Search Documents

By Paul C. Merritt

The story now unfolding on the Mesabi Range is more than just another chapter in the continuing history of iron mining. It is an epic of foresight, research and pioneering instinct just now culminating in the rapid growth of a new industry hardly more than ten years old. And that growth has been impressive. In the decade that has passed since Reserve Mining Co. blazed a trail as the first successful large- scale commercial taconite operation, the number of plants now existing or soon to be operating has grown to six. From Reserve's initial capacity of 3 million tons of pellets per year, total production capacity on the Range has surged to 31.5 million tons while capital investment has leaped from $300 million in 1955 to almost $1 billion today. During the past year alone, more than $500 million has been committed to these projects, an amount probably greater than that going into any other single mining area in the world. There is more yet to come.

Jan 10, 1965

By E. P. Pfleider

Important economic planning by industries, companies, financial firms and governments is predicated on estimates of future growth potential. Prior to the passage of the Taconite Amendment by the people of the State in November 1964, various admittedly conservative forecasts were made of the future of Minnesota's iron ore industry. Since that date, 3 new taconite plants are under construction, 2 are being expanded, one has just gone "on stream," and a seventh is in the design stage. These plants have a total capacity of about 35 million tons of pellets annually. The pessimism of 1964 today is being replaced by unbounded optimism. In the light of such recent developments, it becomes essential to update forecasts related to production, employment and capital investments. New projections show 50 to 58 million tons of pellet production from Minnesota by 1975. This tonnage, together with some 6 to 8 million of natural ores, should provide an annual full-time employment of 15,000 by the mining companies, plus 700 in related railroad and power producing activities and 1000 in plant construction. Total capital investment in new plants should reach $1.5 billion by 1975 and $2 billion by 1990. These current estimates indicate that the conservative goals as set in 1964 for the year 1990 should actually be achieved by 1975 or 1980. BACKGROUND Various forecasts1,2,3 have been made of the future consumption and production of iron ores within the United States. The most recent of these is by Harrison,1 delivered at the American Mining Congress meeting in Las Vegas in October. His estimates reflect the latest iron ore developments on a world wide front as well as the accelerating trend toward pellets. Minnesota's Natural Resources council prepared a detailed report in 1964 outlining the various factors that affect the future of Minnesota's iron ore industry. It presented estimates predicated on the

Jan 1, 1967

By B. E. Mix

THE present methods of unloading ore and coarse-crushing at the Magna plant of the Utah Copper Co. are the developments of the past five years. Hand dumping and breaking have given way to the rotary car dump and large gyratory; and the bucket elevators and stationary screens have been replaced by conveyors and shaking screens. The coarse-crushing plant may be broadly divided into two units: the primary crushing and the secondary crushing, or primary roll, unit. The former was erected in 1923; with the exception of the excavation, the construction was carried on by forces of the company. Excavation was started in the middle of March, the first concrete was poured July 16, erection of the steel building started Oct. 30, and the unit was com-pleted and operating Dec. 30. The unit has a capacity of over 24,000 tons of ore per day, and cost more than a million dollars. Remodeling of the primary-roll unit extended over a period of years preceding this, some of the work having been done as early as 1918.

Jan 8, 1925

By Milton H. Fies

THE writer wishes to acknowledge at the outset his great sense of obligation to those who contributed so broadly and expertly to the preparation of this paper: Dr. Albert DeSmaele, Chairman of the Board of Directors of Socogaz, Belgium; Dr. Julien Venter, Director of the National Institute of Coal Mining and Coal Utilization Industry, Belgium; Dr. Roger Loison, Chief Engineer, French National Coal Research Board, France; and Dr. Erich Sarapuu of the Sinclair Coal Co., Kansas City, Missouri. Liberal use has been made in this paper of a recent publication on Russian, Italian, Belgian, and French experiments prepared jointly by the Centre d'Etudes et Recherches des Charbonnages de France (Cerchar) and the Institut National de l'Industrie Charbonnière of Belgium (Inichar).1 The writer is particularly indebted to Dr. E. T. Wilkins, of the Fuel Re¬search Station, London, and to Mr. James L. Elder, of the Alabama Under¬ground Gasification Experiment at Gorgas, Alabama, for their unfailing patience and effective support. Not only did these experienced and capable men edit and revise this discussion, but much of its content is a direct con¬tribution quoted in full from their memoranda. Without the assistance of all these colleagues, this presentation would not have been possible. It has been a privilege to serve and work with them and to acknowledge repeatedly their loyal cooperation. The recovery of the energy inherent in coal beds throughout the world has been done through the years by methods that today may be justly regarded as primitive. While the techniques of coal mining through the application of machines and the enhancement of information and skills have advanced to an extraordinary extent, particularly during the past 20 years, the underlying means is essentially the same as when man first began to mine coal. Despite this progress in the manner of coal mining, it continues today as a hazardous, toilsome, grimy task. Coal mining per se has advanced at a greater and more rapid stride in the past 25 years than the mining of any: other mineral, but the basic method is fundamentally the same as it was the first day a miner took a pick into his hands many centuries ago. This situation presents a challenge to mining engineers everywhere, from

Jan 1, 1953

By Marston G. Fleming

CRITICAL pH has been defined by Wark1 as that pH value below which a mineral will float and above which it will not float in solutions containing a given concentration of collector but free from other depressants or activators. - The relationship between mineral, collector concentration, and pH is expressed in the form of a critical pH curve determined by the captive bubble method and is generally regarded as being fundamental to the system involved. The mechanism of alkali depression is currently in dispute, two general theories having received authoritative support. The view expressed in Taggart's chemical theory of flotation2 is that in the presence of a critical concentration of hydroxyl ions, all the metal ions available at the surface of the mineral are present as anions of the form [M (OH)x]-y. The formation of the necessary metal xanthate is thus precluded and, furthermore, any metal xanthate coating previously formed, at a lower pH, will be dissolved. Gaudin3,4 and the Melbourne school under Ware hold that alkali depression is caused by the preferential adsorption of hydroxyl ions by the mineral surfaces. This paper describes an investigation of the mechanism of alkali depression in each of the three systems: cerussite-aqueous carbonate-xanthate, cerussite-xanthate, and galena-xanthate. Alkali Depression of Cerussite by Control of Carbonate Ion Concentration Wark and Cox6 have shown that in the presence of Na2CO3 a higher concentration of xanthate is required to effect bubble contact with a cerussite surface than in carbonate-free solutions. To examine this depression of cerussite by sodium carbonate, a series of captive bubble tests was carried out on cerussite specimens in solutions containing 100 mg per liter of potassium ethyl xanthate and varying concentrations of sodium carbonate. The pH was adjusted by the addition of dilute hydrochloric acid. For purposes of comparison a standard critical pH curve was determined for cerussite with potassium ethyl xanthate in solutions containing no added carbonate. In the latter case pH adjustment was made with sodium hydroxide. The experimental method employed in both series was broadly similar to that described by Wark and Cox;7-9 a number of refinements suggested by Sutherland10 were adopted, and the procedure was further developed and modified to suit the particular requirements of the investigation. The relationship between pH and the concentration of potassium ethyl xanthate necessary to induce bubble contact at cerussite surfaces is shown in Fig. 1. The relationship between pH and the concentra- [ ] tion of sodium carbonate necessary to prevent bubble contact at cerussite surfaces in the presence of 100 mg per liter of potassium ethyl xanthate is shown in Fig. 2. Comparison of the. two curves shows that at a collector concentration of 100 mg

Jan 1, 1952

NEW MEMBERS The following list comprises the names of those persons who became members during the period Nov. 10 to Dec. 10, 1914: Members ABBOTT, Louis BENJAMIN, Chief Engr., Consolidation Coal Co., Elkhorn Div., Jenkins, Ky. ANDERSON, THORVALD J 542 S. Seventh Ave., Mt. Vernon, N. Y. BALFOUR, A. M P. 0. Box 301, Republic, Wash. BOUDWIN, WALKER J., Mech. Engr 159 Pierpont Ave., Salt Lake City, Utah. BRADLEY, LINN, Engr Research Corpn., 63 Wall St., New York, N. Y. BROKAW, ALBERT DUDLEY, Asst. Prof., Min. and Econ. Geol., Univ. of Chicago, Chicago, Ill. BUTLER, PERCIVAL PACE, Asst. Supt., Copper Queen Reduction Wks., Douglas, Ariz. CAJANDER, SVEN G., Mech. Engr Braden Copper .Co., Rancagua, Chile.

Jan 1, 1915

By R. Laird Auchmuty

A NUMBER of factors, of varying importance, should be considered in assessing mineral land-here specifically coal land -for tax purposes. (1) Is the coal developed or un- developed'! (2) If undeveloped, is it surrounded by active development in adjoining properties? (3) Accessibility of railroad facilities to the property. Sometimes the railroad itself is of importance. (4) Cost of development of the seam under consideration, whether it can be developed by a drift from the outcrop or whether it will require deep and costly shafts or slopes. (5) Is it definitely known that the coal seam being assessed is of minable thickness and quality on the property? (6) Thickness of recoverable coal from the seam. (7) Quality of the coal. (8) Is the seam being assessed likely to be more costly to mine than other seams in the same district? (9) Is there an active market demand for coal from the seam being assessed? (10) Is the coal held only as protection to surface structures and in such location that it can never be mined? (11) Is the coal in a solid piece or is it in pillars that have been standing for some years as a result of incomplete mining in an old operation? (12) Expected percentage of recovery from the seam being assessed as determined by present mining practice in similar properties. (13) Value of the seam as indicated by sales of similar mineral rights in the immediate district. (14) Current royalty rates of leases for coal in the seam under consideration.

Jan 1, 1939

By J. J. Rutledge

I. DESCRIPTION OF THE CLINTON ORES AND ASSOCIATED POCKS. THE Clinton rocks in Stone Valley comprise (1) thick layers of deep-red shale, (2) layers of reddish-gray shale interspersed with beds of sandstone and thin beds of extremely fossiliferous limestone, and (3) yellowish-gray shales alternating with thin layers of olive-colored shales. These layers of shale are all very thin and are at no point massive.1 A bed of gray-white sandstone varying in thickness from 12 to 30 ft. occurs in the tipper portions of the Clinton shales. This is called the ore sandstone, a name given to it by Rogers and adopted also by the Second Pennsylvania Geological Survey. It is the surest guide to the presence of the iron-ore bed, which lies below its lower edges at distances varying from 10 to 20 ft. This ore sandstone, in the valley, lies generally at very low inclinations, but often is locally subjected to considerable changes in dip. At its outcrop this ore sandstone weathers very easily and yields a rather sparse crop of stone, Composed of dirty, iron-stained boulders. Upon the roads these boulders are quite well rounded, and bear fragments of crinoid stems and other fossil forms. The ore sandstone exhibits at many points evidences of former movements in this very great thickness of the shales, and as it is the only stratum in this series competent to transmit and record stresses due to structural movements, its evidence is of considerable importance in determining the occurrence of movements in times past.

Nov 1, 1908

By CHARLES DORRANGE

INTRODUCTION. CULM is a general term used in the anthracite regions for many years to denote a mixture of coal, bony coal and impurities which is sent to the refuse-banks. Thus, 35 years ago culm contained the pea and buckwheat sizes of anthracite; but to-day, and as mentioned in this paper, culm is used specifically to denote the material which passes through the smallest screen in the anthracite-breaker. The smallest size of commercial anthracite is known as No. 3 buckwheat, barley, or bird's-eye coal, and is ordinarily made through a round-punched plate having openings 3/16 in. in diameter, and over a round-punched plate with opening 3/32 in. or 1/16 in. in diameter. Thus culm will consist of coal, bony coal, slate, gravel, iron pyrite, etc., ranging in size from ? 3/32 in. down to dust. Other local terms for culm are " slush," " silt," and " dirt." The first experiments towards the utilization of anthracite culm by briquetting, and the first briquetting-work done in this country, were in 1872 at Port Richmond Piers, Philadelphia, Pa., by E. F., Loiseau.1 Clay was used as a binder and the finished briquette was water-proofed with shellac, etc. Excessive cost was given as the reason for discontinuing work at this plant. The Delaware k Hudson Co., in 1876, built a plant at Rondout, N. Y., which operated until 1880. Gas-tar was used as a binder, and anthracite screenings were briquetted for engine fuel. Excessive cost, poor results in firing, and the tendency of the fuel to cut the boiler-flues were given as the reasons for discontinuing the manufacture. The next plant was built by E. F. Loiseau about 1878, at Nesquehoning, Pa., near the No. 1 or Nesquehoning colliery

Sep 1, 1911

F. E. Thompson, Pottstown, Pa.: If Mr. Webster's endeavor to open up the subject of cast-iron should prove as prolific of results as did the discussion on " The Physics of Steel," he must certainly feel richly repaid. His remarks concerning the secretiveness and conservatism of the average foundry-man recall to my mind an instance of a blast-furnace which, for the last year, has been producing much foundry-iron. The manager, believing that much more of his product could be used in the foundry than the foundry-men were will-

Jan 1, 1896

By W. F. Flanagan, K. R. Evans

The strong temperature dependence of the flow stress of bcc metals has prompted considerable interest and controversy concerning the rate-controlling influence of the Peierls' mechanism upon deformation in metals.18"21 Dorn and Rajnak" recently offered a criterion for Peierls' deformation utilizing a line-energy model of a dislocation to estimate the saddle-point activation energy, for the nucleation of a pair of kinks as a function of applied stress and the height and shape of the peierls9- barrier. It was found that Un/2Uk, where 2Uk is the excess energy of a kink pair, was a unique function of where is the effective stress and 7p is the Peierls' stress as defined in Fig. 20. The relationship found by Dorn and Rajnak between is given by the solid line of Fig. 21. may be expressed in terms of the temperature of test, T, and the athermal temperature, T,, defined in Fig. 20. Considering the strain-rate, 9, of deformation for a thermally activated process to be given by where fo is nearly a constant, it is seen that and the theoretical relationship of Fig. 21 may be examined with readily obtainable experimental parameters. Dorn and Coworkers analyzed a large amount of experimental data in this manner for materials sus-

Jan 1, 1967

1-Coal and Coal Products HOWARD N EAVENSON, Chairman C. T. HAYDEN, Vice-chairman, Bituminous Coal Production CHARLES DORRANCE, Vice-chairman, Anthracite Coal Production HOWARD P. ZELLER, Vice-chairman, Evaluation of Coal for Coke Making W H. FULWEILER, Vice-chairman, Gas and Its Manufacture J. B. MORROW, Vice-chairman, Coal Preparation Bituminous Coal Production C. T. HAYDEN, Chairman R L ADAMS EUGENE MCAULIFFE HAROLD J ROSE WILLIAM H BLAUVELT H E NOLD D. A RUSSELL GEORGE W EVANS JAMES W PAUL CECIL W. SMITH JOHN A GARCIA GEORGE S RICE SAMUEL A TAYLOR H. S GEISMER W F RITTMAN F G TRYON FRANK HAAS J D ROGERS FRANCIS R WADLEIGH CHESTER M LINGLE

Jan 1, 1923

By AIME AIME

THE third award of the W. L. Saunders Gold Medal for "distinguished achievement in mining" has been made to John Hays Hammond. The two previous awards were to David Brunton and Herbert C. Hoover. Since the principal events in Mr. Hammond's interesting life are not only well known to his many friends but were summarized in the April, 1928, issue of MINING AND METALLURGY (p. 186), it seems most appropriate to reproduce here the chronological record of his career that appears on pages 67 to 69 of "A Student Reverie," by his friend and fellow Freiberg student, F. G. Corning. Mr. Hammond, after graduating from Yale in 1876, was a student at Freiberg for the next three years.

Jan 1, 1929

By E. Mille, R. Geffken

The Mg-Si and Mg-Ge phase diagrams were rede-levtnined by thermal analysis, and the existence of a single congruent melting compound in each system was confirmed. The melting points of the two compounds Mg2Ge and ,Wg2Si were found to be 1117.4° and 1085.0°C respectively. The euteclics for the Mg-Ge system occur at 635.6°C (1.15 at. pcl Ge) and 696. 7°C (64.3 at. pct Ge); for the Mg-Si system the eutectics are at 6376°C (1.16 at. pct Si) and 945.6°C (53.0 al. pcl Si). The phase diagrams and known thermodynamic data were used to calculate activity values for both systems. The activities calculated for the Mg-Ge system agreed very well with those previously published. Partial molar enthalpy values for the Mg-Si systetn were calculated from the phase diagram for the composition region where no experimental values have been reported. THE phase diagram for any system is an important source of thermodynamic data. Steiner, Miller, and Komarek1 have derived equations which permit calculation of the activity in binary systems with an inter-metallic compound! if the liquidus and enthalpy data are known. The thermodynamic properties of the Mg-Ge and Mg-Si systems have recently been determined in this by by an isopiestic method, and it was considered that it would be interesting to compare these directly determined values with those computed from the phase diagram. The basic features of the Mg-Ge and Mg-Si systems are essentially similar. The one intermediate compound present in each system. Mg2X, crystallizes in the antifluorite structure and melts congruently. Raynor4 has accurately determined the temperature and composition of the magnesium-rich eutectic in both the Mg-Ge and Mg-Si systems. Klemm and West-linning5 investigated the entire Mg-Ge liquidus, employing sintered alumina crucibles; the purity of the magnesium and germanium starting materials was not reported. The melt was not stirred, and the temperature was automatically recorded to an accuracy of ±3°C. The authors reported large weight changes due to magnesium evaporation between 50 and 67 at. pct Mg. The Mg-Si system has been studied by a number of investigators, and the results have been compiled by Hansen and Anderko.6 Significant discrepancies exist between the two principle investigations of voge17 and Wohler and Schliephake.8 Two different grades of silicon were used by Vogel, one of 99.2 pct purity and the other quite impure, containing 6 pct Fe and 1.7 pct Al. The magnesium purity was not specified. The melts were contained in graphite crucibles with porcelain thermocouple protection tubes under an atmosphere of hydrogen. Samples weighing 10 g were rapidly heated to 50° to 100°C above the liquidus: held, and then rapidly cooled without stirring. Accuracy was ±1 at. pct which is equivalent to a maximum error in temperature of ±18°C. Wohler and Schliephake used 97.9 pct Mg and 99.48 pct Si. The graphite crucibles contained a stirrer and the 15-g samples were melted under an atmosphere of streaming hydrogen. The samples were chemically analyzed after each run. Because of the scarcity of the data, the impurity of the starting materials, and the resultant uncertainty and inconsistency in the published liquidus values, it was decided to undertake a reevaluation of the Mg-Ge and Mg-Si phase diagrams by thermal analysis. EXPERIMENTAL PROCEDURE Alloys were prepared from 99.99+ pct Mg (Dominion Magnesium Ltd.) with impurities in ppm: 20 Al, 30 Zn, 10 Si, <1 Ni, <1 Cu. <10 Fe; 99.999 pct Ge (United Mineral and Chemical Corp.), and 99.999 pct Si (Wacker Chemie Corp.). All graphite parts were machined from high-density (1.89 g per cu cm) G-grade graphite obtained from Basic Carbon Corp. with a total ash content of 0.04 pct. Boron nitride parts were machined from rods of National-grade H.B.N. boron nitride. All graphite and boron nitride pieces were baked out under running vacuum at 1100°C for 24 hr before us Cylindrical graphite crucibles (1; in. OD, 23/4 in. long, l3/8 in. ID) were tightly closed with threaded graphite covers which had 21/4-in.-long thermocouple wells and 1/4-in.-diam off-center holes for stirrers. The cover and thermocouple well were machined from a single piece of graphite. A stirrer was made from a flat cylindrical graphite plate perforated with five 3/16-in.-diam holes and a 1/2-in.-diam central hole, and was held parallel to the crucible bottom by a 1/4-in.-diam. 4-in.-long graphite rod which screwed into the plate and extended up through a tightly fitting hole in the crucible cover. An iron core enclosed in a glass capsule was attached to the stirrer with an 18-in.-long molybdenum wire, so that the stirrer could be magnetically raised and lowered from outside the system. One crucible and stirrer with essentially the same dimensions given above was made entirely of boron nitride. Chunks of magnesium were premelted, cast into 11/2-in.-diam. rods, and then cut into lengths varying from a to 1 in. A 5/16-in. hole was drilled through the center of each piece to accommodate the thermocouple well and the individual pieces were then cleaned and rinsed with acetone. The total weight of an alloy was 50 to 70 g in the Mg-Ge system and 40 to 60 g in the Mg-Si system. The pure components were weighed to an accuracy

Jan 1, 1969

By Douglas Bunting

(Wilkes-Barre Meeting, June, 1911.) 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 o for the roof. Under the pillar-and-chamber system, almost exclusively followed in the Northern 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 fits 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 most 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

Sep 1, 1911

By GUY B. HUNNER

THOSE magnetite ore bodies comprising the Clifton Mines are situated in the south central part of St. Lawrence County, New York, on the western slope of the Adirondack Mountains. The topography is made up of gently rolling hills densely wooded with second-growth maple, birch, and hemlock. Drainage is north and west to the St. Lawrence River.

Jan 1, 1943

By Samuel H. Dolbear

NOT WITHSTANDING the extremely low ebb of business activity, the nonmetallic industries have fared somewhat better than some other branches of mining. The average price level in nonmetallics, although far from satisfactory, has resisted any cataclysmic fall such as has been suffered by copper and other metals. Stocks, also, though too abundant, represent no corresponding overhang of the market, which should result, in some branches, in a more rapid recovery. Research work has continued in some industries with interesting developments.

Jan 1, 1933

By H. O. Hofman

(Canal Zone Meeting, November, 1910.) I. INTRODUCTION. IN a previous paper,1 The Behavior of Calcium Sulphate at Elevated Temperatures with Some Fluxes, we published the results of our investigation, into the behavior of calcium sul-phate in air; that, is, under oxidizing conditions. The results as a whole hold good for a neutral atmosphere, or one which has no chemical effect upon calcium sulphate. The present paper deals with the behavior of calcium sulphate under reducing conditions, the reducing-agents employed being carbon monoxide and carbon, and the changes the calcium sulphide formed undergoes when it is subjected to an oxidizing-roast. Technical literature deals very sparingly with the subject. The information furnished by the recent leading chemical manuals is general. Thus Dammer,2 Moissan,3 and Abbegg,4 quote Berthier as having reduced, in 1823, calcium sulphate with carbon, and Stammer as having done the same in 1851 with carbon monoxide; Gmelin Kraut 5 add Leplay and Laurent (1848) as third investigators. Metallurgical authorities confine their statements to the known fact of reduction by carbon. II. PREPARATION OF RAW MATERIALS. 1. Calcium Sulphate.-This salt was prepared in the same manner as described in our previous paper,6 and was found to be chemically pure. * Professor of Metallurgy, Massachusetts Institute of Technology. + Metallurgical Instructor, Imperial Technological Institute. 1 Trans, xxxix., 628 to 653 (1909). 2 Handbuch der anorganischen. Chemie, vol. ii., part 2, p. 309 (1894). 3 Traité de Chimic Minérale, 1904, vol. iii., p. 543. 4 Handbuch der anorganischen Chemie, vol. ii., part ii., p. 116 (1905). 5 Handbuch der anorganischen Chemie, vol. ii., part 2, p. 233 (1909). 6 Trans., xxxix., 630 (1909).

Nov 1, 1910

By Everett D. Jackson

The largest deposits of chromite in the United States occur in tabular layers in the lower part of the Stillwater Complex, Montana. Nearly 900,000 long tons of chromite concentrates have been produced from mines and mills in this area, and reserves equivalent to more than 2,500,000 long tons of Cr2O3 remain in the ground. Layered concentrations of chromite (known as chromitite zones) occur only within olivine-rich layers of the Peridotite member of the Ultramafic zone of the Stillwater. As many as thirteen chromitite zones, lying conformably one above another, have been found, but only five of them are thick enough to be of any conceivable economic interest, and only two of them have been mined to date. Individual chromitite zones can be recognized by their distinctive internal character and by their stratigraphic position in the Peridotite member. The chemical composition of chromite varies consistently between and within the different chromitite zones. The zones range in thickness from about an inch to more than 12 feet, and the Cr2O3, content of their chromite ranges from 36.6 to 49.2 weight per cent. In any given area, the G chromitite zone is the thickest, but the chromite of the H chromitite zone has the highest grade. Laterally, the thickness of the chromitite zones is related to the thickness of the Peridotite member; in general all are thicker in the eastern part of the Stillwater than in the western part.

Jan 1, 1968

By J. R. McWilliams

Several of the current concepts of brittle fracture involve consideration of the existence of defects or flaws. Griffith 1 observed that the tensile strength of brittle materials was several orders of magnitude less than might be expected from the calculated strength of the molecular bonds. He was able to show that the stress concentration at the tip of an elliptical crack is sufficient to raise the local stress level many fold. He therefore proposed that real materials do contain such stress risers in the form of minute cracks and that this accounts for the rather large difference between theoretical and actual strengths of brittle materials. The degree of stress magnification depends to a great extent upon the orientation of the crack with respect to the general stress system, and to the general geometry of the crack.

Jan 1, 1967