Search Documents

By W. E. Rice

THE ceramic industry has to do with forming or molding articles of clay, and imparting to them their characteristic properties of permanence, strength and color by subjecting them to heat treatment in kilns. This heat treatment is commonly known as "firing" or "burning." The art of firing clay ware antedates the industrial use of coal; hence wood was the fuel first used, and the earliest kilns were built to burn wood. The majority of the clay ware produced today is fired in periodic kilns that are essentially enlarged structures similar to the earliest enclosed kilns; a kiln consists of a large chamber 16 to 32 ft. dia. (or rectangular) in which the ware is charged and fired either in direct contact with flame or hot gases from the fuel, or in saggers which shield the ware from direct contact with the gases. The fuel is burned in a number of small furnaces built in the thick .wall that encloses the chamber; these furnaces are from 16 to 36 in. wide and from 3 to 5 ft. long; some of them have horizontal or sloping grates, but many have no, grates; the last are called "dead-bottom" furnaces. The temperature necessary to mature the charge in the kiln varies for different clays; for some it is as low as 1800°, and for others as high as 2700° F. Wood was an ideal fuel for the low temperatures that were used in early kilns; it burns freely and produces along, clean flame, which promotes uniform distribution of the heat in the kiln; it does not coke and so restrict the passage of air through the fuel bed; the ash does not melt in the fuel bed to form clinker; and the ash that is carried into the chamber and deposited on the charge does not contain iron and sulfur to discolor the clay. The extent of the ceramic industry now precludes the general use of wood in kilns, so that today this industry is one of the large users -of bituminous coal, burning more than eight million tons annually in kilns. Manufacturing plants usually are at or near the source of clay, with deposits of coal suitable for kilns occurring near the clay. The cost of coal is the largest single item of expense in the manufacture of clay ware, and, because all coals are not equally suitable for the process, the careful selection of coal is important. In this selection the desidera-

Jan 1, 1932

IN order that you may be expert in every phase of this art of parting, I wish to caution you upon twelve points, all necessary, so that you may know beforehand what may happen to you and what provision you must make to obtain good results. Understand that the gain derived from this art lies in carrying out the work that you wish to do accurately and with profit, which means nothing other than working with certainty and seeing that there is no loss of gold or silver [69v] or substance of anything that is valuable in itself. For, since everything lost is silver or gold, it is valuable however little. it may be, and many little things obviously make one large one; for gain increases gain, and loss, loss. Therefore, you will have as your first task to seek to have the materials that you are to use for making the aqua fortis of as perfect a nature as possible. I tell you this because the saltpeter is often weaker and of worse quality than the alum. For this reason it is necessary to make an effort to have it not only well refined and free from earthiness but also of a nature as powerful as pig [dung] saltpeter, the color of which tends strongly toward yellow. In order to work better, you also refine this again before you put it to use, as I shall tell you in its place. Likewise, see if you can get alum of the red kind that comes from the Levant or from Cartagena, for it costs less and is more powerful. Furthermore; see that you have a good number of well-formed cucurbits of properly purified glass not too large and of, equal content and size. Above all, see that they contain, no indentations, bubbles, folds, or scratches, especially, in the, body or in the bottom, because they break easily and the nature and great force of the acid pierces them by eating away, and makes a little hole. For this reason, cucurbits. are always divided into four grades; one is taken for making aqua fortis, another for drying the waters when they are laden with silver, the third for recovering the acids from the separations, and the fourth are either rejected as useless or dangerous, or are saved for recovering the whites or for some other service. Whoever wishes to practice this art well should remember to keep a large supply of cucurbits, receivers, and alembics. The third is the precaution which one must take in luting them; first, to

Jan 1, 1942

By W. F., Netzeband

SEVERAL criteria have been used more or less successfully in finding and developing orebodies in this district. Among them is the relation of the fracture to the orebodies. It has been known for some time that there was some relation, but it has been but recently. that careful study has been given to this subject. Like all the criteria used in this district, this relationship must be used with caution, for what-is true in one part of the district is not necessarily true in another.

Jan 1, 1928

By G. Richards Gwinn

Graphite is one .of the so-called minor nonmetals that have become of major importance during the present conflict. Requirements of the domestic industry for graphite are relatively small and uses have changed so greatly in the past 25 years that different types, with few exceptions, are more nearly interchangeable in use. However, the quality of the graphite required for crucibles is exacting and under present manufacturing practices only a small part of the world production can be utilized economically by this industry. Graphite, for the domestic crucible industry, was supplied almost entirely by imports from Madagascar and Ceylon before the present war. The quantity required has been increased by the war and the greatest demand is for Madagascar flake. The fall of France to Germany in 1941 and the Japanese expansion southward threatened our supply of crucible-grade graphite, therefore the domestic industry was revived to supplement imports and to act as insurance in the event of complete stoppage of imports. Two plants were constructed for the Government, one at Ashland, Alabama, called the Crucible Flake Graphite Co., operated by Haile Mines, Inc., and one near West Chester, Pennsylvania, operated by the Benjamin Franklin Flake Graphite Co. Loans were made by the Reconstruction Finance Corporation to the Alabama Flake Graphite Co. at Ashland, the Ceylon Graphite Co., at Goodwater, Ala., and the Southwestern Graphite Co., at Burnet, Texas. However, by the end of 1943 the import situation had improved and it was no longer necessary to continue Government assistance to mine and stock-pile the high-cost domestic material. Graphite is found in virtually every country of the world. Most occurrences are of no economic importance because of their poor quality, prohibitive mining and milling costs, inaccessibility, and poor location with reference to the industrial centers where graphite is used. The principal commercial deposits are found in Ceylon, Madagascar, Chosen (Korea), United States, U. S. S. R., Canada, Germany, and Mexico. Graphite is a typical international mineral in that it is utilized mainly in countries noted for industrial activities, whereas the two islands, Ceylon and Madagascar, which produce the major part of the world's high-grade graphite, are virtually devoid of any manufacturing establishments requiring its use. The industry thus is violently disturbed by wars. Types oF Graphite Used in Crucibles Graphite of various kinds and from many countries has been used for making crucibles. Passau (Bohemian) flake was first used in Europe, but by the middle of the seventeenth century crucible formulas were established largely on Ceylon graphite and German Klingenberg clay. Ceylon graphite is granular and the grains are subdivided into more or less

Jan 1, 1948

By Harry H. Power

WHILE the attention of all engineering branches is focused today on changes and improvements in the several curricula, we are concerned here with the many questions arising in industry and college concerning the preparation for the petroleum engineering profession. The succession of problems confronting the petroleum engineer demands a degree of versatility not ordinarily encountered in other professions. As a consequence, the student cannot acquire mastery of all the several engineering branches represented in field practice, and as an alternative, he has learned to appreciate the value of thorough training in the basic or fundamental sciences as a preliminary foundation for professional work. While our attention is directed primarily to the training of engineers in educational institutions, it is necessary that the ultimate education of the professional engineer be kept in mind. It is apparent that his success as an engineer is limited not only by his educational and job experience, but by his native endowments: mentality, personality and health. The engineer's lack of job experiences often proves as much a handicap as the lack of adequate college training. If the final product proves to be satisfactory, both industry and technical institutions are to be congratulated. However, if the product is found wanting, the one most responsible should be apprehended. FITNESS FOR PROFESSION For the most part, students of engineering colleges have had their interests in applied science awakened at a relatively early age. Many of them entered college as a convenient means of acquiring a technical education, but undoubtedly would have had better prospects in a shorter program offered by institutions of noncollege grade. The activating motive of practical interest is also strong in students suitable for college training, but in many cases an intellectual interest must be awakened in order that it may survive graduation. Numerous engineers believe that this progressive process of selection and determination of career aims is one of the essential services rendered by the engineering colleges. Industrialists, who are more interested in quality than quantity of petroleum engineering graduates, have pointed out the good that should result if more emphasis were given to the work of eliminating the misfits. They also point out the protective measures that have been taken by other professions for the betterment of professional standards. Mention has been made of the beneficial results to be obtained by collective determination of the market for the products of engineering schools, particularly in that the students unsuited to the profession would have an opportunity to prepare themselves for employment more suited to their personalities and other qualifications. In the event that such a system of coordinated personnel direction is developed by the

Jan 1, 1941

By E. C. Harder

J. T. SINGEWALD, JR., Baltimore, Md.-I might say a word further in regard to the Miguel-Burnier district. I am sorry Mr. Harder is not here. I had hoped to be able to ask him a few questions in regard to some of the features of those deposits. This Miguel-Burnier district is the other district that I referred to that lies in the State of Minas Geraes. The interpretation that Mr. Harder gives to, these deposits is the same as that given by Harder' and Chamberlin in a previous paper, and that is that they consist of lenticular deposits of sedimentary origin in a series of rocks that also include lenses of limestone, but they seem to think that the manganese ores occur at a definite horizon and at a horizon different from the limestone. We found that the manganese ores occur at several horizons, as also do the limestone lenses, and the limestone masses are of the same shape as the manganese orebodies, so that our own feeling was that these manganese ores simply represent concentrations within the zone of oxidation formed as. a result of decomposition and replacement of this limestone. To the east of Miguel-Burnier there is a big bend in the railroad which cuts through, at right angles to the strike, this series of rocks. Here is shown very well both the local character of the limestone lenses and the local character of the manganese lenses in that, though the distance across the bend is only a couple of hundred yards, the limestone and. ,manganese lenses do not match up at all, and it also shows plainly that both occur at more than one horizon.. Consequently there is no reason why the manganese ore could not represent alterations of this limestone. The mine workings do not as yet give very definite information on this point in that they are all comparatively shallow and within what would be the zone of oxidation so that what they will grade into below that depth, we do not know. On the other hand, the character of the ore suggests that it may, represent such -a product in the zone of oxidation in that it tends to be quite earthy; whereas, if the manganese had been there originally as such it would have undergone the same metamorphism that the other rocks have undergone and one would have expected that it would have been converted into a hard manganese ore that would today stand out pretty much as do the iron ores of that same region and not have the soft, earthy texture which is a prevailing characteristic.

Jan 4, 1917

By G. Richards Gwinn

Graphite is one .of the so-called minor nonmetals that have become of major importance during the present conflict. Requirements of the domestic industry for graphite are relatively small and uses have changed so greatly in the past 25 years that different types, with few exceptions, are more nearly interchangeable in use. However, the quality of the graphite required for crucibles is exacting and under present manufacturing practices only a small part of the world production can be utilized economically by this industry. Graphite, for the domestic crucible industry, was supplied almost entirely by imports from Madagascar and Ceylon before the present war. The quantity required has been increased by the war and the greatest demand is for Madagascar flake. The fall of France to Germany in 1941 and the Japanese expansion southward threatened our supply of crucible-grade graphite, therefore the domestic industry was revived to supplement imports and to act as insurance in the event of complete stoppage of imports. Two plants were constructed for the Government, one at Ashland, Alabama, called the Crucible Flake Graphite Co., operated by Haile Mines, Inc., and one near West Chester, Pennsylvania, operated by the Benjamin Franklin Flake Graphite Co. Loans were made by the Reconstruction Finance Corporation to the Alabama Flake Graphite Co. at Ashland, the Ceylon Graphite Co., at Goodwater, Ala., and the Southwestern Graphite Co., at Burnet, Texas. However, by the end of 1943 the import situation had improved and it was no longer necessary to continue Government assistance to mine and stock-pile the high-cost domestic material. Graphite is found in virtually every country of the world. Most occurrences are of no economic importance because of their poor quality, prohibitive mining and milling costs, inaccessibility, and poor location with reference to the industrial centers where graphite is used. The principal commercial deposits are found in Ceylon, Madagascar, Chosen (Korea), United States, U. S. S. R., Canada, Germany, and Mexico. Graphite is a typical international mineral in that it is utilized mainly in countries noted for industrial activities, whereas the two islands, Ceylon and Madagascar, which produce the major part of the world's high-grade graphite, are virtually devoid of any manufacturing establishments requiring its use. The industry thus is violently disturbed by wars. Types oF Graphite Used in Crucibles Graphite of various kinds and from many countries has been used for making crucibles. Passau (Bohemian) flake was first used in Europe, but by the middle of the seventeenth century crucible formulas were established largely on Ceylon graphite and German Klingenberg clay. Ceylon graphite is granular and the grains are subdivided into more or less

Jan 1, 1948

HAVING previously told you about lodestone, azure, emery, and manganese, things that perhaps belong more properly to the stones than to the metals, the desire has come to me to describe rock crystal as well and, with this, some of the better-known gems, although they are things that are not classified with metals, as far as I know, or even with the serniminerals. In truth, they are neither one nor the other, but they are stones and have the nature of stones. But let them be what they may, in order to avoid straying from the subject we have begun, we shall say that they have mineral properties in their species, as for instance, in respect to their colors. It would seem an error to me if, while I am giving you information on so many other useful things, I did not give you this as well, if for no other reason than because it is a fine accomplishment for a gentleman to have some knowledge concerning gems. Thus since I have proposed to do this, beginning to speak of crystal, I tell you that it is a lustrous, transparent, and limpid stone, composed by Nature with a predominance of wateriness. Thus, contrary to the order of natural things, it has been believed by many that Nature has generated it of pure water by means of a powerful and continuous coldness that always exists in those mountains and places where it is found, for the waters and snows can never melt there because of the great cold. They have tried to prove this opinion of theirs by saying that crystal still retains the nature of frozen water, and that this is. shown not only in its appearance but also in the fact that, like ice, it floats without sinking to the bottom when it is put in water. They also say that it is customary to put it under the tongues of the thirsty on account of the, coldness and moistness which it gives, and that it quenches the dryness of their thirst. But even if all these things were true, as they are not, they do not upon consideration bring us to the conclusion that it is water, because the diamond and the beryl would do the same. Therefore it does not seem to me credible [391 that it is as they claim, for it is a stone generated as such by Nature. And then, if this were true in those places where it rains often and many snows are formed, and these should freeze and never melt but

Jan 1, 1942

By T. A. Rickard

Freeman, the English historian, said that he had learned from Macaulay "never to be afraid of using the same word or name over and over again if by that means anything could be added to clearness or force. Macaulay never goes on, like some writers, talking about `the former' and `the latter', `he', `she', `it', and `they', through !clause after clause, while his reader has to look back to see which of several persons it is that is so darkly referred to". He might have learned also from Macaulay that a preposition makes a' feeble ending for a serious statement. How often do you hear `the former' and `the latter' in conversation? Good conversation is a guide not to be dis¬dained. When you must use `the latter', do not confuse it with `the last'. Cheap labor, cheap fuel, and cheap transport are essential, the latter [last] being particularly important in base-metal mining. The apparatus may be used to determine such gases as hydrogen sulphide, chlorine, oxides of nitrogen and some others, as well as sulphur dioxide. In the case of the latter two, etc. The two last mentioned are "sulphur dioxide" and "some others". He should have stated which two, and he should have avoided that wretched phrase "in the case of ". Pinning may be partly prevented by use of chalk, oil, or turpentine. The latter [last] is best. I "Partly prevented" is a poor phrase. `Prevented! is a word that does not lend itself readily to modification. He means `checked' or `corrected'.

Jan 1, 1931

By J. B. Hiskey, G. Ramadorai

This paper presents the kinetics of leaching pure wulfenite (PbMoO,) with sodium sulfide solutions. Wulfenite dissolves in the presence of sulfide ion according to the following reaction: PbMoO , + S= PbS + MOO;. Synthetic galena is formed and molybdenum is released to solution as molybdate ion. The reaction follows the shrinking core model and the leaching mechanism involves mixed kinetics where both diffusion and chemical reaction are rate controlling. Molybdenum extraction is explained in terms of temperature, particle size, and sodium sulfide concentration.

Jan 1, 1977

By James W. Clark, William C. Dixon, Kevin L. Wiley, Thomas R. Couzens

This paper is a description of certain geologic aspects of the Inspiration mining operation. It is not a geologic description of the Inspiration deposit nor an exposition of all the geologic work. Involved in the development and exploitation of the deposit. A few specific examples of the application of geology and geostatistics to the mining process are given to illustrate the progress that has been made in making the Inspiration Mine more efficient and cost effective, and a statement of the philosophy of geologic applications is offered.

Jan 1, 1985

By Evert Hoek

An empirical criterion is presented for use in estimating the strength of heavily jointed or weathered rock masses. This criterion incorporates the intact strength of the rock material and introduces two dimensionless parameters m and s which characterize the behaviour of the interlocking particles in a jointed rock mass. The use of rock mass classification systems for estimating the values of these parameters is discussed. The application of this non-linear failure criterion to the analysis of slopes is illustrated by means of a worked example.

Jan 1, 1983

MODERN mining is based on drilling--everything else stems from that. U. S. and Canadian mining companies and suppliers of mining equipment are constantly seeking new techniques and tools for increased drilling efficiency. Ultimate goal is increased bit-against-rock time. Just how important their efforts are can be seen through a legitimate extension of available statistics. In Canada some 70 million ft of drilling per year is required. While the number of feet actually drilled in the U. S. is unknown, it is possible to compare U. S. and Canadian mineral production, excluding oil and coal. In the U. S. mineral value was $4200 million and across the border $836 million. Taking into account the number of drilled feet required to produce Canada's total, the U. S. total would be 365 million ft in 1953. A saving of 14 per ft would reduce over- all metal and nonmetal mining costs some $3.65 million per year. This does not take into account other indirect savings. Methods have changed and there have been structural and metallurgical advances in blasthole practice in the last ten years. The Editors of MINING ENGINEERING in the articles on the following pages present the most significant innovations coupled with results in the field. Environment plays an important role in selection of materials and equipment -but while unanimity does not exist throughout the industry in reference to drilling problems, a definite trend is evident. For the story of that trend read the articles on the following pages.

Jan 9, 1954

By P. Somasundaran, Yee-Ho Wang

Carrier flotation of anatase impurities from clay using oleate is compared in this study with conventional flotation. The role of relevant parameters such as carrier size, carrier amount, dissolved calcium and carbonate, conditioning speed, pH, time and temperature and collector concentration is investigated in order to elucidate the mechanisms involved in carrier flotation. Addition of coarse calcite particles is found to contribute towards the enhanced separation of anatase from clay mainly due to aggregation of anatase fines with calcite and the subsequent flotation of the aggregates.

Jan 1, 1980

By George W. Becker, Blair R. Benner, Ronald J. Roman

A computer model for the leaching of oxide copper heaps is developed and verified in laboratory tests. The model can be used to scale-up laboratory test data and to simulate the results of various operating philosophies on heap leaching. The model is applicable to leaching processes where the rate of leaching is controlled by a deficiency of reactant or diffusion of a reactant through the leached rock. Several examples are presented showing the sensitivity of the recovery vs. time curve to the variables in leaching.

Jan 1, 1975

By Robert L. Braun, Arthur E. Lewis

A contained nuclear explosion is proposed to produce a chimney of broken ore well below the water table. After the chimney is filled with water and reaches hydrostatic equilibrium, oxygen, under pressure, is introduced near the bottom of the mass of broken rock. The increase in solubility of oxygen at high hydrostatic pressure is sufficient to initiate the oxidation of the primary sulfides, chalcopyrite and pyrite. The oxidation and dissolution of these sulfides produces enough heat to increase the temperature of the ore and water in the chimney as much as 100° to 150°C. The rate of dissolution of chalcopyrite exposed to oxygen-saturated solutions becomes so rapid under these conditions that the rate of copper extraction becomes limited by the accessibility of chalcopyrite to the solutions. Means of solution control and distribution of oxygen in the chimney have been examined in some detail. A calculational model utilizing experimentally determined solubility rates is utilized to estimate the temperature rise in the chimney and the rate of copper recovery as a function of initial temperature, sulfide content of ore, and size of ore particles. It is concluded that the high pressure and high temperature obtainable in a large ore mass broken by nuclear explosives promise the rapid and economic recovery of minerals not considered soluble by conventional in-situ leaching methods. The method should be broadly applicable to the kinds of deep deposits that must be depended on for future production and with minimum disruption of the environment.

Jan 1, 1974

By Robert F. Mehl

METALLURGISTS are properly interested in papers dealing with subjects ranging from the theories of the metallic state to very practical details on the use of metals. A review of theoretical metallurgy naturally excludes papers concerned with the usefulness of metals and limits itself to those concerned purely with the science of metals. Even in this more limited field the reviewer is confronted with thousands of papers dealing with facts and theories of metallic behavior, from which he can select only a few hundred. The selection is necessarily highly arbitrary. Certain classes of papers definitely fall within the scope of this review. Thus, papers on the deformation of metals, on the structure of alloys, on recrystallization, to select only a few, are fully represented. Other subjects, which verge on what is generally accepted as the science of metals, such as the magnetic properties of metals, the less familiar electrical properties and theories, adsorption of gases by metal surfaces, and the like, are treated scantily. Particular attention is given to reviews of subjects that fall with in the groups chosen; in a summary such as this, which is largely in the nature of a progress report, capably written reviews are of importance, particularly since the monograph and text book literature is so meager in this field. THEORIES OF STATES OF AGGREGATION The electron theory of metals and the relation between possible electron structure and the magnetic properties have been discussed (317.307)† the electron theory of liquid metals, especially with respect to electrical conductivity, has been considered(306).

Jan 1, 1935

A. W. TRACY' AND D. H. THOMPSON (presented in written form).-Messrs. Ed¬munds, Anderson and Waring are to be commended for their excellent work in so painstakingly reducing to constants, the variables believed to be associated with these types of stress corrosion. An ammonia stress-cracking apparatus similar in many respects to the one described has been assembled. Certain aspects of its construction may be of interest. Tensile loading of the specimens, which are usually Y8-in. diameter wires, is accomplished by springs acting in compression in compact, portable, tubular assemblies weighing less than 10 lb. The lower end of each assembly contains the test specimen and is passed into the test chamber for exposure to the moist ammoniacal atmosphere. There is considerable convenience in being able to load the specimens in a portable unit, which may be warmed to prevent condensation, and then thrust instantaneously into the test chamber. Spring loading obviates the use of weights, but it has the disadvantage of allowing some relaxation of the load if the specimen elongates during corrosion. The maximum loss in load found so far was 20 per cent. The air supplied to the test chamber is humidified by passing it through thermo¬statically controlled water. The air is dispersed into fine bubbles by being forced through a porous ceramic filter at the bottom of a tank containing about 10 liters of water. The temperature of the water is controlled independently of the tempera¬ture of the test chamber. Thus, with the

Jan 1, 1945

By John A. Oberteuffer, Ionel Wechsler

High gradient magnetic separation, first introduced in 1968 as a means for the removal of very fine magnetic contaminants from clay, is no longer a new technology. Applications to a number of solid-solid and solid-fluid separation problems are now being exploited. Recent pilot tests and full-scale industrial installations of high gradient magnetic separators and filters have established their commercial viability and have demonstrated their superiority both in efficiency and cost effectiveness to conventional prior-art methods for water treatment and mineral processing. In high gradient magnetic separators, the magnetic trapping force for fine, even weakly magnetic particles is greatly increased over those available in previous magnetic separators. These very strong trapping forces are created on the edges of fine filamentary ferro- magnetic materials in the presence of a strong background magnetic field. In the basic cyclic high gradient magnetic separator, particles are trapped in the filamentary matrix until it is loaded, at which point the magnetic field is turned off, and the filter is back- flushed. Applications of cyclic high gradient magnetic filtration for water treatment include filtration of waste and process waters in steel mills. Treatment of hot rolling mill scale pit effluent and cold rolling mill emulsion has been successfully tested, and 2-meter diameter SALA-HGMF® magnetic filters have been installed at Japanese steel companies for the treatment of recirculating scrubber waters in a de-gas- sing system. A second major application in water treatment is for the polishing of recirculating waters in power and steam generating systems. High gradient magnetic filters can operate under conditions of

Jan 1, 1980

By H. C. Lay

No one who knows the conditions of altitude, difficulty of access, shortness of working-season, etc., under which the work of the U. S. Geological Survey in the Rocky Mountains is carried on, can fail to be impressed with the magnitude and accuracy of the results obtained. Yet, in the necessarily brief time given to any one district, maily minor features must remain unregistered. The purpose of this paper is merely to place on record observations, made during nearly eighteen years, of local phenomena in themselves unimportant, which, however, if collated with similar memoranda from neighboring regions, may lead to something of value. The Telluride quadrangle in southwest Colorado is a tract of fifteen geographical minutes on a side, the NW. corner of which is in Long. 108º W., Lat. 38º N. Topographically it is essentially a lofty horseshoe, some of the peaks of which rise more than 14,000 ft. above tide, the interior (with an average altitude of 9000 ft.) being the eastern edge of the rolling Colorado-Utah plateau, and, in its turn, deeply cut by the canons of the San Miguel river and its tributaries. The range-summits are mainly igneous rocks, partly intrusive (such as massive " stocks" and laccolites, left bare by superior erosion), but mostly bedded volcanics of an average vertical thickness of 3500 ft., which change with changing height from andesite to rhyolite. The lowest series of these volcanic rocks, known as the San Juan formation, lies conformably upon the sedimentary San Miguel conglomerate, which dips gently east., and is attributed provisionally to the

Jan 1, 1902