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By L. S. Darken

It has long been recognized that the driving force in an isothermal diffusion process may be regarded as the negative gradient of the chemical potential (partial molal free energy) of the diffusing substance. This equivalence is exactly analogous to the fact that a mechanical force is the negative gradient of the potential energy. Nernstl in 1888 expressed these driving forces in terms of osmotic pressure, which is directly related to the chemical potential and free energy. The relationship between the diffusivity, mobility and activity coefficient of electrolytes in aqueous solution has been developed in great detail by Onsager and FuossZ with results which have recently been confirmed experimentally by Harned and Nuttall. This interpretation of the driving force in isothermal diffusion has been discussed earlier by the author,4,5 who pointed out4 that "for a system of more than two components it.is no longer necessarily true that a given element tends to diffuse toward a region of lower concentration even within a single phase region"; departure from the behavior of an ideal solution may be so great that the concentration gradient and the chemical potential gradient, or activity gradient, may be of different sign, thus giving rise to "up hill" diffusion. Since, in the discussion of that paper,4 some skepti- cism as to the validity of this conclusion was expressed, it seemed desirable to adduce experimental evidence to support it. Evidence of such diffusion in nonmetallic systems has already been reported. For example, it was observed by Hartley" in a series of experiments in which sodium chloride diffused from a saturated solution in about 5 pct acetone in water into a solution virtually free of salt but with essentially the same acetone concentration. Specifically, a wine glass containing crystalline salt and saturated solution was totally immersed in a large glass cylinder containing the acetone-water solution without salt; the difference in specific gravity was such as to prevent convection. Diffusion of salt was accompanied by a marked depletion of acetone in the wine glass; after 3 days the acetone concentration decreased from 5.37 pct at the top of the glass, where the salt concentration was low (0.93 pct), to 1.84 pct at the bottom where the salt concentration was high (21.64 pct). This "uphill" diffusion was found to be such that the partial pressure (activity) of acetone was essentially equalized throughout the system when a steady state gradient of sodium chloride was maintained. In order to demonstrate the existence of such diffusion in metals, a series of four weld-diffusion experiments was made. In these measurements pairs of steel of virtually the same carbon content, but differing markedly in alloy content, were welded at the end and held at 10500 C for about two weeks. Subsequent analysis showed that carbon had diffused so as to produce an inequality

Jan 1, 1949

By Howard R. Keil

Prior to World War 11, approximately 500 tons per day, or one-third of the ore being shipped to the Golden Cycle Mill at Colorado Springs, Colo., from the Cripple Creek district, was being treated in the flotation plant. The remaining 1000 tons, along with the flotation concentrates, received the usual roasting and cyanide treatment. Since it was not possible to make a disposable tailing on the general run of Cripple Creek ores by flotation alone, it was necessary to treat the flotation tailings by cyanidation, along with the roasted ore. By this treatment, an overall economical tailing could be made. The gold ores from the cripple creek district are the highly siliceous sulphotel-lurides with approximately 3 pet total sulphides. In the lower grade ores most of the values are found in very finely disseminated pyrite. In some of the higher grade ores the tellurides, calaverite and sylvanite, can be recognized. After several years of floating cripple Creek gold ores, having a very low mineral content, it was concluded that this type of ore required a type of flotation machine that would give a rather violent aeration and agitation. This action produced a fairly deep froth column which contained not only the free mineral, principally pyrite, but also most of the middling product; this was most essential in producing a low grade tailing by flotation alone that could be discarded to waste. After investigating most of the current types of flotation machines, it was decided to convert to the Fagergren type. During World War 11, the flotation plant was modified to handle lead and zinc ores. The treatment of these ores was carried on until April 1946. At this time the flotation plant was again reconverted to handle the Cripple Creek ores. Selecting Ores for Flotation During the period of reconversion a considerable amount of experimental work was carried on in the laboratory to determine if a disposable tailing could be made by flotation alone, on certain selected Ores from the Cripple Creek district. It was found that most of the dump Ores and a few of the low grade mine ores could be treated by this method with very satisfactory results. After the preliminary test work in the laboratory, it was decided to carry on further work On a plant-size scale to see if similar results could be obtained. On April 15, 1946, a part of the plant was put into operation as a pilot plant. The various ores that had been found suitable for treatment by this method in the laboratory, were run through the pilot plant at the rate of 200 tons per day for a period of several months. Results were so encouraging that it was decided to put the plant in full-scale operation. Since that time plant capacity has been the from 500 to 1000 tons per day. This has been done not by the installation of more machinery but by (I) crush-

Jan 1, 1949

By A. S. Eve

This paper gives a brief summary of theory on radiation waves and describes experiments by the United States Bureau of Mines and by the Canadian Geological Survey at the Mammoth Cave, Kentucky. Dr. D. A. Keys, Dr. F. W. Lee and the writer did the experimental work, and Dr. L. V. King some of the theoretical work. In prospecting for orebodies underground it is a usual practice to employ a loop of wire carrying an alternating current that induces an electromotive force in good conductors beneath the earth. The conduc-tars are usually of such low resistance and impedance that currents of considerable magnitude may result in these orebodies or conductors, and these currents in turn induce an electromotive force in a detecting coil placed near the surface of the ground. By using amplifiers it is possible so to enhance the effect of this electromotive force in the detecting coil that the currents in the underground conductors are detected in spite of the powerful effects coming directly to the coil from the primary loop. In such cases distance or depth has primary importance, for the effects have to pass down to the orebody and the secondary effects induced therein have again to pass back through the ground to the observer's detection coil on the surface. However, there is another factor to consider—absorption of the electromagnetic disturbances by the intervening soils and rocks. This absorption will vary with their composition. Thus the main factors that determine the possibility of detecting ore underground are: 1. Its depth beneath the surface, 2. The magnitude of the conducting orebody, 3. The excellence of the conductivity of the orebody as compared with that of the surrounding strata, 4. The frequency of the alternating current,

Jan 1, 1932

By Perry Harrison

THE mixing of fine ores with fuel and burning under induced draft is called sintering in iron-ore practice and either sintering or roasting in copper and lead metallurgy. The first development of sintering was the Huntington-Heberlein process, in which fine sulfide ores mixed with fuel were charged into a pot and burned under the action of an upcast current of air introduced at the bottom of the pot. Modern development has resulted in two types of machine; the continuous Dwight-Lloyd machine, which consists of heat-resisting grates and a moving beltlike type of hearth constructed of metal, in which air is drawn downward through the bed, and the stationary Greenawalt batch process, which operates in the same manner as the Dwight-Lloyd except that the hearth is stationary. The recent rapid increase in the use of sintering has been the result of two great advances in metallurgical process-flotation and the treatment of flue dust. The Dwight-Lloyd machine is used in many plants for agglomerating flotation concentrates and at the same time reducing the sulfur content to furnace requirements and it has become standard practice to treat iron blast-furnace flue dust by either the Greenawalt or Dwight-Lloyd machine, so that it can be recharged into the furnace. The mining of low-grade magnetite iron deposits and their beneficiating by grinding and magnetic separation resulted in a fine concentrate extremely detrimental to blast-furnace practice. These concentrates are sintered and made into ideal material for blast-furnace use. Incidentally, coke savings of about 400 lb. per ton of pig are reported.

Jan 1, 1930

By Clarence P. Linville

It is recognized that, in all metallurgical operations, the greatest possible uniformity in all conditions is essential to the best results. It is the constant aim of metallurgists to secure this uniformity, and their success in this respect has been in many cases the cause of great advances in the metallurgical processes themselves. Of all such processes, the production of pig-iron in the modern blast-furnace is perhaps the most complex and the most difficult of control. Notwithstanding the great progress of recent years, there is still great difficulty in obtaining anything like uniform conditions and results in the manufacture of pig-iron. The reason is not far to seek. The ores vary in composition and physical properties, often carrying a varying amount of gangue material, unevenly distributed. The coke may vary in composition, cell-structure, moisture, hardness, and other properties. The blast, the weight of which far exceeds that of any other constituent, varies in its moisture-content and consequently in its chemical composition; and its temperature, by reason of the method of heating, is not uniform, but, besides varying from time to time with atmospheric and other outside conditions, has a maximum and minimum for every changing of stoves. Now, with regard to these variations, the modern demand for uniformity has been greatly aided. Chemical analysis has made it possible to select ores so that the composition of the charge can be kept fairly constant; coal may be obtained from a given field, and coked in the same kind of ovens under conditions which make the fuel practically uniform; the Gayley

Jan 1, 1911

By D. I. Bardos, Paul A. Beck

A MARK, Boren, and westgrenl in their X-ray diffraction study of manganese alloys with 10 to 25 at. pct Si reported the occurrence of a phase with complex crystal structure in a Mn + 14.5 at. pct Si alloy after annealing at 800° or at 600°C. Recently Kuzma and Gladyshevskii2 identified Mn6Si as an R phase,3,4 the second binary R phase known. (A binary R phase was previously found in the Mn-Ti system by Waterstrat.5) A ternary R phase was reported earlier0 near the Mn-Si binary border of the Ti-Mn-Si system at 1000°C. In the light of the Russian work it now appears that this "ternary" R phase may be in fact an extension of the binary (Mn6Si)R phase. As already found by Amark, Boren, and Westgren, the corresponding binary phase is absent at 1000°C; the effect of titanium is evidently to stabilize it up to higher temperatures. In a recent paper Wieser and Forgeng7 gave the boundaries of this binary phase, which they referred to as 6. In order to re-examine the identity of this phase, an alloy of Mn + 14.5 at. pct Si was prepared by induction melting in helium atmosphere and annealed for 3 days at 800°C. The powder pattern, taken with an asymmetrical focusing camera of large dispersion and shown in Table I, was found to be that of the R phase, in agreement with the results of Kuzma and Gladyshevskii. For comparison, Table I also lists

Jan 1, 1965

By D. L. Pentz

The use of "probability" methods in slope stability analysis has been practised by geotechnical engineers for some time. This paper questions whether the term probability has been properly used since in many cases the term is used in a classical statistical manner only to describe frequency distributions of data. This paper proposes a definition of the term probability which includes the engineers' subjective state of confidence for all critical parameters. Such parameters may include the slope stability model, undetected structural features, strength, groundwater pressure distribution, and geological framework. This process does not make a "better" analysis but it does force the geotechnical engineer to state his opinion in such a manner that others can understand and if necessary, criticize the assumptions. Further, this technique forces the question, "What impact would failure of a slope have on the production schedule or costs of a mine?" The level of risk taken in adopting a particular slope design strategy in a mine is not a question of what Factor of Safety should be selected but, rather, what trade-offs result from steeper slopes and the associated costs of failure. This paper also presents several simple probabilistic techniques which, though widely used in other fields have been all but ignored in geotechnical slope stability analyses. These include the use of stratified sampling to optimize Monte Carlo uncertainty approaches, and the use of simple functional approximations rather than complex stability models in probabilistic analysis. The author presents a case example of this technique and discusses the results in comparison with more traditional methods.

Jan 1, 1983

By Henry Hibbard

This paper aims to put into useful form the information, at hand regarding temperatures of molten steels, covering all carbon contents up to 1.5 per cent., in the hope that if the assumed ideal temperatures tabulated are incorrect the right ones may . be brought to light. THE finishing and casting temperature of steel is a subject concerning which there is so much to learn that some temerity is required to write about it. This paper is, therefore, suggestive rather than dogmatic, though the views expressed seem now to the writer to be correct; nevertheless, they may be modified by future developments or discoveries but until then may be of some utility, particularly to beginners in the making of steel. Consideration is limited to the temperatures of simple steels, also called carbon steels,, which are those in which carbon is the only element introduced for the purpose of conferring desired physical properties. Alloy steels will receive attention later. Three things largely determine what a batch of steel really will be; viz., composition, casting temperature, and rate of teeming. Each has its advocates as being the most important; but they are so interrelated that each must conform to or be right for the, other two. When one is claimed to be the most essential to have right, that, probably, was the one that it was found had to be of a certain description, within narrow limits, to suit the already adopted variations of the other two; that is, it was the last of the three to be brought into line.

Jan 12, 1924

By J. Bruce Wagner, Roger L. Levin

Lattice-parameter measurements have been carrzed out at room temperature on undoped and on chromium-doped wustite as a function of oxygen concentration. Results may he expressed by for the undoped uustite. These data are in agreement with and extend the work of' Jette and Foole. The present results are consistent with the cation vacancy model. for metal- deficil wuslite, both undopen and doped. SEVERAL investigators have made room-temperature lattice-parameter measurements on wustite as a function of composition. Jette and Foote used these measurements to determine the phase limits of wustite. Willis and Rooksby and Benard have also measured the lattice parameters of wustite at room temperature, but their values of a. vary considerably from those of Jette and Foote. In conjunction with Studies' on the transition from surface-controlled heterogeneous reactions involving CO-C02 oxidation of metals, lattice-parameter measurements on undoped and chromium-doped wiistite have been performed. These studies are especially needed to clarify conflicting data of previous authors, to extend the data to the stability limits of wiistite as determined by Darken and urr,6 which include a larger range of existence than that found by Jette and Foote, and to test by X-ray means the role of chromium as a vacancy producer in wiistite.

Jan 1, 1967

By L. W. Eastwood

THE recrystallization and grain-growth phenomena of cold-worked metals have considerable industrial importance because of their role in the fabrication of metals. For this reason, and because of the general academic interest in the subject, these processes have received consider-able attention since the time of J. E. Stead's paper1 on The Crystalline Structure of Iron and Steel. There are considerable differences of opinion regarding the theory of recrystallization and grain growth and also differences in the observation of the modus operandi of the processes themselves. The value of many of the existing quantitative data on grain growth is decreased by the failure to distinguish between the grain sizes just after complete recrystallization and those resulting from coales-cence. For this reason also, the value of the diagrams of stable grain sizes, like those produced by Bass and Glocker2 and Czochralski3, is lessened in so far as an interpretation of the facts of recrystallization is concerned. It is hoped that the data presented in this paper will help to clarify and to simplify the facts relating to recrystallization, and at the same time will show that some of the present-day conceptions are unten-able. A paper on the theory of recrystallization, with additional data, will be completed in the near future by the writers.

Jan 1, 1935

By J. J. Scott

During 1967 wage settlements and strikes ran at a high rate and it has been estimated that union gains of 4 ½ % to 5% per year have been attained in many instances. When these events are coupled with the need to operate on lower grade and deeper deposits the need for technological advances in underground mining becomes obvious. Emphasis has been on ways to reduce the number of manshifts required per ton of product. Most significant advances have been made in rapid advance of primary mine openings, development of mobile, rubber-tired, front-end loaders and hauling units, high-speed drilling with rotary and rotary percussive units and a serious look at nuclear devices to break large volumes of ore cheaply. Boring Developments in bit manufacture and metallurgy have allowed continuous boring-type machines to be used on harder formations than was deemed feasible in the past. Hecla Mining Co. has been using raise boring equipment for drilling 9 7/8 in. diam pilot holes in raises which are later reamed in an upward direction to 48 in. diam. The machine used is a J. S. Robbins model 42R raise borer which has been used successfully to drill two relatively long 48-in. diam holes. The first was 520 ft in length at a dip of 76º, and the second was a 785-ft vertical hole.

Jan 2, 1968

By Emil Gathmann

Through wide experience at numerous mills in the United States I have found that there is a decided difference of opinion among the producers of steel as to what constitutes commercially sound steel. Some metallurgists insist upon having, for certain grades of steel, a so-called "open steel," i.e., one which contains numerous blowholes of varying sizes, and consequently but little volume of true pipe, although a central intermittent shrinkage cavity often extends well down into the ingot. Such blowholes and pipe are expected to weld up during rolling or forging, because their surfaces are not oxidized. Even if this be admitted, included slag particles and a high degree of segregation are bound to be present in the product of such ingots. Blowholes are the result of an oxidized heat, and all heats, unless made or finished in the electric furnace, are oxidized more or less. Subsequent deoxidation, or rather degasification, either in the ladle or in the mold, or in both, is necessary to reduce or prevent blowholes. As is well known, a greater yield of billets or sheets is secured from the ingot by allowing the formation of blowholes, but this is undoubtedly obtained at the expense of the quality of the product. Decided segregation of carbon, phosphorus, and sulphur, as well as small included slag particles, undoubtedly exists throughout the greater portion of all so-called open steel, and cannot be removed by cropping. The first requisite for sound, homogeneous steel ingots and blooms is therefore, in my opinion, so to treat the steel in the furnace, ladle, and mold that "piping" steel is produced. The line of demarcation between harmful and so-called "harmless" blowholes is practically impossible to define. Is it not, therefore, the better and safer practice to use means for the elimination of blowholes and correspondingly reduce the segregation and allow the formation of a well-defined shrinkage cavity or pipe at the upper end of the ingot? With the relatively cheap deoxidizers available at the present time, e.g., ferro-silicon, aluminum and titanium, there is no commercial reason why all steel should not be thoroughly degasified. A degasified steel being provided, the method of freezing or solidifying the liquid steel into an ingot, and the subsequent working into blooms and thence into various products, is of great importance in reducing the crop or scrap portion of the steel, due to segregation and pipe. How to ac-

Jan 1, 1916

By Colin C. Harris, Nathaniel Arbiter

Flotation kinetics is the study of the variation in amount of froth overflow product with flotation time, and the quantitative identification of all rate con- trolling variables. With such variables maintained constant, the algebraic relationship between the proportion of mineral floated and flotation time is a flotation rate equation. This contains the constant values of all the rate determining variables implicit in one or more rate constants which must be evaluated from experimental data. The form or forms of the rate equation can either be deduced from assumptions or facts established about the mechanism of the process, or, more commonly, determined empirically or by analogy with similar rate process. (See pages 215 to 226 and 221 to 227.) The earliest contributions on flotation kinetics came in the mid- ~hirties,3,27,31 with interest increasing in recent years. The broad goal of these studies is the reduction of the total variability of a flotation system to equations among the individual variables, or among a lesser number of dimensionless groups.43 Although the goal is at present far from realization, its attainment will yield many practical advantages. The quantitative under- standing of the process should lead to improvements both in metallurgical results and flotation cell design. It should lead also to reliable methods for scaling-up* of laboratory and pilot plant results to full size operation without the need for expensive and time-consuming mill tests on commercial machines. Finally, this type of quantitative information will assist the automation of flotation cells and circuits as well as control by computers. Replacing present methods of control based on post-mortem assays and trial and error manipulation of reagents and cell variables by control through exact process analysis should yield higher levels of grade and recovery through reduction of the large variability of present-day results. CHEMICAL ENGINEERING ANALOGIES: Industrial flotation always involves flow of pulp through a series of stirred tanks. It thus bears formal analogy to many chemical engineering processes, the kinetics of which in flow systems has been studied for many de- cades.38,39,73,83 It will be useful to review these studies before considering flotation kinetics. Batch and continuous treatment are classified in Fig. 1. Batch operation in the true sense is unknown in flotation where concentrate is always removed

Jan 1, 1962

By A. G. Roach

THIS paper deals with a plant built under joint agreement between the Canadian and United States Governments to supply the strategic mineral, corundum, at a time when African production was dwindling and war demands still increasing. The operation began in September 1944, retreating the tailings from a former mill by gravity methods, and is still in production (Sept. 1946). In 1942, when it became apparent that South African production would be insufficient to meet mounting wartime requirements of corundum, a survey was made of Canadian sources of the mineral, at the request of the Foreign Economic Administration of the U. S. Government. The investigation1 indicated that the quickest and cheapest supply would be obtained by reworking the tailings deposit at Craigmont, Ontario. Sampling of the dump by Canadian Government engineers revealed that 125,000 tons of tailings, averaging 2.96 pct corundum, was available for retreatment. Ore-dressing tests on representative samples of the deposit were conducted by the writer in the laboratories of the Industrial Minerals Section of the Department of Mines and Resources at Ottawa; and gravity concentration on tables was indicated as the most suitable method of treatment. Decision to proceed with the project was deferred until May 9, construction was begun on May 18, and tune-up operation of the plant begun on Sept. 15, 1944. CORUNDUM-ITS PROPERTIES AND USES Corundum is an oxide of aluminum having the formula A1203. Its outstanding characteristic is its great hardness; 9 in the Mohs scale. In this respect it is second only to the diamond. It usually occurs in barrel-shaped crystals in nepheline syenites and other feldspathic rocks deficient in silica. It has a specific gravity of 3.95 to 4.1 and is insoluble and infusible. In color it ranges from white, through red and green, to black. Ruby and sapphire are gem varieties of corundum. Corundum is supplied to industry as closely graded grain.2 The coarser products (10 to 24-mesh) are used largely for snagging wheels in the steel-castings industry, while the finer grades (from 70-mesh down to flours) find a wide application in the polishing of lenses and for other uses in the optical trade. All sizes of corundum grain are in close competition with synthetic abrasives such as carborundum and alundum. In some fields of application the artificial product is superior to the natural grain but not in all. The shape of the particle and its behavior under conditions of actual use determine its sphere of employment. The United States is the largest consumer of corundum, annual consumption in peacetime approaching 3000 tons; during the war this figure was roughly doubled. During the years 1902 to 1913, the bulk

Jan 1, 1946

By R. J. Wysor

MORE than two years ago, in making efficiency tests on our hot-blast stoves, I was surprised to discover a marked difference in temperature as indicated by a pyrometer inserted near a stove on blast, and the regular recording instrument situated near the bustle pipe. A special hole was then provided at the junction of the hot-blast main and the bustle pipe, and portable pyrometers with bare fire ends were used, both at this point and in the stove necks. I had occasion to make a number of such tests on various stoves on the same furnace, the difference in temperature as shown by the two pyrometers varying chiefly with the temperature of the blast, and the distance of the stove from the furnace. However, this difference was always great, often averaging above 100° F., for the entire period of a stove on blast. Later, experiments were made with similar results on another group of stoves. The pyrometers used were properly standardized and checked against each other before and after each test. However, there was still a slight doubt as to the accuracy of the results obtained, as subsequently explained. But I have obtained corroboration of the magnitude of heat losses from blast-furnace hot-blast mains from two independent sources. I have learned from H. A. Brassert, Superintendent of Blast Furnaces, South Works, Illinois Steel Co., that he has recently clone some experimental work similar to my own, and found temperature drops of the same order of magnitude. Also, C. R.. Lyle, of the Armstrong Cork Co., has made some interesting theoretical calculations, based oil the thermal conductivity of fire brick, for the particular conditions of several of our tests; and his theoretical interpolations, all facts being considered, check our actual observations fairly well. I understand that Mr. Lyle expects to publish the results of his investigations.

Jan 10, 1915

By Douglas Bunting

With the gradual exhaustion of the upper veins in the anthracite coal-fields, the problem of mining at greater depths acquires increasing importance and demands the consideration .of a number of important factors, one of which is the greater earth-pressure and the consequent necessity of stronger support for the roof. Under the pillar-and-chamber system, almost exclusively followed in the Northen anthracite-field, the highest economy in mining is generally secured by leaving, on first mining, pillars only sufficient to support safely the overlying strata. As to the necessary size of such pillars, the opinions of mining experts are widely divergent. In establishing the width of chambers and pillars, the thickness of vein and its depth below the surface have received little consideration. For instance, it has been quite usual to work both overlying and underlying veins with the same width of chambers and pillars, when the lower vein was two and one-half times as thick, and twice as far below the surface, as the upper. In view of the generally-accepted theory that the crushing-strength of coal-pillars of the same base-area becomes less with increased height, it is probable, in this instance at least, that the moat economical mining has not been secured. This question of adequate pillar-support is economically less important down to about 800 ft. than at greater depths; and it is with reference to mining at these greater depths that the study of the subject here presented has been prompted. The necessity of leaving larger pillars, involving greater mining-costs per ton and also smaller yields per acre, is one of the troubles of deep working. To mine without leaving adequate pillar-supports will result, sooner or later, in a squeeze. Limited areas, it is true, have been mined at certain widths of

Jan 1, 1912

By Caner Zanbak

Deformational behavior of a rock mass can be modeled with a constitutive law based on theories of elasticity, and plastic and viscous deformation. For most rock types, such constitutive models may be used with confidence because there is no geochemical interaction between the rock-forming minerals and water. However, if volume changes in the rock due to geochemical phase transitions are predicted, then these volume changes must be converted into mechanical parameters and must be included into the constitutive laws. In this paper, geochemical phase transitions between anhydrite (CaS04) and gypsum (CaS04-2H20) will be brought to the attention of researchers in rock mechanics. Hydration of anhydrite to gypsum may yield volume increases up to 62.6 percent and dehydration of gypsum to anhydrite may cause volume decreases up to 38.5 percent. If one were to convert the volume increases in hydrating anhydrite into strains and calculates the required stresses to restrict the expansions, the magnitude of stresses will be found at GPa levels. Such stresses on swelling anhydrite layers cannot be provided by geologic media. Therefore, the host rocks will deform under these high stresses. On the other hand, under increasing stresses, geochemical transition of anhydrite to gypsum may stop after some hydration of anhydrite, and anhydrite and gypsum systems may become stable. Changes in temperature and solution compositions in the anhydritel gypsum system also control the stability of the geochemical system, and therefore, the extent of volume changes. A similar conceptual model can be drawn for volume decreases due to dehydration of gypsum to anhydrite. In this case, stress reduction on the gypsum layer may cause extensive fracturing and changes in the state of stress in the host rock mass.

Jan 1, 1984

By Lawrence A. Garfield

From the bottom of a shallow fissure the near-naked man-animal hacked out a hand-sized chunk of the red rock-like material which, when beaten between other rocks, could be drawn out to the thinness of a flint edge. Unlike flint, however, it did not shatter when, as the point for a spear or arrow, it went wide of its mark and fell amongst the rocks or tree branches. For this reason and others, the red rock- like material was specially sought and much time was spent in trying to break it from the real-rock in which it grew. Only infrequently was a chunk broken free and in this area many hours had been spent to win but a few fragments. What luck then, to remove so large a piece where before nothing but real-rock dust formed whenever the stone hammer hit. Why did this happen? Did yesterday's food-fire, burning on top the red rock, loosen its hold on the real-rock and cause it to break free when struck with the stone hammer? It is not a matter of recorded history that the first heat assisted extraction of native copper from country rock occurred under the circumstances or in the manner related but it is known that crude mining operations using these techniques were conducted by primitive man. In this way the first mineral production may have been achieved and the original example of rapid excavation unknowingly given to the world. Rapid excavation is not a method of removing rock or ore from an insitu condition nor does the term refer to a type of equipment which is specifically employed in mining and underground construction. It is, rather, the nomenclature currently popular in industry-primarily in underground construction- relating to a comparative rate of performance.

Jan 1, 1971

Curlisle: We're sitting in Don McLaughlin's office overlooking Sun Francisco Bay, and he's going to be good enough to talk about early days in California, maybe about his college years in Berkeley, maybe about his first jobs in mining. Now, Don, would you be good enough to start it off by saying something about George Hearst. I know he died before you were born, but you know a lot about the family, and you no doubt know a lot about him. Certainly, nobody in those days matched his record in getting hold of good mines. McLaughlin: George Hearst died, actually, in the very year that I was born, but as I more or less grew up in the home of Mrs. Phoebe Hearst, who was his widow, I heard a great deal about the mines and the people that made the great fortune she inherited and used so wisely. But I don't think I really had an appreciation of George Hearst until 1 became active in the mining world and realized how many major mines he had picked in the early days and how much he had to do with their development into successful properties. He started in the California gold fields, but it was on the Comstock that he made his first big stake out of his substantial share in the Ophir mine. I believe at that time he was on his own. Then, later, he became associated with Ben Ali Haggin and Lloyd Tevis in San Francisco, and scoured the West for them. They made a little money in Eureka, Nevada, and other places, but the first important property that he obtained for them was the famous Ontario Mine in Park City, which became one of the great silver lead producers of the Continent. Then later he acquired the Homestake.

Jan 6, 1965

By Oscar Weiss

An aerial magnetometer survey was carried out by the author's geophysical organization over the Vrede-fort dome, where Witwatersrand beds are wrapped around a granite plug 25 to 30 miles in diameter. The well-known magnetic shales of the Lower Witwatersrand system (to be referred to as LWW) give strong negative anomalies over the out-cropping portion of the dome. This is unusual as these same shales cause positive anomalies over large areas near Johannesburg and along a strike of 130 miles between the East Rand and Klerksdorp. It is suggested that the negative polarization is caused by heat and stresses connected with the doming. This negative polarization of sedimentary beds, which usually cause positive anomalies, is a good example of how "magnetic lows" can be caused by "structural highs." Significant magnetic anomalies should be tested by gravity and seismic methods irrespective of the sign of the anomalies. South of the line X — Y (see Fig 2 and 5) the gravity anomalies are smaller over the LWW beds and magnetic anomalies suddenly become weaker and even change signs by becoming positive. This peculiar phe- nomenon is explained by faulting which truncated the nearly vertical LWW shales at depth. In this manner the mass of the LWW beds has been reduced together with the weakening of the negative magnetic anomalies by positive poles which occupy the truncated end. Furthermore, by uplifting the beds between f1 and f2 (Fig 2) positive polarization becomes predominant, thus causing positive magnetic anomalies. General The Vredefort dome is one of the internationally known geological structures, which, among others, has attracted the attention of numerous American geologists. It was for this latter reason that our aerial magnetic results over this structure were selected by us for publication in the United States of America. A full description of the exposed geology has been published by Nell and readers are referred to this paper, which is subject to only slight modifications inasmuch as the subdivision of the Witwatersrand system (to be referred to as WW system) is concerned. The Vredefort dome is about 60 miles southwest of Johannesburg. The WW system, after being exposed in the vicinity of Johannesburg, plunges with southerly dips beneath the Vent-ersdorp and Transvaal systems. Along a section southwest from Johannesburg, as shown diagrammatically in Fig 4, we see no outcrops of the WW system until the Vredefort dome, where the Upper and Lower WW system are wrapped around an almost circular arc of a granite mass. Here the whole WW system is overturned around the perimeter of a huge granite plug 25 to 30 miles in diameter. Less than half of the circumference of this dome is exposed; the rest is covered by Karroo dolerites and sediments. The determination of the exact shape of the concealed part of the dome has been a matter of considerable interest and obviously geophysical methods were eminently suited for obtaining this information. A good start was made with a gravimeter survey carried out by the Geological Survey of the Union of South Africa.2 Our own

Jan 1, 1950