PEGMATITE DIKES OF THE BRIDGER MOUNTAINS, WYOMING*

      THAD G. McLAUGHLIN, University of Kansas, Lawrence, Kansas.

      CONTENTS

ABSTRACT.............................................................................. 46
INTRODUCTION.................................................................... 47
COUNTRY ROCK................................................................... 49
   Pre-Cambrian.......................................................................... 49
      Area.................. ................................................................... 49
      Description................ ........................................................... 49
      Structure............................................................................... 50
      History ................................................................................. 50
PEGMATITE DIKES................................................................. 51
   Distribution............................................................................... 51
   Age.......................................................................................... 51
   Older Dikes.............. .............................................................. 52
      Description............................................................................ 52
      Structural relationships.......... ....... ................................ . ..... 53
      Contact metamorphism.......................................................... 53
      Mineral descriptions............................................................... 54
   Younger Dikes............................ ............................................ 55
      Description............... ............................................................ 55
      Structural relationships.......... ................................................ 56
      Contact metamorphism............................. ............................ 56
      Mineral descriptions............................................................... 56
   Zoning ..................................................................................... 60
   Banding.............. ..... ... . .......................... ................. ............ 62
   Paragenesis............ .............. ................................................... 64
REPLACED SCHIST.................................................................. 65
HISTORY OF MINERALIZATION ......... ................................ 67
REFERENCES........................................................... ................ 68

ABSTRACT

      The central area of pre-Cambrian rocks in the Bridger Range of central Wyoming consists predominantly of black hornblende schist. The schist has been invaded by large masses of coarsely crystalline granite. The granite invasion was followed by two distinct intrusions of granite pegmatite. The older pegmatite dikes were intruded along a steeply-dipping joint set which is parallel to the plane of foliation of the schist. The younger pegmatite dikes have a lesser dip and were intruded along a joint set which was formed subsequent to the intrusion of the older dikes.

      The intrusion of the older dikes was followed by an invasion of silica-rich and potash-rich hydrothermal solutions which replaced the black schist with quartz and sericite. The intrusion of the younger dikes was followed by an invasion of hydrothermal solutions which partially replaced the original dike minerals and precipitated cleavelandite, muscovite, tourmaline, beryl, garnet, columbite, tantalite, chalcopyrite, lepidolite and petalite.

      Several of the hydrothermal minerals were deposited in parallel bands which strike and dip nearly parallel to the strike and dip of the dikes. The semi-concentric arrangement of these bands and their uniform parallel arrangement indicates a rhythmic precipitation similar to "Liesegang rings."

      After a removal of the overburden by erosion, several supergene minerals were formed, including quartz, sericite, malachite and limonite.

      The purpose of this study was to determine the history of the country rock, the structural relationships between the dikes and the country rock, the relative ages of the two types of dikes, and the paragenesis of the minerals within the dikes.

      INTRODUCTION

      The area which was studied lies in the northeast part of Fremont County near the center of the state. The pegmatite dikes that were mapped are intruded into the pre-Cambrian rocks east of the Wind River Canyon. They are 15 miles northeast of Shoshoni and about 20 miles southeast of Thermopolis. Field studies were made during the summers of 1936, 1937, and 1938, and were supplemented by laboratory work in the spring of 1937 and the fall and winter of 1938.

      A map of sections 27 and 28, range 93 west, township 40 north, was made by a plane table survey in which both the geology and topography were mapped. The map shows the topography, with a contour interval of 20 feet, and the areal geology. Rock types mapped include the pre-Cambrian schist, pegmatite dikes, and Pleistocene (?) alluvium.

      I wish to express my sincerest thanks to K. K. Landes and H. T. U. Smith for the help rendered in the field, in the laboratory, and in the theoretical discussions. Mr. LeRoy Fugitt, Mr. Bruce F. Latta, and Mr. Kenneth E. Corr assisted in the field work during the summers of 1936, 1937, and 1938, respectively. Mr. Ed Crabb and Mr. Denny Thoren of Shoshoni and Mr. Val B. Maghee of Lander furnished valuable information about the area studied.

      The first mining in this area was done in 1906 when Mike Crowley filed a claim in dike 85. Some mica was taken out, but the project was soon abandoned. The present claimants, Jake Stephenson and others, filed in 1910 and mined a small amount of feldspar, beryl, and mica in dikes 3, 7, 25, 81, and 85. One of the claimants leased and did some work on dike 7 in 1928, taking out feldspar and lepidolite. The claims at present are leased by Mr. Val B. Maghee of Lander, Wyoming, who has been working on dike 86 since early in 1937 and has marketed several hundred pounds of tantalite. He is also mining beryl from the same dike.

      COUNTRY ROCK      

PRE-CAMBRIAN

      AREA

      The pre-Cambrian rocks outcrop in a roughly triangular area nearly 20 miles long and 10 miles wide, elongated in an east-west direction. They are also exposed several miles west of this area in the bottom of Wind River Canyon in a triangular area 1 mile wide and 12 miles long (6).

      DESCRIPTION

      The black country rock is a fine grained schist composed dominantly of hornblende and labradorite. The schist resembles a fine grained igneous rock, but the crystals are elongated parallel to the schist's foliation. Thin sections of the country rock show that it is a hornblende schist.

      Hornblende constitutes 70 to 85 per cent of the schist. It occurs in elongated lath-like subhedral crystals, which are crossed and interpenetrating. The color in thin section, because of strong pleochroism, varies from light to medium blue-green. The laths are 0.1 to 0.5 of a millimeter long and generally about one-fifth as wide. A few crystals of hornblende are slightly altered to chlorite. The labradorite variety of plagioclase feldspar constitutes 10 to 20 per cent of the black country rock. It forms irregular masses and subhedral crystals with prominent albite twinning lines. It is colorless and has a low relief and low birefringence. The individuals were generally 0.8 to 1.0 mm. in diameter. Common accessory minerals are magnetite, chlorite, quartz, actinolite and zircon. None of these minerals ever constitute over two per cent of the rock. Magnetite forms very irregular masses which are subrounded, tabular, or trellis-like. It is often slightly altered to hematite and limonite which are nearly opaque to transmitted light, but are reddish brown to ocher yellow in reflected light.

      Many thin sections of the country rock contain a very fibrous or asbestiform mineral, which is probably an alteration of hypersthene to actinolite. The fibers are very coarse and irregular to wavy. Pleochroism is very strong, with a color variation from pale grayish brown to brownish black. The birefringence is high, but the colors are slightly obscured by the color of the mineral. Zircon is present in every thin section that was made of the country rock. It may be rounded, ellipsoidal, or rod-like with rounded ends. The extinction is parallel, the relief and birefringence are very high, and the color is gray to colorless.

      The granite outcrops in the eastern third of the pre-Cambrian area. It is a medium to coarse-grained rock containing over 90 per cent pink feldspar with only a small amount of quartz and muscovite. Near the granite-schist contact the granite has a gneissic structure and a much greater percentage of muscovite and quartz. Joints extend from the granite into the schist without offset. The granite erodes into rounded knobs and hills of small relief. Granite also outcrops in Wind River Canyon (6) and near Birdseye.

      STRUCTURE

      The schist in this area has very distinct foliation and jointing. The strike and dip of the foliation of the schist varies considerably from one locality to another. The strike is "not more than 10° from east to west" in Wind River Canyon (6 p. 1419), but in the two sections that were mapped the strike varied from S. 40° W. to S. 83° W. with an average of S. 60° W.

      The dip of the foliation in Wind River Canyon ranges from 30° to 60° S. with an average of 45° S. (6 p. 1419) while in the area that was mapped, the average dip was 66° S. with a maximum of 70° S. and a minimum of 58° S. The jointing in the schist dips against the foliation of the schist. The dip varies from 40° to 50° N. with an average of about 45° N. It is along these joints that many of the pegmatite dikes were intruded. The granite in the eastern part of the pre-Cambrian area, and many of the larger pegmatite dikes, also show distinct jointing.

      HISTORY

      A considerable controversy has taken place concerning the origin of hornblende schist and similar metamorphic rocks. Many conclusions have been based on the presence of quartz, the range of the plagioclase feldspars, and the physical properties of the zircon grains.

      The zircons in some hornblende schists are smoothly rounded spheroids and ellipsoids, which closely resemble the rounded grains in sedimentary rocks. It has been inferred by some that. this is sufficient proof of a sedimentary origin. Later investigators have pointed out that many of the zircon grains are oblong as well as rounded, and that rounded grains can be found in igneous rocks, such as diorite, dolerite and basalt (22). Zircon crystals from the Bridger mountain schists have length to width ratios as high as 8 to 1. Grains of such shape are probably not of sedimentary origin.

      Carlson (3) believes feldspar and quartz are indicators of the former character of metamorphic rocks. A narrow range in the plagioclase feldspars is indicative of an igneous origin, for derivation from sedimentary rocks would result in a wide range in plagioclase feldspars, and also an excess of quartz.

      The schists of the Bridger Range contain elongate grains of zircon, have a narrow range of plagioclase feldspars, and are almost devoid of quartz. These criteria point to an igneous origin, probably from a diorite or basalt.

      The red, coarse-grained granite at the east end of the pre-Cambrian area was intruded after the formation of the black schist, for it shows very few signs of regional metamorphism. One jointing system passes through both schist and granite, which means that this granite is older than the nearby pegmatite dikes which were intruded along the jointing system in the schist.

      Another area of red granite is exposed near Birdseye at the Gold Nugget mine. The pegmatite dikes in this locality have been intruded into the schist parallel to the near-vertical jointing system. These dikes also intrude the red granite which is probably the source of the later pegmatite dikes.

      PEGMATITE DIKES

      DISTRIBUTION

      The black schist in Wind River Canyon and in the pre-Cambrian area east of the canyon has been invaded by many granite pegmatite dikes. These dikes outcrop in all parts of the schist area and in the granite area near Birdseye, but none were found in the granite at the east end of the pre-Cambrian area.

      The simple dikes (those which have not suffered later hydrothermal replacement) are evenly distributed throughout the area of black schist. Most of them strike and dip concordantly with the foliation of the schist, but a few in Wind River Canyon are parallel or subparallel to a joint system which dips against the foliation.

      The complex dikes, all of which dip in a direction opposite to the dip of the foliation of the schist, are concentrated in a small area on the south side of the mountains in range 93 W. Most of them were included in the map of sections 17 and 28. Dike 86 is the only dike containing hydrothermal minerals that does not lie in either of these two sections. It outcrops in the southeast quarter of section 21.

      AGE

      The pegmatite dikes are truncated by the Deadwood sandstone in Wind River Canyon. This indicates a pre-Cambrian age for the dikes. The relationships of the pegmatite dikes and the Paleozoic sediments are not readily observable along the south side of the Bridger Range because of the extensive overlap of the Wind River formation. The writer was unable to find any dikes that were intruded into sedimentary rocks.

      OLDER DIKES

      DESCRIPTION

      The older dikes in this area are those which strike and dip parallel or subparallel to the strike and dip of the schist's foliation. Only 19 per cent of the dikes in sections 27 and 28 are of this type. Many of these dikes are small parallel bands intruded in a lit-par-lit manner. Others are large, coarsely-crystalline masses. Dike 38 is 50 meters wide and 170 meters long. The large area of altered schist adjacent to this dike indicates that if the dike were entirely exposed it would be nearly 350 meters long. Nearly 50 per cent of the older dikes are more than 35 meters in length and 90 per cent of them are 20 meters or more long.

      The older dikes are all elongated bodies but where insufficient country rock has been eroded from the dike the outcrop area may be roughly circular. One can roughly determine the actual dimensions of the dike, however, by measuring the length and width of the zone of altered schist which invariably overlies the buried part of the dike. The length is always 5 to 10 times as great as the breadth.

      STRUCTURAL RELATIONSHIPS

      The average strike and dip of the dikes is S. 61° W. 65° S., which is almost identical to that of the foliation of the schist (S. 61° W. 66° S.). The strike of the dikes ranges from S. 40° W. to S. 80° W. The maximum dip of the dikes is 70° S. and the minimum dip is 58° S. The strike and dip of the dikes coincide much more closely with the strike and dip of the foliation in this area than they do in Wind River Canyon.

     
FIG. 2. Older dikes intruded parallel to the foliation of the schist.

     
FIG. 3. Jointing in the older dikes.

      CONTACT METAMORPHISM

      The intrusion of the older pegmatite dikes was accompanied by alteration of a large amount of country rock. The altered zone always lies above the dike and never extends more than one meter on either side of it. Silica-rich solutions, which travelled beyond the dike itself, followed the easiest passageway, which was parallel to the foliation of the schist. Thin sections of this altered country rock show considerable change from the original hornblende schist by the addition of a large amount of quartz and some muscovite. Hornblende and plagioclase, and the accessory minerals magnetite and zircon are present. The percentage of hornblende is greatly reduced and quartz is the predominant mineral.

       Megascopically, the metamorphosed schist is brown to brownish green. It is more micaceous, softer, less brittle, and can be readily distinguished from the original black schist even* at a considerable distance.

     
FIG. 4. Pegmatite dikes in the black schist in Wind River Canyon. (Photo by H. T. U. Smith)

      MINERAL DESCRIPTIONS

      Microcline. The most abundant mineral in the older dikes is microcline. It constitutes more than 70 per cent of the rock which is made up entirely of quartz and feldspar. The microcline is dominantly white but may also be various shades of gray, light blue and rarely pink. It is coarsely crystalline with some individuals more than a meter in diameter. No euhedral or subhedral crystals were observed. A perthitic intergrowth of albite and microcline was found in several dikes. The color of both the albite and microcline is white, making the perthite difficult to distinguish from the specimens of pure microcline. Quartz is the most common mineral associate of microcline but tourmaline may rarely be present where a younger dike transects an older one.

      Quartz. Quartz is next in abundance and constitutes almost 30 per cent of the rock in the older dikes. It is always anhedral and its color is generally pale milky but locally is dull gray. The quartz is evenly dispersed throughout the dike and is seldom concentrated in lenses as it is in the younger dikes. It frequently forms veins, varying in width from 1 to 30 cm. which penetrate the country rock in a lit-par-lit manner. These veins often contain a small amount of microcline.

      Tourmaline. Black tourmaline is found in an older dike (no. 28) where it is cut by a younger dike which dips in a direction opposite to that of the foliation of the schist. The tourmaline-bearing solutions obviously came from the same parent magmas as the younger dike. These solutions followed the sides of the younger dike and deposited tourmaline in small veins and clusters in both the older and the younger dikes. The tourmaline is found in subhedral crystals less than 2 cm. long. These elongated crystals cut both the quartz and the microcline.

      YOUNGER DIKES

      DESCRIPTION

      The younger dikes are those that dip north against the foliation of the schist. Eighty-one per cent of the dikes that were mapped in sections 27 and 28 are of this type. These dikes outcrop only in a small area on the south side of the Bridger Mountains near the contact of the black schist and the alluvium. Most of the younger dikes are elongated, tabular masses with a length 5 to 20 times as great as the thickness. The average thickness of the dikes in sections 27 and 28 is 5 meters and the average length is 84 meters. The mean ratio of length to thickness is 16 to 1. The feldspar in the dikes is white, gray, or pale blue and gives the dike a decidedly white color which is amplified by the background of black schist. This contrast of color makes the dikes visible for a great distance.

     
FIG. 5. Sharp contact of a younger dike with the country rock.

      The younger dikes do not weather to any characteristic forms. There is no distinct jointing, but the dikes fracture easily causing them to erode as rapidly as the schist. The rate at which they erode is controlled by the rate of weathering of the schist.

      STRUCTURAL RELATIONSHIPS

      One of the outstanding features of this area is the almost perfect control on the position of the dikes exercised by the structure of the country rock. All of the older dikes strike and dip parallel to the foliation of the schist. Variations in strike and dip of these dikes is caused by variations in the strike and dip of the foliation. All of the younger dikes have been intruded parallel to the jointing in the country rock. The directions of jointing are more uniform than the foliation of the schist, therefore, the strike and dip of the younger dikes is more uniform than that of the older ones.

      The maximum dip measured in sections 27 and 28 is 62° N. and the minimum is 30° N. The average of all the dips measured is 45.5° N. Eighty-five per cent of the dikes dip to the north at an angle within 40° and 50°, and 50 per cent of the dikes dip north at an angle within 3° of the mean (45.5°). The average strike of the dikes is S. 43° W. which is nearly the same as that of the foliation of the schist.

      CONTACT METAMORPHISM

      Metamorphic effects of igneous intrusion were noted in all but two of the younger dikes. The otherwise hard, black hornblende schist has been altered to a dark bluish-green, friable material which megascopically looks very different from the original material. The contact of the altered schist with the unaltered material is always a joint plane. The metamorphosed rocks are coated at the surface by brown iron oxides. This has probably been caused by oxidation of the iron in the schist to limonite, followed by incomplete dehydration to hematite and perhaps turgite.

      Microscopic examinations of thin sections of the metamorphosed material have revealed very little mineralogical change from the original unaltered schist. It contains over 70 per cent of hornblende, 5 to 15 per cent of quartz, and lesser amounts of magnetite, actinolite, and zircon.

      MINERAL DESCRIPTIONS

      Microcline. Microcline is the most abundant mineral in the younger dikes as well as in the older ones, but the percentage is much smaller in the younger dikes. The microcline in these complex dikes is white to pale blue and gray. It is coarsely crystalline, some individuals being over 2 meters in diameter.

      The dikes originally were intruded as liquid masses of microcline and quartz with the quartz concentrated in irregular lenses at the center of the dikes and microcline along both walls of the dikes. Later invasion and replacement at the center and along the walls left only small zones of microcline about one-third of the way in from each wall. Microcline is almost always found in these zones; and in a few dikes where there is no quartz lens, it may be found at the center of the dike.

      Microcline is relatively soluble and has been attacked by hydrothermal solutions and partially replaced by later minerals. Some individuals have been almost entirely replaced by cleavelandite and muscovite. Perthitic intergrowths of white albite and white to gray microcline were found in dike 25. They occur in a narrow zone near the top wall of the dike.

      Quartz. Quartz is found in three places within the complex dikes: (1) in lenses near the center of the dikes, (2) in a small zone near the walls of the dike, and (3) rather evenly disseminated throughout the rest of the dike.

      The quartz is white, milky, dull gray or, in a few instances, rose. It is always anhedral. Dikes 79 and 82 are made up almost entirely of feldspar, but grade into pure white quartz dikes or veins which stand above the rest of the rock because of greater resistance to erosion. Conchoidal fracture is most common, but a few specimens display a poor cleavage. The luster is vitreous, except for the gray quartz which has a dull luster.

      Lepidolite. The lithium minerals are not very abundant. They were found in dikes 7, 11, and 86. The lepidolite was deposited in irregular zones at the center of the dikes where it is commonly associated with quartz, sericite and some cleavelandite. The mineral occurs as small plates or thin books less than 6 mm. in diameter which are rather evenly disseminated through. the central part of the dikes. The plates are pale lilac in color, irregular in outline, and slightly fractured.

     
FIG. 6. Pseudomorph of albite after petalite. One-half natural size.

      Petalite. Dike 86 is the only one where petalite was collected. However, pseudomorphs of albite after petalite are rather abundant in dike 7. A lens of milky quartz lies at the center of 86 and is overlain by a zone containing cleavelandite and a pseudomorph of albite after petalite. The pseudomorph, however, is absent from the zone of cleavelandite which lies immediately below the quartz lens. Number 7 has no lens of quartz at the center, but there is instead a belt of lepidolite, cleavelandite, and some sericite and quartz. This central area is both overlain and underlain by a pseudomorph of albite after petalite.

      The petalite individuals are as much as 20 cm. long. They are extremely brittle and therefore highly fractured. The cleavage is good both on c(001) and x(201). The basal cleavage is not perfect as many textbooks describe it. The color and diaphaneity closely resemble that of pale milky quartz except along the basal fracture or cleavage planes where it is white and less translucent.

      The replaced petalite resembles to some extent the original mineral. The basal cleavage lines are very distinct (Fig. 6), but traces of cleavage along o(201) are poor or absent. The cleavage plates are 1 to 3 mm. thick and are always normal to the dike walls. The zones of replaced petalite lie immediately above and below the central area of lepidolite in dike 7. These zones are very irregular and are at no place more than 1.5 meters thick. Apparently the replacement was complete for no specimens of residual petalite could be found. Specimens collected from the Bridger Range closely resemble those from Londonderry in Western Australia except that the former contain more sericite and have preserved the original structures somewhat better.

      Amblygonite. Although no amblygonite was collected from any of the dikes, it has been reported by Mr. Maghee to be present in dike 7.

      Cleavelandite. Cleavelandite is the most abundant hydrothermal mineral present in the younger dikes. It is white to cream colored, but may be stained black by manganese oxide. The cleavelandite is subhedral with individuals varying greatly in size. The specimens having a sugary texture contain crystals as small as 1 mm. Several crystalline aggregates taken from dike 7 were more than 15 cm. wide. They consist of radiating tabular crystals of cleavelandite elongated parallel to the side pinacoid. Albite twinning is noticeable.

     
FIG. 7. Black tourmaline transecting cleavelandite. Three-fourths natural size.

      The cleavelandite occurs in irregular zones near the walls of the dike. These zones are very indefinite. A wide variety of minerals is associated with cleavelandite, including microcline, quartz, muscovite, garnet, beryl, columbite and tantalite.

      Tourmaline. The ordinary black variety is the only type of tourmaline found in these dikes. It commonly occurs as elongated and striated crystals which were deposited in small veins and lenses within the dike. The tourmaline bearing solutions rose along the contact of the dikes and precipitated tourmaline crystals both in the dikes and in the schist. The mineral is subhedral and very brittle and is found replacing or cutting into quartz, microcline, cleavelandite, and schist.

      Braunite. The mineral braunite always occurs in very irregular nodules surrounded by coarsely crystalline blades of greenish muscovite. The nodules in dike 85 were all found at or near the base of a large lens or band of muscovite and cleavelandite. The mineral is dark brown to nearly black and is submetallic to earthy. The submetallic part of a specimen generally shows good cleavage in one direction, probably parallel to p(111). The earthy part is browner and breaks with an uneven, slightly conchoidal fracture. Single nodules are as wide as 12 cm. but the average is about 5 cm.

      Muscovite. The muscovite found in these dikes is pale green and highly fractured. Hydrothermal solutions have deposited this mineral in three distinct zones within the dikes: (1) at the contact of the dike with the country rock, (2) in lenses between the quartz lode at the center of the dike and the upper contact with the schist, and (3) in small books and flakes associated with the cleavelandite.

      Muscovite is comparatively resistant to attack and is only replaced by one mineral, sericite. Small books of muscovite may be replaced along their borders, and a few specimens show small veinlets of sericite cutting entirely across books of muscovite.

      Apatite. Several small aggregates of anhedral grains were identified by oil immersion methods as apatite. Such aggregates were never over 1 cm. wide, and individual grains were less than 1 mm. in diameter. These sub-rounded grains were all found in specimens collected from dike 25. However, much larger individuals are reported by Mr. Maghee to have been found in dike 86.

      The mineral is dark bluish-green, translucent, and shows a sub-vitreous luster. In every specimen that contained apatite, the small grains were imbedded in a mass of white saccharoidal cleavelandite with fine flakes of muscovite and a small amount of residual pale milky quartz.

      Beryl. Cleavelandite and muscovite are always associated with beryl. The beryl may occur as light green to pale blue elongated euhedral prisms imbedded in the cleavelandite zones near the center of the dike, or as aggregates of small blue green anhedral blebs in the bands of sugary cleavelandite found in the banded dikes. Some of the euhedral crystals of beryl are aquamarine and of gem quality.

      The individuals vary in size from small rounded masses less than 1 mm. wide, to euhedral prisms 20 cm. in diameter. The beryl crystals cut through masses of sugary clevelandite, but are unaltered by any of the hydrothermal solutions.

      Columbite-tantalite. Both the nearly pure niobate and the nearly pure tantalate often are found in the dikes in this area. This mineral is found associated with sugary cleavelandite in a zone just above the center of the dike. The zone varies greatly both in thickness and in concentration of columbite-tantalite. Only dikes 85 and 86 contain it in considerable quantities. Specimens belonging at both ends of the isomorphous series occur in these two dikes.

      Columbite is almost entirely confined to dike 85, but a few crystals were collected from 86. It crystallizes in very thin tabular crystals elongated parallel to a(100). The crystals are generally less than 1 mm. thick, about 1.5 cm. wide, and 5 to 10 cm. long. The most common forms present are two prisms, brachypinacoid and macropinacoid. The mineral is also commonly in anhedral masses with quartz and cleavelandite. Chemical analyses by the Fansteel Metallurgical Corporation show that the columbite is very low in tantalum, and that one specimen from dike 86 was almost pure columbite. It is always black and displays a metallic luster. The specific gravity is 6.21, which is high for a mineral of this series so low in tantalum.

      Tantalite is much more abundant than columbite, but is almost entirely limited to dike 86. A few euhedral crystals, which closely resemble those found in dike 86, were found in dike 7. Tantalite is in most cases euhedral, although a few individuals are subhedral or anhedral. Crystals of this mineral are equidimensional and maybe as large as 5 cm. in diameter, but have an average width of 2 to 3 cm.

NOTES

      * A portion of a Ph.D. thesis, University of Kansas, 1939.