steel with high carbon, and low nitrogen and oxygen contents, after solution annealing and after cold working. Many electron microscopical studies of sensitization of 18-8 stainless steels have already been made. Most of them utilized extraction replicas, from which the morphology and composition of the precipitated carbides could be established.'-" ry stripped replicas were seldom used for such studies, although it was demonstrated that they are applicable, in general, to EurveYs of the general microstructure of many alloys' and, in particular, to the observation of precipitated carbides in austenitic stainless steels.3

Jan 1, 1963

By M. J. Sheriff, D. H. Smith, W. Macdonald

The paper outlines the basic procedure used in electrical system planning and design. Consideration is given to the use of applicable codes and standards, development and use of a system One Line Diagram, power availability, load requirements and power factor. Also discussed are the factors influencing the selection of power supply and distribution arrangement, voltage levels, system grounding, distribution and utilization equipment such as switchgear, transformers, motor control, cable systems, motors. Short circuit calculation methods and system protection are discussed.

Jan 1, 1978

By Martin E. Chenevert

The early work of Terzaghi1 showed that the true stress state of a rock can be expressed by the principle of effective stress. This principle states that effective stress is equal to total applied stress minus the pore pressure. Since the introduction of this principle, a great effort has been made to measure the pore pressure of rocks so that the true stress state can be determined. For clean, permeable sandstones, it is usually found that the effective stress principle can easily be applied. Direct measurement of the pore pressure is achieved with a hydraulic pressure device and the effective stress state thereby computed. However, for fine-grained rocks which contain large amounts of clay (referred to herein as shales), pore pressure measurements are usually quite difficult because of the adsorptive forces of the clay platelets, which produce negative internal pressures. No suitable device is known for measuring internal pressures of dry shales; the effective stress approach is therefore not feasible and data are usually presented in another form. As an example, data from compressive strength tests are presented as a function of rock wetness, rather than effective confining stress. The work of Belopoloskii2 and Colback and Wiid3 are examples of such data. It was pointed out by Lambe and Whitman4 that a better way must be found to express the effective stress of argillaceous rocks. The information presented herein is an attempt to fill that need. Various authors have been able to measure moderate negative pore pressures (0-240 psi) within rather wet, argillaceous rocks by means of various pore pressure devices. In 1961 Lambe5 resented information which shows that pore pressures in argillaceous rocks are indeed negative ; he presented values of suction pressure as low as - 14 psi. Likewise,

Jan 1, 1970

By G. D. Cody, D. S. Beers, B. Abeles

The thermal diffusivity (thermal conductivity divided by specific heat) of Armco iron has been measured over the temperature range 30º to 1025ºC. The results are in good agreement with the thermal diffusivity computed from published values of thermal conductivity and specific heat, except near the Curie point and at the a to y phase transition, where the measured diffusivity undergoes rabid variations. The differences observed are ascribed to a rapid variation in the electronic thermal conductivity near the Curie point, and a decrease in the lattice thermal conductivity, at the a to ? phase transition. A major difficulty in measuring the thermal conductivity of materials at high temperatures is the determination and elimination of radiation losses. It has been shown, however, that in measuring thermal diffusivity D (thermal conductivity/specific heat) radiation losses can be taken into account exactly.' In order to test the performance of a newly designed apparatus for measuring thermal diffusivity of solids,' we measured Armco iron to an accuracy of 2 pct in the temperature range 30" to 1025°C. This material was selected since both the thermal conductivity,3'4 k, and specific heat,5 C, are known accurately in this temperature range and thus the measured values of D could be compared with D computed from the pub-

Jan 1, 1962

By C. F. Cigliana

TO accommodate a large tonnage at high speed from a single level at San Manuel, a very flexible haulage system was designed to give maximum access to stoping areas, minimize delays, and pro- vide for safety. With these requirements as a guide, the haulage was laid out as a simple double loop with one-way traffic of continuous pull. These loops consist of two parallel drifts 100 ft apart, center to center. Haulage drifts are numbered from north to south, the northernmost being the No. 1 drift. This holds true on both the North and South haulage loops. Haulage drifts are connected at 1000-ft intervals with crossovers so that it is possible to bypass any given section that may be inaccessible. Two of these turnouts are double diamonds, one located on the approach to the rotary dumps at the hoisting shafts and the other on the departure from the dumps. The turnout on the approach or North haulage is 835 ft in front of the rotary dump, and the one on the departure, or South haulage, is 1058 ft from the dump. This type of turnout allows trains coming down either haulage drift No. 1 or No. 2 to be routed into either shaft and expedites movement by avoiding delays due to congestion in the dumping area. The double diamond on the return haulage loop facilitates returning empty ore trains to the panels.

Jan 5, 1958

By V. G. Mooradian

THE diffusion of metal atoms into the crystal lattice of another metal and the assumption of regular positions to form a homogeneous alloy are some of the most fundamental phenomena of structural metallurgy. The mechanism of this process has been the subject of a great deal of study from many viewpoints, but the picture is still far from complete. It is generally believed that if one metal is to diffuse into another, the first must have some solubility in the second1. This means that the solvent must be able to accommodate the stranger atoms, either on points of its own lattice or in the interstices between, without losing its own crystal-lographic identity. It is also a general observation that cold-working is a distinct aid in the diffusion process, causing it to take place with greater rapidity at the same temperature or with the same speed at a lower temperature. There is considerable doubt, however, as to the real significance of the deformation in its effect on the diffusion mechanism. Several factors associated with the cold-working may be responsible for the increased diffusion rate. The plastic deformation causes a distor-tion of the solvent lattice, which may be an aid in the assimulation of atoms of another kind. There are many references2 in the literature that suggest that some distortion or "loosening" of the lattice is necessary for atomic migration and that diffusion cannot take place in the undis-

Jan 1, 1935

By Zay Jeffries

It has been known for centuries that iron and steel could be hardened by cold hammering and that the metal could be restored to the normal condition by heating to a red heat arid cooling, either rapidly or slowly with nearly pure iron, but slowly with steels containing considerable carbon. The art of tempering steels is also very old. When a medium- or high-carbon steel is quenched from a cherry-red heat, the hardness is increased; this hardness can be reduced by heating to various temperatures below the lower thermal critical point,, which is near 700" C. The present paper will not consider these old and well known changes in iron and steel below the thermal critical range but will confine itself to the more unusual changes, some of which have been studied recently. Stromeyer,1 for example, In 1886, reports that both working steel and heat treatment of steel in the blue-heat range arc highly injurious. Recently, however, some metallurgists believe that the straiglltening of warped steel forgings at a blue heat produces a beneficial effect on the metal2 The Engineering Division of the National Research Council has constituted a committee on Physical Changes in Iron and Steel below the Thermal Critical Range, which has the following personnel: K. R. -Abbott, H. C. Boynton, William Campbell, J. V. Emmons, F. B. Foley, H. J. French, H. M. Howe, Zay Jeffries (chairman), F. C. Langenberg, J. A. Mathews, P. D. Merica, A. H. Miller, J. H. Nelson, G. A. Reinhardt, W. E. Ruder, H. F. Wood. This committee is studying several aspects of the physical changes in iron and steel. The results, reported in the literature and obtained by recent experimentation, will be given; this will be followed by a general theoretical discussion of the more important

Jan 1, 1922

By Zay Jeffries

It has been known for centuries that iron and steel could be hardened by cold hammering and that the metal could be restored to the normal condition by heating to a red heat arid cooling, either rapidly or slowly with nearly pure iron, but slowly with steels containing considerable carbon. The art of tempering steels is also very old. When a medium- or high-carbon steel is quenched from a cherry-red heat, the hardness is increased; this hardness can be reduced by heating to various temperatures below the lower thermal critical point,, which is near 700" C. The present paper will not consider these old and well known changes in iron and steel below the thermal critical range but will confine itself to the more unusual changes, some of which have been studied recently. Stromeyer,1 for example, In 1886, reports that both working steel and heat treatment of steel in the blue-heat range arc highly injurious. Recently, however, some metallurgists believe that the straiglltening of warped steel forgings at a blue heat produces a beneficial effect on the metal2 The Engineering Division of the National Research Council has constituted a committee on Physical Changes in Iron and Steel below the Thermal Critical Range, which has the following personnel: K. R. -Abbott, H. C. Boynton, William Campbell, J. V. Emmons, F. B. Foley, H. J. French, H. M. Howe, Zay Jeffries (chairman), F. C. Langenberg, J. A. Mathews, P. D. Merica, A. H. Miller, J. H. Nelson, G. A. Reinhardt, W. E. Ruder, H. F. Wood. This committee is studying several aspects of the physical changes in iron and steel. The results, reported in the literature and obtained by recent experimentation, will be given; this will be followed by a general theoretical discussion of the more important

Jan 1, 1922

LIST OF MEMBERS AND ASSOCIATES ARRANGED ACCORDING TO STATES AND TOWNS. ALABAMA. Aldrich, Aldrich, W. F. Anniston, Noble, A. E Bessemer, Fergusson, V. Bir¬mingham, Aldrich, T. H., Jr, Bowron, C. E, Bowron, J., Davis, F H, Dimmick, F. D., Hillman, Ludwig, C. P., McCarthy, M. E, McCormack, G B., McCrery, Ramsay, E, Shook, A. M. Ensley, McDonald, S., Shook, J. W. Gadsden, Lee, R. H., Jr. Gannts Quarry, Hiller, E Helena, Squire, J. Huntsvalle, Moss Le¬high, Toulmin. Ozmoor, Greenamyer, A. G. Rock Run, Garvin, J M. Selma, Saylor, B. F. A. Sheffield, Miller, D, I. Tuscaloosa, Lodge, J. Woodward, Wood¬ward, A. H. ALASKA Coppermount, Wiswell, H. J. Douglas Island, Macdonald, J Ellamar, Stevens, B. Fairbanks, Hamilton, A L Haines, Le Wald, E A. Jualin, Hoggatt, W. B. Juneau, Denby, Hampton, W. H , Thane, B L , Whipple, J. R. Ketchikan, Bar¬ber, J C , Parker, J. L., Reid, G D Nome, Garfield, C D. Snettwham, Tisdale. Teller, Hirshberg, M. R. Treadwell Caty, Kinzie, Stow, H. P. Valdez, Esterly, G. M, Hazelet.

Jan 1, 1910

By Chas. H. Behre, Arbiter. Nathaniel

Two outstanding factors characterize most undertakings in the mineral industries: (1) mineral deposits are not equally distributed over the earth but are localized by natural processes, and (2) the mineral industries are largely based upon wasting assets. Neither of these factors is invariably true, for a few mineral materials, such as water, are essentially world-wide and others are replaced periodically by processes not unlike crop production. Indeed, with some, such as the salts in the sea, the supply is cyclic in short or long periods, whereas others, like water, though locally or periodically in short supply, are essentially inexhaustible. But to the extent that most minerals are limited to certain areas and are "permanently" consumed, any industry founded upon them, by implication, demands great care in planning for the most effective extraction-the essence of wise and conservative expenditure. Several less unusual yet also truly fundamental factors in the economics of the mineral industry are those involving (1) the various kinds and properties of mineral materials, which influence their use; (2) their geo- logic and geographic availability; and (3) the techniques unique to their discovery, exploration, and utilization. This chapter is designed primarily as a brief background for later ones and as a summary of the several un- usual or at least special factors and of their effect upon the economics of the mineral industry. The social and political and especially the economic aspects of the industry are the main subjects of this volume and are dealt with in extended fashion in subsequent chapters. Fundamental Characteristics of Mineral Deposits DEFINITIONS Minerals are naturally occurring compounds of nonliving matter, each technically defined as having (1) a chemical composition that may vary

Jan 1, 1959

By W. C. Ellis, M. E. Fine

WHEN certain binary Fe-Ni alloys are worked cold and then stabilized by a stress-relief anneal, their Young's moduli are nearly invariant over a substantial temperature range determined by composition and work-anneal history.' In the present investigation addition of molybdenum to such alloys (besides increasing the yield strength) was found to diminish the sensitivity of the thermal coefficient of Young's modulus to composition changes. Such alloys are of interest for applications requiring 1 a metal with controlled, low temperature coefficient of Young's modulus and substantial magnetic permeability.' Young's modulus ordinarily decreases with rising temperature. In ferromagnetic alloys a change in modulus on heating due to loss of ferromagnetism modifies the .temperature dependence. Far below the Curie temperature ferromagnetism alters the modulus in two ways: a—Addition of the energy of magnetization, a function of interatomic distance, changes the relation between interatomic energy and distance.' This lowers the modulus in the Fe-Ni and Fe-Ni-Mo alloys of this investigation. b—An applied stress changes the ferromagnetic domain arrangement so that linear magnetostriction contributes to the strain and further lowers the modulUs.~ Since in the alloys of this investigation both effects lower the modulus, loss of ferromagnetism on heating changes the slope of the modulus-temperature curve in a positive direction. With increasing temperature the slope of the modulus-temperature curve, due to progressive loss of ferromagnetism, becomes less negative, goes through zero (the modulus is minimum), becomes positive, and resumes its normal negative value above the Curie temperature.' The increase in modulus and decrease in curvature in the modulus-temperature curve occurring on work hardening have been attributed to a reduction in effect (b).'. Vn the absence of ferromagnetism work hardening through introduction of residual strains would be expected to decrease the modulus.' Alloy Preparation—Test Methods Four series of alloys having nominal molybdenum contents of 5, 7, 9, and 10 pct and containing 47 to 58 pct Fe (the analyzed compositions are given in Table I) were melted in air and cast as bars weighing from 2 to 6 lb. The charge consisted of electrolytic nickel, Armco iron, molybdenum (99+ pct) or ferromolybdenum (62 pct Mo), manganese, and aluminum as a deoxidizer. The bars were swaged cold to the desired diameters with intermediate anneals. Alloys in this composition range are reported" to be face-centered cubic and ferromagnetic at room temperature. The Curie temperatures" decrease with molybdenum. For example, keeping the Ni/Fe ratio at unity and increasing the molybdenum from 0 to 17 pct decreases the Curie tempera- ture from approximately 500°C to room temperature. (The 17 pct Mo alloy requires quenching to retain a single phase.~) Young's modulus at a series of temperatures from —50" to 100°C was determined by a dynamic method previously described.', " In this method Young's modulus is calculated from the resonant frequency (in this case approximately 50,000 cycles per second) of a rod sample (approximately 2 in. long) vibrating longitudinally in the first mode. The densities of the samples at room temperature, Table I, were determined by the standard method of weighing in air and reweighing in redistilled bromobenzene. The linear coefficients of expansion, Table I, needed to calculate the sample length and density at the temperature of measurement, were obtained by observing the change in length (22" to 100°C) of an 8 in. gage distance with a two microscope cathetometer. The values of density and coefficient of expansion in the few samples checked are independent of treatment to the accuracy reported. This was assumed to be general for all the samples. The modulus values are accurate to &0.05x101' dynes per sq cm. However, values of thermal change in modulus are accurate to 20.005~ 10" dynes per sq cm. Young's Modulus Values at 25°C: The modulus values of O', 5, and 10 pct Mo alloys at 25 °C are plotted in Fig. 1 as functions of nickel. After working cold, the samples were either recrystallized at 950" to 1000°C or given a low temperature stabilizing anneal at 400° C. The 400°C anneal does not reduce the hardness nor cause recrystallization. Annealing at 400 °C does

Jan 1, 1952

By Frank C. Aplleyard

The two calcium sulfate minerals-gypsum and anhydrite-occur in many parts of the world, and gypsum has long been of economic importance in the family of industrial minerals. Gypsum, the dihydrate form of calcium sulfate (CaSO4•2H2O) and anhydrite, the anhydrous form (CaSO4) are frequently found in close association, and it is seldom that a calcium sulfate deposit will consist exclusively of one mineral or the other. Although known gypsum deposits are extensive, anhydrite makes up the largest part of total calcium sulfate reserves. However, it has very minor economic use, and most of the following discussion will be devoted to gypsum. Calcium sulfate is one of the principal constituents of evaporite deposits, and when pure, has the following composition: [Sulfur Combined Lime Trioxide Water (CaO) (SO,) (H2O) Gypsum 32.6% 46.5% 20.9% Anhydrite 41.2% 58.8% -] Deposits of pure gypsum or of pure anhydrite which are large enough to be considered commercial have never been found because of both the metastable relationship between the two minerals, and the presence of impurities such as calcium or magnesium carbonates, chlorides, other sulfate minerals, clay minerals, or silica. As a result most mine production of gypsum will range between 85 and 95% pure. Often it is used as mined, although in certain cases, one or more forms of mineral beneficiation are employed to upgrade the product. End Uses The largest use for gypsum is based upon the unique property which calcium sulfate has of readily giving up, or taking on, water of crystallization. With the application of a moderate amount of heat in a process known as calcining, gypsum is converted to plaster of paris (the hemihydrate of calcium sulfate, CaSO4 ½ H2O) which when mixed with water will set or harden as the calcium sulfate returns to the dihydrate form. This semifinished product, usually called stucco, is then manufactured into a large variety of plasters, wallboard, and block for construction use, or into plasters for industrial applications. About 75% of the gypsum used in the United States is calcined for these purposes. Uncalcined uses of gypsum are principally as a retarder for portland cement, as a soil conditioner, as a mineral filler, and other minor industrial applications. About 25% of the gypsum mined in the United States goes into these markets; however, in less developed countries where construction and other industrial uses may be quite limited, the use of uncalcined gypsum for portland cement retarder may well be the dominant-if not the only-market for the mineral. It has often been noted that calcium sulfate constitutes the world's largest reserve of sulfur, and minor use of both anhydrite and gypsum has been made to produce sulfuric acid or other sulfur compounds such as ammonium sulfate. However, because sulfur is readily available at lower cost from other sources, use of calcium sulfate has been limited to only a few locations where it could compete economically. No such use is reported in the United States at this time. Mineralogy Anhydrite, CaSO4 because of its geologically rapid conversion to gypsum, and relatively high solubility (about 0.2 g per 100 g H2O) is not often found outcropping in climates wet enough to support abundant vegetation except on steeply dipping slopes or other places where hydrated material is continuously removed. Anhydrite rock is most often light to bluish gray in color, and under the microscope, varies

Jan 1, 1983

By Chauncey L. Berrien

The United Verde Copper Company's mine is at Jerome, Ariz. The orebodies arc of the schist replacement type, the main sulfide mass being a large lens-shaped body approximately 7 acres in cross-sectional area. In general, the mineralization is found along the iron schist contact, with the orebodies extending about 1000 ft. along this contact and varying from a few feet to 250 ft. in width. Three types of ore are mined; schist porphyry and the so-called "heavy" or "massive sulfide" ore. Approximately 50 per cent. of the total tonnage mined at present is of the latter class. A representative analysis of this ore indicates 42 to 46 per cent. sulfur. Because of the high sulfur content, blasting in this ore is an extremely hazardous operation. not only does it endanger the lives of men who may be in the mine at blasting time but it also becomes a potential fire menace when timber is in close proximity to shots being fired in these massive sulfides. The origin of the fires of 38 years ago, embers of which are glowing to this day, may perhaps be attributed to dust explosions due to blasting in the massive sulfides of those early square-set stopes. Dust Explosions Dust explosions in coal mines are a common source of mine disasters. That such explosions may be of common occurrence in metal mines is not generally known, but it is known that metal-mine dust will explode and such explosions have been investigated by the U. S. Bureau of Mines at the Pittsburgh Experiment Station. Samples of "heavy sulfide" ore from the United Verde mine were used in the tests. After studying the results of the tests it was concluded1 that: 1. Dust explosions were initiated in sulfide ore dust in the gallery with a charge of as low as 75 g. of 60 per cent. gelatin dynamite. 2. The exploding dust generated considerable pressure.

Jan 1, 1930

By D. J. Jr. McAdam

RESULTS of a general survey of corrosion-fatigue phenomena at the Naval Engineering Experiment Station were presented in five previous papers.1-5 In these papers references were given to the work of Haigh in 1917.6 The five papers mentioned gave a general survey of the behavior of metals when tested to failure under simultaneous corrosion and cyclic stress. As a result of this survey the fourth paper gave a tentative analysis of the corrosion-fatigue process. According to this analysis, if the initial stress is below the endurance limit but above the corrosion-fatigue limit, the corrosion-fatigue process is divided into two periods. The first period is the formation of pits. As the initial stress is below the endurance limit, the pitting is assumed to be due to corrosion, not to fatigue. The corrosion pitting, however, is not the ordinary pitting of stressless corrosion, but is an accelerated pitting due to the effect of the cyclic stress on the corrosion. As the pit progresses, both the actual stress and the effective solution pressure increase. The increase of actual stress is due to increasing stress concentration at the bottoms of the pits. The increase of effective solution pressure is due to the increase of actual stress. The mutual intensification of stress and solution pressure continues until the actual stress at the bottoms of the pits surpasses the endurance limit. The second period of corrosion-fatigue then begins. In this period the process is merely fatigue accelerated by the corrosion. The cracks advance at a continually accelerated rate until the specimen breaks.

Jan 1, 1930

By C. W. Davis, R. S. Dean

AMONG naturally occurring inorganic compounds, magnetite has many unusual and interesting properties, and it is the purpose of this paper to call attention to these properties and review possible uses of magnetite that might be based on them; in other words, to see what uses may be found for magnetite as a physical rather than as a chemical entity, as a mineral rather than as an iron ore. The Mineral Physics Studies initiated in the Metallurgical Division of the U. S. Bureau of Mines a few years ago gathered together much information on mineral properties, and they are therefore the principal source of material for this paper. Magnetic Properties Of Magnetite And Possible Uses As A Magnetic Material Magnetite is unique among minerals for its great magnetic susceptibility, approximately one-third that of soft iron, and some magnetites are permanent magnets, or lodestones. The literature contains many measurements of magnetic properties of individual magnetite crystals, but no systematic survey of the effect of mechanical preparation and heat-treatment was available until the recent investigations of the Bureau of which included not only susceptibility but coercive force and remanence as well. For the purpose of orientation, Table 1 gives the magnetic properties of several samples of magnetite and of a few other substances3. This table indicates that when ordinary magnetite is powdered it decreases in susceptibility and increases in coercive force and remanence, approaching the properties of lodestone. The properties of lodestone are in fact probably due to the development of very large internal surface on fine grain size4.

Jan 1, 1937

By H. O. Hofman

With the exception of the exhaustive paper on the Father de Smet mill, by its designer, Mr. A. J. Bowie, Jr. (Bans., x. 87), nothing, so far as the writer is aware, has as yet appeared on the stamp-mills of the Black Hills of Dakota, although they have been steady producers for the last ten years. Within the small area of about 6000 by 1600 feet, $2,271,341.14 was produced in 1887† from rock averaging $4 per ton in free gold. I. The Ores. Character of the Ores.—The gold of the district is found in quartz and pyrites finely distributed through vast bodies of mica- and amphibole-schists, argillites and phyllites, and also impregnating the schists themselves. The " Belt," embracing the section of Lead City, Terraville and Central City, is the only part worked at present. The principal associated mineral is iron pyrites, with some arsenopyrite and chalcopyrite, garnet and asbestos. The ores from open cuts and upper levels are more free-milling. than those from underground workings below water-line. Hence

Jan 1, 1889