BWwm umm w M Mw ww Mmw Naoarmonncc GEOLOGY AND ORE DEPOSITS OF THE RANDSBURG QUADRANGLE OF CALIFORNIA BULLETIN Na. $5 ISSUED BY THE CALIFORNIA STATE MINING BUREAU FERRY BUILDING, SAN FRANCISCO THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA DAVIS FRONTISPIECE. ^'l HIGH GRADE SILVER ORE. From 6th level, California Rand Silver Mine. Drusy cavities and angular schist inclusions with the schistosity oriented at random are characteristic of the ore. Natural size. 37841 CALIFORNIA STATE MINING BUREAU FERRY RUrr.DTNG. SAN FR\NCIS(JO LLOYD L. ROOT State Mineralogist San Francisco] BULLETIN No. 95 [March, 1925 Geology and Ore Deposits OF THE Randsburg Quadrangle CALIFORNIA By CARLTON D. HULTN, Ph. D. LIBRARY UNIVERSITY OF CALiFORNIA; DAVIS CALIFOKNIA STATE PRINTING OFFICE JOHN E. KING, State Printer SACRAMENTO, 1925 LIBRARY UNlVERSliY OF CALIFORNIA DAViS f\ CONTENTS. Page INTRODUCTION H PART I. GENERAL. GEOGRAPHY AND GEOLOGY 13 Location 13 Accessibility Ip Literature 1^ Bibliography 15 Topography 1^ The Rand Mountains 1^ Red Mountain 1'^ Lava Mountains 1^ El Paso Mountains 1^ Summit Range 1^ Valleys 18 Climate 1^ Vegetation 19 Water Supply 19 Geology 20 General Outline 20 Rocks of Archean Age 21 Johannesburg Gneiss 21 Petrology 21 Origin 23 Structure 23 Rand Schist 23 Petrology , 23 Origin 26 Structure 27 Relation of Rand Schist to Johannesburg Gneiss 38 Conditions of Metamorphism of Johannesburg Gneiss 38 Conditions of Metamorphism of Rand Schist 39 Age of Johannesburg Gneiss and Rand Schist 39 Undifferentiated Strata of Paleozoic Age 31 Litholog>' and Structure 31 ]Mode of Accumulation 33 Age 33 Atolia Quartz Monzonite— 33 Petrology 3 4 Relation to Adjacent Formations 37 After Effects of the Bathonthi'c Invasion 38 Correlation of the Three Areas of Quartz Monzonite 3S Mechanics of Intrusion 39 Age 1 42 The Rosamond Series 42 Lithologj' 43 Weathering 45 Structure and Thickness 45 Relation to Adjacent Formations 46 Conditions of Accumulation 47 Age and Correlation with Adjacent Regions 47 Rhyolite-Latite Series, of Intrusives 48 Petrology 49 Effects on Intruded Rocks 50 Age 51 Weathering , 52 Diabase-Basalt Series of Intrusives 52 Field Relations 52 Petrology 53 Weathering 54 Age 54 Red Mountain Andesite 55 Field Relations , 55 Mechanics of Extrusion _ 56 Petrology 57 Structure and Thickness 58 Age 58 915S5 6 CONTENTS. Page Black Mountain Basalt 58 Field Relations 58 Petrology 60 Age 60 Alluvium 60 Ancient Gravels 61 Other Alluviums 61 Structure 61 The Garlock Fault I 62 The Lava Mountains 64 The Rand Mountains 64 Geologic History 65 PART II. MINERAL DEPOSITS 69 General Statement 69 The Tungsten Deposits 70 Field Relations 70 Minerals of the Deposits 72 Grade of the Ores ; 74 Ore Textures and Paragenesis 74 Nature of the Solutions Which Deposited the Ores 76 Possible Structural Control of the Ore Deposition 76 Age and Genesis of the Denosits 77 Possible Extension of the Tungsten Deposits 78 The Gold Deposits 79 Field Relations 79 Structure of the Deposits 80 Nature of the Veins 83 Mineralogy 83 Ore Textures and Paragenesis 84 Oxidation and Enrichment 86 Age and Genesis of the Deposits 88 Structural Control of the Ore Deposition 88 Future Possibilities of Gold Mining 91 Tlie Silver Deposits 92 Field Relations 92 Structure of the Deposits 95 Nature of the Vein Filling 97 Mineralogy 97 Grade of the Ores 99 Ore Textures and Paragenesis 99 Alteration of the Wall Rock 102 Order of Formation of . tlie Minerals 103 Age and Genesis of the Deposits 104 Origin of the Veins , 104 Structural Control of the Ore Deposition 105 Possible Extension of the Deposits 105 Conclusion 106 PART IIT. HISTORY OF MINING lOS PART IV. MINES AND PROSPECTS 110 California Rand Silver Mine , 110 Northeast Veins 112 Footwall Veins 112 Shaft Vein 112 Antimony Vein 112 Alpha Vein 113 Williams Vein 113 North-South Veins , 114 Blanck Vein 114 Frog Vein ■ 114 Treasure Box Vein 114 Rourke Vein 115 Jameson Vein 115 No. 3 Stope , 115 518 and 530 Veins 115 Sill Vein 116 Harrell Vein 116 Cow Trail Vein 117 Hughs Vein 117 Grady Lease Vein , 117 Grady West Vein 117 Wliirk Vein 118 Unnamed Vein 118 Nosser Vein 118 Number Six Workings 118 COXTENTS. 7 Page l-*roduction and Costs 119 Yellow Aster Mine 121 Mine Workings 122 Orebodies 122 Cost and Production 124 Properties of the Atolia Mining Company 125 I'nion Mine 125 Amity Mine 126 Attilla Mine 126 Acaley Mine 126 Papoose Mine 126 Mahood Mine 126 Flat Iron Mine - 127 Par Mine 127 Goldstone Mine 127 Rainstorm Mine 127 PrudiR'tion 127 Other Properties 128 Baltic- 128 Beehive 128 Bekher Extension 128 Ben Hur 129 Ben Hur Extension 129 Bevis Divide 129 Big Four 129 Big Gold 130 Big Six 130 Black Hawk 130 Bray and Bisbee.- 131 Bully Boy 132 Butte 132 Chicken Hawk 132 Cima Bimetallic 132 Consolidated 132 Coyote 133 Cuve 134 Flat Tire 134 Fox- Lease ' 134 Garford Lease 135 Hard Tack J 135 Hummer . 135 Julius Shades 136 Kelly Rand Extension 136 King So'omon 137 Little Butte 137 Miniuhalia 137 Mizpali Montana 138 Mizpah Nevada 138 Monarch Rand 138 Nancy Hanks 139 Navajo and Swastika 139 Oney Lease 139 Operator Divide 140 Rand Contact 140 Rand Mountain 140 Randsburg Associated Mines, Inc. ^ 140 Silver Basin 141 Silver Bell 141 Silver Giant 141 Silver Glance Lease 142 Silver King 142 Sliverton ^ 143 South Rand 143 St. Lawrence Rand 143 Treasure Hill 143 White Horse Rand 144 Other Lode Properties 144 Placer Deposits 144 Atolia 'Spud Patch' 145 Huelsdonk Placer 145 Oro Fino Placer .- 147 Summit Placer Gold and Rock Company, Inc. 148 Other Economic Deposits 148 ILLUSTRATIONS. rage Plate 1. Geologic Map of the Randsburg Quadrangle In pocket Plate 2. Underground Workings of the California Rand Silver Mine and Adjacent Properties In pocket Plate 3. Underground "Workings of the Properties of the Atolia Mining Company In pocket Plate 4. Underground Workings of the Yellow Aster Mine In pocket Plate 5. High Grade Silver Ore, California Rand Silver Mine Frontispiece Plate 6. A. Main Street of Randsburg Looking West. B. Randsburg and the Yellow Aster Mine 16 Plate 7. A. Hornblende-plagioclase gneiss. B. Biotite-albite schist 24 Plate 8. A. Biotite-hornblende-albite schist. B. Chloritic-actinolite-albite schist 25 Plate It. A. Outcrop of Rand Seliist Northwest of Randsburg. B. Southern Front of the Bl Paso Range 30 Plate 10. A and B. Quartz Monzonite 34 Plate 11. A. Thin Section of an Orbicle. B. Quartz-epidote Rock 36 Plate 12. A. Orbicular Quartz Diorite. B. Weathering in the Rosamond Series. 41 Plate 13. A. Rosamond Sandstone. B. Rhyolite 48 Plate 14. A. Rhyolite with "Vermicular Intergrowth. B. Basalt 50 Plate 15. A. Rhyolite Dike. P.. Red Mountain from the West 55 Plate 16. A and B. Pyroxene Andesite from Red Mountain 58 Plate IT. A and B. Black Mountain Basalt 60 Plate IS. A and B. The Garlock Fault Line 63 Plate 19. A and B. Thin Sections of Scheelite Ore 74 Plate 20. A and B. Scheelite Ore 76 Plate 21. A. Thin Section of Gold-bearing Vein Matter. B. Gold Ore from the Sunshine Mine 86 I'hilf :;2. A. Stope at the Top of the Rand Vertical Vein, Yellow Aster Mine. B. The 'Silver Camp' from the East 92 Plat<' 23. A. Vein Matter. 6th Level, California Rand Silver Mine. B. Vein Matter, 11th Level, California Rand Silver Mine 96 i^hitt' L'4. A. Miargyritc Filling Cavities in Earlier Quartz. B. Banding of Pyrite and Finely Crystalline Quartz 100 Plate 25. A. Silver Ore, Showing General Relations of the Minerals. B. Miar- gyrite Surrounding and Replacing Stylotypite 101 Plate 26. A and B. General Relations of Minerals of the Silver Ore 102 Plate 27. A. Silicilication of the Schist Wall Rocks of the Silver Veins. B. Alteration of Metallic Silver Minerals 103 Plate 2 8. Map of Mining Claims in the Vicinity of Randsburg In pocket Plate 29. A. lleadframe of No. 2 Shaft and Mill of the California Rand Silver Mine. B. New and Old Shafts of the Union Mine Near Atolia 111 Plate 30. Map of Mining Claims in the Vicinity of Atolia In pocket Plate 31. A. Dredge Installation on the Norden Placer Claims. B. Stebbins Dry Concentrator on the Property of the Oro Fino Mining Company 14 6 Figure 1. Sketch Map of California Showing Location of Area Covered by this Report 14 Figure 2. Orbicles Whose Appearance Suggests That They Once Formed a Single Mass 36 Figure 3. Portion of the Randsburg Quadrangle in Which the Atolia' Quartz Monzonite Would Outcrop if- the Tertiary and Later Deposits Were Removed 40 Figure 4. Fault System of the Yellow Aster Mine 88 Figure 5. Hypothetical Cross-section of the Ore Bodies of the Yellow Aster Mine 90 Figure 6. Structural Control of Mineralization Along a Fracture 91 l'"'iguri' 7. Idealized Section Tlirough Silver Deposits 94 Figure S. Flow Sheet of the California Rand Silver Mill 120 LETTER OF TRANSMITTAL. To His Excellency, Honorable Friend \\yi. Kichardson, Governor of the State of California. Sir: T luive tlic lioiior to herewith transmit Bulletin No. 95 of the State ^Mining Bureau, relating to the geology and ore deposits of the Randshurg Quadrangle. This area, consisting of some 243 square miles, has one of the most interesting histories of any mining area in the State of California, and a production of over $37,000,000 in appruxiiiiately thirty years' time. ^lininiT started in 1895 as a gold (•aiii[), ami ()\rr .$15,000,000 was pi'ddurcil. Diii-ing the World War period it came into pi-omiuenee as a producer of tungsten, from wliieli ])et\veen $10,000,000 and $1-,- 000,000 were i)roduced; and in 1917. the famous Hand Silver Mine was discovered and the production in silvei- from 1917 to 1925 is in excess of $10,000,000. This I'.iilletiii is presented to tlie puhlic so that the detailed informa- tion on the geology and ore deposits will aid and assist in a better understanding of this area. Respectfully submitted. Lloyd L. Root, State Mineralogist. INTRODUCTION. Perhaps no miiiiiijr region of the west has had a more varied or fortunate career than has the re^iion surroimdinfr Randsburg. Initially located in 189o as a jrold camp, S(SK.YOU|^J,PO^. r-i;.— :r$^, ..--"-V -^i-ro v ,fO ° \ \ \ y \ \ I \ ^^V"N *Nt* BARBARA! M-^ \ t^ SAN B ERN AROI N -Ir--— Figure 1. Sketch map of California showing location of area covered by this report. certain sedimentary strata in nearby portions of the desert has attracted the attention of palaeontologists. The' folloAving bibliography, though possibly not complete, is believed to include all of the more important papers which have been published concerning the Eandsburg quadrangle or immediately adjacent portions of the desert province. A number of the papers listed are popular or semi-popular in nature. — 15 — BIBLIOGRAPHY, C. Ia Baker— Notes on the Later Ceiiozoic History of the -Mojavo Desert Region. T'niv. of Calif. Bull. Dept. Geol. Vol. G (1011) pp. 333-3S3. riiysiography and Structure of the Western El Paso Range, and the Southern Sierra Nevada. Tniv. of Calif. Bull. Dcpt. Geol. Vol. 7 (1912) pp. 117-142. Walter W. Bradley- loth Rept. of the State Mineralogist, 1915-191G. Calif. State Min. Bureau, pp. 830-839. (1. Chester Brown — 14th Rept. of the State Mineralogist, 1913-1914. Calif. Stale Min. Bureau, pj). 483-4 84. J. A. Carpenter — • Th." Kelly Silver Mine at Randsburg, California. Eng. & ^fin. Jour. Vol. 108 (1!H9) 11]). UM) UVA. (A popular aceount of the mine). Shaft Sinking in the Randsburg District. Pac. Min. News of the Eng. & Min. Jour.-Press Vol. 2 (1923) p. 2(il. (An account of the mining practice). S. II. Dolbear— The Occurrence of Tungsten in the Rand District. Eng. & Min. Jour. Vol. 90 (1910) pp. 904-905. II. W. Fairbanks- Red Rock, Goler and Summit Mining Districts. 12th Ann. Rept. Calif. State Min. Bureau. 1893-94, pp. 456-458. C. II. Fry— The Story of Randsburg. Pac. :\Iin. News of the Eng. & Min. Jour.-l'ress. A'ol. 1 (1922) p. 101. (A popular account of the district). II. E. Gregory and L. F. Noble- Notes on a Geological Traverse from Mojave, California, to the Mouth of the San Juan River, Utah. Ainer. Jour. Sci. A'ol. 5 (1923) pp. 229-239. F. L. Hess— (Jold Mining in tlic Randsburg Quadrangle, California. U. S. G. S. Bull. 430 (1909) pp. 23^7. J. C. Merriam — A Collection of Mammalian Remains from Tertiary Beds in the Mojave Desert. Univ. of Calif. Bull. Dept. Geol. Vol. G (1911) pp. 1G7-1G9. New l'rotohipi)ine Horse from Tertiary Beds (vn the Western Border of the Mojave Desert. Univ. of Calif. P.ull. Dept. Geol. Vol. 7 (1912) pp. 435-441. Tertiary Mammalian Faunas of the Mojave Desert. Univ. of Calif. Bull. Dept. Geol. A'ol. 11 (1920) pp. 438-585. J. N. Nevius — Notes on the Randsburg Tungsten District. Min. & Eng. World. Vol. 45 (191G) pp. 7-S. R. W. I'ack— Reconnaissance of the Bar.stow-Kramer Region. U. S. G. S. Bull. 514 (1914) pp. 141-154. A. B. Parsons — The California Rand Silver Mine. Min. & Sci. I'ress. Vol. 123 (1921) pp. UG7-4j75: 855-859. Vol. 124 (1922) pp. 11-17. (A semi-popular account of the history, geology, and the mining and milling methods). J. E. Spurr — Descriptive Geology of Nevada South of the 40th Parallel and Adjacent Por- tions of California. U. S. G. S. Bull. 208 (1903). Succession and Relation of Lavas in the Great Basin Region. Jour. Geol. Vol. 8 (1900) p. 636. W. II. StOlTUS — • Geology of the Yellow Aster Mine. Eng. & Min. Jour. Vol. 87 (1909) pp. 1277-12S0. W. B. Tucker— 17th Rept. of the State Mineralogist, 1920. Calif. State Min. Bureau, p. 361. — 16 TOPOGRAPHY. The general topographic features of the Randsburg quadrangle are not dissimilar to those which characterize the Mojave Desert in general. Irregular mountain ranges are the rule rather than tlie strikingly parallel ranges of the region to the north and east. The major portion PLATE 6. A. MAIN STREET OF' RANDSBURG LOOKING WEST. B. itA.\J>SBURG AND THE YELLOW ASTER MINE. of the quadrangle lies at rlcvations between 3000 and 4000 feet. The lowest point in the quadrangle, at the west end of the valley to the north of the Rand jNIountains, has an elevation of slightly less than — 17 — 2350 feet, while the hig-liest point, the suiiiinit of Red Mountain, is 5270 feet above sea level. THE RAND MOUNTAINS. Sevel-al distinct units may be roeognizcd in the mountain ranges pre.sent in tlie quadrangle. As will be shown later, they are not merelj^ topojzraphie units ])ut are physiographic and geologic units as well. Among these, one of the more prominent is the Rand Mountains. Rising near the center of the cpiadrangle, these trend to the south- westward, passing out of the quadrangle and continuing on for some fifteen or twenty miles. The Rand Mountains reach their maximum elevation in Government Peak (4755 feet), a mile and a half southwest of Randsburg, gradually decreasing in height both to the northeast and southwest from this point. The range is notictvibly asyniinetrie in form and possesses a straight front. The north slope is quite steep and rugged throughout, a drop in elevation of some 1500 feet occurring within a mile and a half to two miles. The south slope on the contrary, is quite gentle and regular up to the very crest. This is especially true towards the western edge of the quadrangle where the slope is quite even and registers a decrease of 1400 feet in some six miles. This southern slope is quite smooth and practically undissected, so much so that when viewed from a few miles to the south, the presence of a mountain range is not apparent. A few miles to the southwest of the quadrangle a low rounded hill with rather steep slopes, stands out prominently on this otherwise even southern slope of the Rand ]\[ouii1ains. RED MOUNTAIN, Located several miles to the east and southeast of Johannesburg, Red Mountain stands out as a prominent landmark visible for miles over the surrounding desert country. Its upper slopes are extremely steep and rugged. About two-thirds of the way down the mountain, however, the slopes flatten out and become rounded though somewhat deeply dissected by canyons arranged radially around the mountain. When viewed from a distance, the top of Red Alountain is seen to possess a semblance of a flat crest-line. LAVA MOUNTAINS. The Lava Mountains lie to the north and northeast of Red Mountain and topographically form a unit witli it, though the two are separated by a low divide. The Lava ^Mountains possess extremely rugged slopes and a very noticeable' flat summit. This flat summit has locally been cut into b}^ sharp ravines and deep canyons and is nearly everywhere covered with lava boulders of all sizes, so that making ones way across the surface is an arduous task. The northern end of the Lava Moimtains culminates in a straight and very rugged northward-facing escarpment which overlooks a flat alluvium-filled valley. Several miles to the south of this escarpment, the range has been dissected by a series of deep canyons which flow both to the' northeast and to the west from either side of. a divide in the center of the Lava Mountains. 2—37841 — 18 — EL PASO MOUNTAINS. The El Paso Mountains are located across the valley to the north from the Rand ^Mountains. They have a general trend from northeast to southwest. Only a small portion of the total length of the range lies inside the boundaries of the Randsburg quadrangle. The range ter- minates to the northeast in Laurel ^Mountain, just west of Searles. That portion of the range within the Randsburg quadrangle is quite narrow, varying from less than two to nearl}^ five miles across. Both faces of the range are exceedingly steep and rugged and are cut by many small and immature ravines and canyons. The .sunnnit of the range, in con- trast, though locally dissected, appears to po.ssess a mature and rounded topography. The high points of the range, Laurel Mountain and the El Paso Peaks are quite inconspicuous when observed from a distance, merging into the general summit line of the range. SUMMIT RANGE. The region between the El Paso Mountains and' the northern end of the Lava INIountains is occupied by an irregular series of low hills which have been termed collectively the Summit Range. The Summit Range possesses no definite features of its own unless it might be said tliat the very absence of definite characteristics is such a feature. VALLEYS. The most noticeable valley in the region and one which undoubtedly owes its origin to faulting is that which lies between the El Paso Mountains and the Rand Mountains. This valley which is some' five miles broad, heads in a somewhat ill-defined group of alluvium-covered hills near the north-central part of the quadrangle. It extends south- westward some eight miles from the edge of the quadrangle where it culminates in a sink known as Cane Lake at an elevation of about 1900 feet. This valley is deeply filled with alluA'ium on the north side, as is shown b}^ borings which have gone through 600 feet of gravels before ' reaching solid rock. On the south side, however, occasional exposures of the underlying schists project through the alluvium at distances of from one to two miles from the front of the Rand ]\Iountains. A somewhat poorly defined valley which extends well across the quadrangle is found to the south of Atolia and Red Mountain. The alluvium in this valley is ]>robably nowhere of great thickness, though the thickness undoubtedly increases greatly to the eastward where at a distance of from one to two miles from the edge of the quadrangle the vallej^ merges with a sink known as Cuddeback Lake. Another allnvium-fiUed valley is fouiul to the north of the Lava ^lountains. This valley heads in the vicinity of Searles, and trends off to the northeast, emptj-ing into Searles Lake, fifteen miles distant. CLIMATE. The climate of the Randsburg quadrangle is quite arid. The summers are long and hot, with only occasional thunder showers. Parching winds are common. These are sometimes accompanied by sand storms. Daytime temperatures of 100 to 105 degrees occur throughout most of the summer in Randsburg, while in the low or more sheltered por- ' — 19 — tioiis of the quadranple the temperature runs somewhat higher. The ui'jhts are cool and ])leasaiit ho-wever. During the late fall, winter and early spring raw gales occur, some- times accompanied by freezing temperatures. During the first week of January, 1924, temperatures as low as 10 degrees Fahrenheit were recorded, while small ])atches of snow lay on the higher mountains for over a week. Only a few inches of rain falls in the region during the year but this largely comes down during short storms with an intensity approach- ing that of a cloudburst. VEGETATION. because of the general aridity of the climate and of the long hot summers, vegetation is quite scant. What little vegetation doe's occur is largely confined to the lower portions of the' region. Greasewood and sagebrush occur accompanied by a inimber of types of cactus. The only tree otl'ering shade is the yucca or "Joshua tree." This plant was only observed growing in areas underlain by quartz-monzonite or feldspathic sandstones of the Rosamond Series. ElVorts are being made at dry farming in the valley to the east of Ked Jlountain and also north of Laurel Mountain. Some of these efforts appear to be meeting with success. WATER SUPPLY. All of the Avater used for domestic purposes in the towns of the quadrangle and at the various mines is obtained from a number of dtH'i") wells in the area north of Red Mountain. While these wells all start in llie Red JMountain andesite, no data could be ol)tained regard- ing tile rocks penetrated by them. The wat(M' obtained is of excellent <|uality. In times of shortage, water is hi-onglit into the region in lank cars on the Sjinta Fe Railroad and pumped into the mains of the water com]iany. Water foi- mining and milling ]nirposes is obtained from (le(>p wells belonging to the Yellow Aster Mining and Milling Company. Tiiese' wells are located just otf the map and immediately in front of the steej) sonthei-n face of the El I'aso Mountains adjacent to the Garlock Fault. When the wells were first sunk some twenty years ago water was obtained from a depth of -i'^i) feet. Since then pumi)ing has reduced the water level fifty feet, the water now being pumped from a depth of .')(){) feet. Bedroek occurs at a depth of (500 feet. North of the Garlock Fault line water is obtainable in many places at fairly shallow de})ths in the quartz monzonite, though never in great ([uantity. A number of wells have been sunk in the region about Laurel Mountain, fi-om most of which water of good (piality has been obtained. At one well a half mile soutlieast of the Hummer J\line, water of a very poor quality was obtained at a depth of about twenty feet. At the time of the San Francisco earthquake in 1906, the water, level in this well is said to have dropped materially. South of the Garlock Fault the occurrence of water appears to be veiy erratic. At Bedrock Spring in the northeastern part of the Lava Mountains a small flow of water of excellent quality is obtained through- — 20 - out the year from surface lavas. In the valley to the east of Red Mountain, water of not very o;ood quality is obtainable from depths of several hundred feet in p-ravels. (Rosamond Series?) This water is used for irrigation purposes. Water from greater depth is said to be quite poor in quality. The flow of water in. the various mines of the district is somewhat erratic and see'minply is dependent on the presence of open fractures antl other local geologic features. Thus, water stands in the Cima Bimetallic shaft Avhich is only 200 feet deep in Rosamond sandstones. The Big 4 shaft, less than a mile to the southeast has been sunk to a depth of 1100 feet i]i these same sandstones and has made practically no water. The California Rand No. 2 shaft is making rather abundant water on the 14th level, much of it coming from the' shaft vein. The Bray and Bisbee workings are making about 400 gallons per day, the water coming from the 500 and 600 levels, while the Pittsburu- and Mt. Shasta shaft, 1000 feet to the south is making some 4000 gallons of water per day. Water was first met in this shaft when a vein was intersected at a depth of 550 feet from the surface. Water stands in the sump of the Navajo (C4rady No. 2') shaft at a depth of about 1100 feet. It is not known how high this water would rise if it were not kept down by bailing. Water stands in the shaft of the Union No. 1 Mine at Atolia between the 10th and 11th levels or at a depth of about 650 feet below the surface. It is said that no water existed in the Little Butte mine at Randsburg at the 520 level previous to the San Fran- cisco earthquake in 1906. Following the earth(piake water came in and now stands at a depth of 500 feet in the shaft incline. Most of the shafts of the region aside from those mentioned, are dry. GEOLOGY. A wide variety of rock types occur within the area covered by the Randsburg quadrangle. Igneous, sedimentary and metamorphic rocks are all represented. The igneous rocks include plutonic and shallow intrusive bodies as well as surface flows. The sedimentary rocks include l)oth nuirine and continental deposits. The metamorphic rocks which include both gneisses and schists have been derived from both igneous and sedimentary sources. The rocks range in age from Archean to Recent. GENERAL OUTLINE. The Rand ^Mountains are composed of a series of flat-lying schists which have been intruded by a later batholith of (puirtz mon/.onite, and by two much later series of shallow dikes having the composition of diabase and rhyolite-latite respectively. The quartz monzonite out- crops over much of the southern portion of the quadrangle. Just north of the center of the quadrangle a series of gneisses are exposed which are older than the schists of the Rand Mountains. The El Paso IMountains are composed of a series of steeply-dipping Paleozoic marine sediments which have also been intruded by the quartz monzonite. Quartz monzonite also occupies the northern por- tion of the Lava^Iountains and presumably underlies all of the valle.v to the north. Red Mountain, the Lava ^Mountains and a portion of the Summit Range consist of Tertiary sediments of continental origin which are capped by later flows of ande'sitic lavas. — 21 — At several places in the vieiiiity of Siinimit Di^^iiijis, late Tertiary or early Quaternary iiitrusives of basie composition cut the okler rocks. The several valleys interveninji' between the mountain ranges are filled with alluvium which has been derived for the most part within 01- not far distant from tlie (luadrangle. The thickness of this alluvium is (luite variable. Faulting is of importance in the region. One of the major struc- tural lines of California is found in the Garlock Fault which passes across tlic north(M'n part of the (piadrangle. A series of older faults are also known along the north fronts of the Kand .Mountains and the Lava ^Mountains, as well as elsewhere in the' region. Only two fossils have been found within the quadrangle. These were Init poorly preserved. In conse(iuence, all age determinations have been based on seciuence and on petrological similarities with hori- zons of known age in other parts of the desert province. ROCKS OF ARCHEAN AGE. Within the limits of tlie Kandsl)urg quadrangle two groups of rocks are recognized which, owing to their degree of metamorphism are con- sidered to be of Archean age. The older of the.se tw^o groups, which will be referred to here as the Johannesburg gneiss, consists essentially of crystalline limestone and rocks possessing a gneissic texture. The second group for which the name Rand schist is here proposed, is as the name implies, composed predominantly of true schists. JOHANNESBURG GNEISS. Hocks of this groui) outcro]) at oidy two localities within the area mapped. The larger of the two masses is exposed over an area approxi- mately two miles in length by a half mile in width, the' center of which lies two miles due north of the town of Johannesburg. To the north the rocks here exposed, become covered with a mantle of Tertiary sedi- ments and volcanics and Quaternary alluvium, while to the south they appear to be in fault contact vnth the Rand Schist. In the second locality, a small broken xenolith or series of xenoliths are included in quartz monzonite. forming a zone not over one hundred feet wide and prol)al)ly less than one thousand feet long inclusive of breaks. This zone lies just east of and roughly parallel to the main highway one-half mile southeast of Randsburg. Petrology. A variety of rock types are to be found within these two masses. The majority of them are gneisses. In no case do they show a development of a true schistose cleavage. The most characteristic type represented may be termed a horn- blende'-plagioclase gneiss. Rocks of this class show a parallel banding of the light and dark constituents, white bands composed of a fuie granular crj^stalline aggregate whose component minerals are usually .so small that they can not be recognized with a hand lens alternating with equally thick black bands which even with the unaided eye can usually be seen to be composed largely of black hornblende. The thick- ness of these bands varies from a thirty-second of an inch or even less up to more than one-fourth inch. In addition, large and well-formed — 22 — porphyroblasts of black hornblende, usually more or less equant, may be? scattered through the rock mass apparently more or less mdependent of the parallel structure. The banding- is sometimes highly contorted and broken by fractures, giving the effect of a minute 'Appalachian structure.' Parallel cleavage, if present at all, shows only the crudest development. On microscopical examination the white bands are found to be composed of a fine grained, even granular aggregate of andcsine and quartz, with occasional larger jiorphyroblasts of andosiue. The feldspar is usually present in greater quantity than the quartz, while many of the smaller feldspar grains are untwinned, the larger ones commonly show both albite and pericline twinning. The dark bauds consist predominantly of strongly pleochroic, coarsely crystalline, brown hornblende iutergrown with small rectangular plates or aggregates of deep brown biotite. The hornblende crystals are fairly well formed and usually the length is less than three times the diameter. The biotite appears to be orientated at random while the hornblende crystals lie with their long axes in or making only slight angles with the gneissic banding. Rather abundant magnetite in irregular patches, together with a fine-grained dark substance, presumably graphitic in nature, occur scattered through the dark layers. Local areas of indefinite outline which are characterized by abundant closely spaced small colorless crystals of diopside are developed apparently at random in both the white and dark bands. Traces of calcite and small crystals of apatite constitute the only other minerals observed. Microscopically the sharp boundaiw between the white and black bands vanishes, the minerals characteristic of either baud being irregularly iutergrown within a limited zone which is free from any trace of parting. In the field varying types are to be found, from rocks composed largely of material found in the white bands with only a minor develop- ment of the ferromagnesian minerals, through intermediate types con- taining- roughly equal development of the white and black bands, to massive holocrystalline rocks composed as far as can be told with the unaided eve, entirelv of coarsely crystalline black hornblende. This last type may be termed a hornblende gneiss. Microscopically the hornblende gneisses are found to be composed almost wholly of large interlocking crystals of hornblende which appear to be oriented at random or nearly so. The hornblende is strongly pleochroic chiefly in shades of deep reddish brown, but the brown color may grade into shades of light green within the same crystal. The hornblende crystals are cut and surrounded by irregular veinlets and patches of magnetite. Traces of plagioclase are the only remaining mineral. Interbedded with these varying types of gneisses there occur rather important quantities of a massive and quite coarsely crystalline marble. The beds of marble are from a foot to twenty feet in thickness. The rock is almo.st snow white in color and is composed of practically pure calcite, although locally small flakes of black graphite occur scattered through it. There sometimes occurs intercolated with the limestone in thin layers a light-colored gneiss which may be hard to ditt'erentiate from the lime- stone. This gneiss is massive and usually without parallel structure and is composed of scattered porphyroblasts of actinolite, augite and light brown garnet set in a matrix composed of fairly large interlock- ing grains of plagioclase (albite to oligoclase). A more or less pure quartzite is the only other rock type to be observed. It occurs in subordinate amount interbedded with the marbles and gneisses. It may be massive or coarsely banded. The massive variety is composed almost wholly of pure" quartz though — 23 — rarely a feldspar cleavage face is to be seen. A relic clastic texture, composed of rounded medium-sized sand grains can usually be made out with a hand lens. With an increase in amount of the impurities present in the rock a banding is dovelopod, ])eing the result of the formation of thin layers of light brown mica possessing a parallel orientation. Origin. The pJohannesburg gneiss has clearly been derived from a series of rocks which were predominantly if not entirely sedimentary, probably a series of marine sediments. The quartzites are the metamorphosed equivalents of original bedded sandstone, in general quite pure or nearly so. The 2narl)les are similarly the metamorphic product of slightly carbonaceous but otherwise pure limestones. The derivation of the hornbleiide-plagioclase gneiss and the hornblende gneiss is slightly more obscure, but their composition and the fact that they occur interbedded with other i-ocks which were originally limestones and sandstones would suggest that tliey were derived from ferruginous shales. The light-colored actinolite-pyroxene-garnet-plagioclaise gneiss which occurs in places with the crystalline limestone is especially sug- gestive of a metamorphosed shale. It can not be denied, however, that some portion of these rocks, especially the hornblende gneisses, may have been derived from igneous rocks. Structure. The structure of the gneisses exposed to the north of Johannesburg, aside from minor crumpling is quite simple. The beds have a general east and west strike with dips to the north of from forty to sixty degrees, though locally the attitude may differ radically from the figures given. The thickness is unknown, neither the top nor the bottom of the group' being exposed. A thickness of approximately 2500 feet of these gneisses actualty outcrop. THE RAND SCHIST. Rocks of this group compose the bulk of the Rand Mountains. As mapped, they cover an irregular area whose maximum extent is about ten miles from northeast to southwest and over four miles from north- west to southeast. An occasional outcrop projecting through the allu- vium to the northwest of the Rand ^Mountains indicates that the total extent of the Rand Schist is probably much greater than would be' indicated by the mapping. Petrology. Over most of the area occupied by the schists, but especially in that portion lying east of a north and south line passing through Govern- ment Peak, the predominant type of rock represented is a mica-albite' schist. This rock shows a highly developed schistosity. It is usually a dark silvery-gray in color, but where weathered may be stained by yellowish-brown iron oxides set free' by the decomposition of the biotite and other ferromagnesian minerals. As viewed on a schistose cleavage surface the rock appears to be composed largely of micaceous minerals, chiefly a golden-brown biotite, though some are light colored, — 24 — arranged with a parallel orientation. It is these micas which give to the rock its silvery sheen. When vieAved perpendienlar to the cleavage however, the niicaceons minerals are fonnd to be subordinate, the pre- dominant minerals being lenticular grains of feldspar and quartz, everywhere separated by thin micaceous layers. These lenticular grains are so oriented that the short axis of each is perpendicular to the schistosity. The feldspar in general shows no twinning. Microscopically the mica-albite schist is found to be composed predominantly of anhedral grains of albite and quartz, the former being either without twinning or ver.v poorly twinned. The proportions of albite and quartz vary considerably. The allntc tends to form larger grains than the other constituents and characteristically acts as a poikilitic host for many of the other minerals. Graphitic material especially is to be found included within the albites. Though nearly or entirely absent from some crystals, in others the graphite becomes so abundant as to make the all)ite appear almost black. Stubby crystals of apatite in minor quantity are also conuiionly observed as inclusions in the albite. In addition to the albite, por- l)liyroblasts of oligoclase showing well developed albite twinning are occasionally found. Surrounding the albite and quartz grains is a matrix consisting chiefl.v of light colored micas, in general a light brown biotite, possessing a noticeable orientation parallel to the schistosity but locally curving around the grains of albite and quartz. Small quantities of actinolite and green hornl)lende, oriented with their long axes at random in the plane of schistosity, irregular patches of magnetite in minor quantity, and occasional traces of chlorite complete the mineral composition. As will be pointed out later, the hornblende and actinolite ai-e believed to indicate, in part at least, a transition towards other schist types. Next in abundance are numerous amphibole schists. These are only slightly subordinate in quantity to the mica-albite schists. They reach their maximum development along the north face of the Rand jMoun- tains from Government Peak southwest to the border of the map, but commonly occur in some quantity in all portions of tlie formation inter- bedded witli the mica-albite schist. Many of the schists of this group consist of i)ractically pure actin- olite. The actinolite, light green in color, occurs in radial bunches or groups of interlocking crystals, the individual prisms being up to an inch or more in length and so oriented that their long axes lie in or make only a small angle with the*plane of schistosity. A closely related type cotisists largely of fine needles of actinolite set in a matrix of talc, the long axes of both minerals lying in the plane of schistosity. Locally the actinolite may disappear leaving beds of practically \nn'e talc which may be up to two or three feet in thickness. In these schists hornblende may take the place of actinolite althougli the latter mineral is by far the more common. The types just described, the actinolite schist, actinolite-talc schist, talc schist and hornblende schist, are regarded as extreme types in a series, and are to be found only in comparative minor amount. The other extreme of the series referred to is the miea-albite schist. Between these limiting members all gradations are to be found. The composition of the major portion of the amphibole schists of the Rand ]\Iountains would fall i-oughly about midwaj^ between those of the two limiting extremes. An average type of amphibole schist then would be from light to dark green in color and would possess a well-developed schistosity. As viewed on the schistose cleavage, the rock would seem to b composed of a fine felt-like intergrowth of amphiboles (in the vast majority of PLATE 7. .' •- "fr^ Q A. HORNBLEXDE-P I^ A G I O C I. A S E GNEISS. H = hornblende ; Br— fine grained hiotite ; Q = fine g-ranular aggregate of quartz and plagioclase. 30 dia. Ordinary light. r>. BIOTITE-ALBITE SCHIST. A = albite ; B = biotite ; Q = Quartz. 54 dia. X nicols. 37841 — facing p. 24. PLATE S. A. BIOTITE-HORNBLENDE-ALBITE SCHIST. From the ^th level of the California Rand Silver Mine. A ^ albite ; B — biotite ; H = hornblende ; M = magnetite. 30 dia. Ordinary light. B. CHLORITIC-A CTINOIjIT E-ALBITE SCHIST. A r- albite ; C =: chlorite ; prismatic mineral is aetinolite. 50 dia. X nicols. 37S41 — facing p. 2.'.. — 25 — eases aetinolito^ and small quantities of micas. Such a surface possesses a soft sheeu due to the many minute crystal faces in it. When viewed across the cleavao-e, numerous lenticular feldspars are seen, flattened parallel to the schistosity, with the flat felty layers of araphiboles tend- iiipr to curve around them. In some cases the feldspars are few and minute, and the ami)hiholes very small, when the rock louks not unlike a jrreen slate. In the amphibole schists however, there is a strong ten- dency for these lenticular feldspars to become enlari)Ie-gr(M'n in color and slightly pleochroic. The mica is probabl.v closely related to fuchsite and mariposite, and possiI)ly intermediate between them. The indices of refraction were intermediate between those of these two minerals, wliile in different flakes 2V varied from 0° to 40°. Dispersion of the optic axes was very strong (r>v). The character of the mineral was negative. Quartzites and limestones occur throughout the series interbedded witli the mica-albite schist and the amphibole schists. They very in thickness from a foot or possibly less to over ten feet. They form con- tinuous beds, lensing out in all directions, at times only a few hundred feet in length ; at other times traceable for over a quarter of a mile. The quartzites vary greatly in purity. In many cases they are massive and consist essentially of pure quartz in w^hich, with a hand lens, a relic texture consisting of rounded sand grains may be observed. In other cases a coarse parallel structure is developed as the result of thin layers of light brown mica which ])arallel the schistosity of the adjoining rocks. The (juartzites are usually white, pinkish or faint brown in color, although in certain cases they are stained black Avith oxides of manganese. — 26 — The limestones are white to lip'ht sjray in color and appear to be slightly carbonaceons but otherwise fairly pure. Quite contrary to what might be expected, these limestones have been but little effected by recrystallization, their texture being aphanitic as observed with a hand lens. Microscopically they show some development of diopside and plagioelase (oligoelase). Limestones are generally considered to be e"xtremely susceptible to metamorphism, but here, interbedded with rocks which have been complete^ recrystallized, they appear to have been one of the most resistant types. The only remaining type of rock found in the Rand schist is a green- stone-schist, quite different in nature from the previously described types. A number of exposures of these greenstone-schists are to be found along the north flank of the Rand Mountains west of Government Peak. This rock, although completely recrystallized, is practically massive with only traces of a schistose texture. It is dark green in color and moderately tine grained. In the field it was observed that this rock was not bedded as were the other types. It occurs in small irregular and isolated masses which quite commonly cut across the bedding of the other tj^^es of schists. Microscopically, its composition was found to be quite simple, consisting of rounded grains of albite set in a matrix of light green hornblende crystals. The crystals of hornblende Avere oriented in all directions. Many of the smaller crystals were included in the albite grains. No relic texture was to be seen. Origin. The massive nature of the greenstone-schist, its composition, and its habit of cutting across the bedding planes of the other schists all denote that it was originally an intrusive igneous rock, now completely recrys- tallized. The lack of any relic textures and the small extent of the areal exposures would tend to indicate that it was a shallow-type intrusion, with a tine-grained or glassy texture, rather than a deep- seated type. Had it been a coarsely-crystalline igneous rock formed under the influence of high pressures, it seems probable that it Avould have remained stable enough to have preserved traces of its original texture during the later period of metamorphism. Of the remaining rocks, the limestone is clearly a sediment which has been but little changed since its deposition, while the quartzites are just as clearly derived from more or less pure and well-sorted sandstones. The composition of the mica-albite schist, the lack of relic textures in it, and the fact that it occurs inter])edded with rocks originally deposited as sandstones and limestones, all indicate that it was originally a fine-grained clay shale. The nature of these three original sediments, sandstone, shale and limestone, their purity and the character of their bedding as well as their areal extent and their thickness all lead to the belief that they represent a former series of marine sediments. The origin of the amphibole schists appears to offer no difficulties. Pure actinolite schists occur sharply interbedded with normal mica- albite schists, indicating sudden changes of short duration from the normal sedimentary sequence. The beds laid dovn\ during such periods are interbedded as are normal sediments; they show no relic textures but are complete!}' recrystallized and schistose, showing an original — 21 — material highly susceptible to metamorphism. In the majority of cases, howevoi'. the composition of the ainplii1inl(> schists suggests an admix- ture of a basic material ^vith the normal shales. The' only explanation which will fit all the facts is that of basic volcanic tuffs interbedded with or mixed Avith normal sediments. In times of intense volcanic activity, bods of nearly pure tuff would be dojiosited, noAV observed as aetinolite or talc schists. During periods of lesser activity the tutfs would become mixed with shales, either by contemporaneous deposition, or through mixing as the result of the action of water currents in the basin of depositioii. Such mixtures we now find as amphibole-albite schists. And in times of volcanic (piiescence we obtain the normal sedi- ments interbedded with the other types. The concentration of the amphibole schists in the central part of the Hand Mountains, with a general feathering out tOAvard tlie east, together Avith the finding of metamorphosed intrusive igneous rocks again in the central portion of the range Avould suggest possibilities as to the location of the former igneous vents. Structure. Perhaps the most striking feature of the Band schists is the con- cordance throughout betAA'een the schistosity and the original bedding inherent in the roeks from which the schists were derived. Where the schists consist solely of mica-albite schists or of amphibole schists the original bedding is difficult of perception. But Avherever a bed of limestone or of quartzite appears, such beds or contacts are alAA^ays parallel to the schistosity. The schists Avhile usually almost flat-lying are not stricth' horizontal but show broad open folds and a regional dip over most of the area at low angles to the south or southeast. The dips on the flanlvs of the folds seldom exceed fifteen degrees Avhile the regional dip is much less. The formation of these open folds appears to be entirely subsequent to the metamorphism. In the region to the north of Johannesburg the beds are in general flat-lying, but approaching the area occupied by the Johannesburg gneiss a rather sharp fold appears, so that on the contact with the gneiss the beds posses.s a northAvard dip of about sixty degrees. Near the contacts Avith the Atolia quartz monzonite intrusive, especially those Avdth the small stock-like mass lying just south of Rands- burg, the schists may be found to stand in any attitude. Local blocks may cA^en be found to stand vertical. Tliis is probably the result of local faulting in connection Avith the batliolithic invasion. Due to faulting in the Rand Mountains no exact statement can be made concerning the total thickness of the Rand schists. The north front of the mountains shows schist exposures through a A^ertical dis- tance of over 1800 feet. AlloAving for a low southerly dip, the total thickness exposed Avould be greater than this figure, providing that no alloAvanee be made for duplication of strata through faulting. A number of hurried trips across the range at various points has con- vinced the Avriter that no great duplication of beds occurs, so that it seems probable that between 1500 and 2000 feet of strata are actually exposed, the true value probably being closest to the higher figure. — 28 — Relations Between the Johannesburg Gneiss and the Rand Schist. The relationship existing l)et\veen the Johannesburg gneiss and the Rand sehist is believed, from petrologieal evidence, to be one of uncon- formity, the Johannesburg gneiss being the older and separated from the Rand schist by an intervening period of metamorphism. The exact relations existing between the two groups can not be directly determined in the field, but they are l)elieved to be in fault contact. As mapped, and as seen in the field, the Johannesburg gneiss appears to overlie the Rand schist, the bedding in the two groups having about the same attitudes. The contact is everywhere covered with talus and alluvium and can nowhere be located more accurately than within from twenty to fifty feet. Within this zone the rocks change abruptly from schists to gneisses so that no possibility of a gradation exists. The mapping indicates that the contact dips to the north at an angle of about sixty degrees. It is inconceivable that the series of gneisses can actually be younger and actually overlie the schLsts. The gneisses represent rock tj'pes which have suffered from the most intense metamorphic processes/ while the schists have suffered from much less intensive action. - The limestones of the Johannesburg group, as has been pointed out, are coarsely cry.stalline. Their bituminous matter has been changed to graphite. Whereas the limestones of the Rand series are micro- crystalline and show only a slight development of new minerals. The position of the xenolith of gneiss im])edded in quartz monzonite southeast of Randsburg is such that it c-ould be easily explained a^ having been lifted up from lower horizons by an intrusive magma, but its position would be very difficult to explain on the hypothesis of its haying sunk from a higher elevation. The structural conditions are such that the possibility of an over- turned fold in the region north of Johannesburg can also be ruled out. The only remaining po.ssibility is tliat the contact represents a .strong reverse fault, possibly a bedding fault, the older Johannesburg gneiss overriding the younger Rand schist. The obscurity of the contact would be in agreement with tliis view. Supporting the possibility also are evidences of faulting observed close to the line of the contact. In three shallow prospect shafts, sunk in the schists and located from twenty to two hundred feet from the contact, strong faults were observed, having about the same strike as the contact and dipping steeply to the north. In the shaft closest to the contact a brecciated fault zone six feet wide is exposed, possessing strong walls and dipping sixty degrees to the north. Conditions of Metamorphism of the Johannesburg Gneiss. The massive gneissic textures developed, together with the utter absence? of any evidence of pneumatolysis, indicate that the recrystalli- zation of these rocks occurred chiefly as the result of ordinary dynamic metamorphism, whereby intense pressures which approached a hydro- static condition, probably aided by high temperatures and solutions, were the active agents. The fact that gneisses were formed instead of ' According to Grubenmann"s sclieme of classification they would be formed in the upper part of the deep or "kata" zone of metamorphism. Dr. U. Grubenmann — Die Kristallinen Schiefer. = They would be formed in the upper part of Grubenmann's middle or "meso" zone of metamorphism. — 29 — schists is suggestive of the intensity of the metamorphie action. The pffects of lateral stresses are shown l)y tlic' contoi-tions sometimes seen in the gneissic banding. Conditions of Metamorphism of the Rand Schist. The most noticeable feature of the Raiul schist is tlie concordance which exists everywhere between the flat-lving schistositv and the bedding. The slight jimount of folding which has effected these schists, occniM'ed subse(|nent to tiie metam()r])hism. Ilenee the schistositv must have been developed hori/ontally and the pressures involved were vertical. The effects of lateral compression are strictly absent. Effects of j)iieumatolysis are likewise absent except in a few places along the contacts Avith the (puii'tz mouzonite where locally the eft'ects of contact metamorphism are superimposed on the older schistosity. The meta- morphism of the Rand .schist can in no sen.se be attributed to the batho- lithic invasion of (piai'tz monzonite, for angular fragments of schist of various sizes are commonly to be found as inclusions in tlie ([uartz monzonite. with their schistosity oriented at I'andom. The oidy process of recrystallization which will account for the observed facts is that of static metamorphism whereby the original I'ocks wei'e deei^ly buried beneath a tremendous load of other sediments, the resulting intense vertical forces, aided by increased temperatures in depth and by solutions trapped in the sediments, resulting in the metamorphism. This ]n'oeess of metamorphism would make necessary not only an unconformity between the Johannesburg gneiss and the Rand schist, but an hitervening period of metamorphism as well, for the gneisses appear to have suffered from lateral stresses, the schists solely fi-om vetrical ones. During the earlier ]')eriod, the metamorphism of the gneisses was largely completed, wiiile during the secontl period of metamorphism the recrystallization of the Rand schist occurred, the underlying gneisses being further effected at the same time. Age of the Johannesburg Gneiss and the Rand Schist. The age of these two groups of rocks is believed to be Archean. The age can be inferred solely from a consideration of their petrology, and comparison with formations of nearby regions. In the El Paso Mountains across the valley to the north, marine sediments believed to be Paleozoic in age, though intruded by quartz monzonite, show practically no effects of metamorphism. Still further north, in the Inyo Mountains, marine sediments ranging in age back to the Pre-Cambrian are similarly l)ut little altered.^ In the desert ranges to the north and northeast, namely the Argus, Slate, Panamint and Funeral ranges, sedimentary strata ranging from Cambrian to Permian in age are practically nnmetamorphosed.- In the region to the south, rocks believed to be of Paleozoic age, while more or less effected by batholithic invasion, have not been subjected 'A. Knopf and E. Kirk. A Geologic Reconnaissance of the Inyo Range and tlie Ea.'^tern Slope of the Sierra Nevada, California. U. S. G. S. Prof. Paper 110. -S. H. Ball. A Geologic Reconnaissance of Southwestern Nevada and Eastern California. U. S. G. S. Bull. 308. (1907.) J. E. Spurr. Descriptive Geology of Nevada South of the Fortieth Parallel and Adjacent Portions of California. U. S. G. S. Bull. 208. (1903.) — 30 — to the same intensity of action suffered by the rocks of the Rand Moun- tains.^ Further east, in the Bristol Range, sediments of Lower and PLATE 9. « A. OUTCROP OF RAND SCHIST NORTHWEST OP RANDSBURG. The schls- tosity is flat-lying ami conforms to the bedding. B. SOUTHERN FRONT OF THE EL PASO MOUNTAINS. The Garlock Fault lies behind the low ridge in the middle distance. Middle Cambrian, and Carboniferous age are known which are but little altered. = Still further east, in the Grand Canyon section, sedi- mentary strata of Algoukiau age- are not metamorphosed.^ 'R. W. Pack, Reconnaissance of the Barstow-Kramer Region. U. S. G. S. Bull. 514. pp. 141-154. (1914.) -C. W. Clark, Lower and Middle Cambrian Formations of the Mojave Desert. Bull. Dept. of Geol. Univ. of Calif. Vol. 13 (1921), pp. 1-7. ^L. F. Noble. The Shinumo Quadrangle, Grand Canvon District, Arizona. U. S. G. S. Bull. 549. (1914.) I — 31 — Since nowhere within this province or in adjacent regions have rocks known to be yonnger than the Archean in age suffered the same inten- sity of metamorphism as have the Johannesburg gneiss and the Rand schist, these groups must, at least until further evidence is forthcoming, be considered to he Archean in age. UNDIFFERENTIATED STRATA OF PALEOZOIC AGE. A series of marine sediments believed to be Paleozoic in age outcrop over a large portioii of the El Paso Mountains in the northwestern corner of the quadrangle. The rocks of this series pass beneath later sediments and alluvium to the northwest and are cut off by later intru- sive quartz monzonite on the northeast. On the south they abutt against a major fault line, tlie Garloek Fault. South of this fault, due to a heavy cover of alluvium, no exposures occur except near the fault in the central portion of the quadrangle where a number of small out- crops project up thi'ough the alluvium. Lithology and Structure. Tlie rocks composing this series include marine limestones, cherts, clay shales, sandstones and conglomerates in a bedded series of strata. Limestones are the dominant rock type represented, being closely folloAved in quantity by shales. In consequence of the limited time available for field work and due also to the absence of fossilif erous horizons, it was not found practicable to subdivide this series into definite formations. Although no exact statement can be made, it seems probable that at least three and possi- bly more distinct groups of rocks are present. The structure of the beds exposed in the El Paso Mountains to the north of the Garloek Fault is broadly simple, although in detail it may be highly complex. The major feature is a steeply-folded syneline whose axis roughly parallels the trend of the range from southwest to northeast. The axis plunges at a steep angle to the northeast. Only part of a fold is represented, the northeastern portion having been destroyed bj' the batholithic invasion of quartz monzonite. Ile'nce a section along the crest of the range from southwest to northeast shows a simple series of strata having a common dip to the northeast, the dips varying from about forty degrees near the western ])order of the map to vertical near the (juartz monzonite contact. The beds outcropping along the front of the range north and northwest from Rand Station, dip to the north and northwest at angles of from ten to thirty degrees. In detail however, the structure appears much more complicated due to minor flexures on the flanks of the major fold. Within the range faulting appears to be decidedly subordinate to folding. South of the fault the beds dip at angles of twelve to fifteen degrees to the north and northwest, an attitude similar to the attitude of the beds on the front of the range just north of the fault. The lowest bed exposed in the quadrangle is a conglomerate which outcrops on the south front of the El Paso Range practically at the west border of the map. Its thickness is unknown. The bed has been tightly squeezed and shows slight effects of recrystallization. No bed- ding is apparent. The component pebbles are chiefly angular and con- sist predominantly if not entirely of schist. They vary from a half to — 32 — about one and a half inches in diameter and are set in a matrix which appears to consist of sand and small schist flakes. The majority of the schist fragments are dark green in color and appear to be fine-grained amphibole schists. But the rock is so altered that these fragments can not be correlated with any degree of certainty with the schists of the Rand Mountains. Approximately the lower fourth of the series exclusive of the con- glomerate at the base, consists of claj'' shale with interbedded cherts, limestones and sandstones. The shale is light gray in color and thinly fissile so that it breaks out and covers the surface of the ground with small thin flakes. These flakes show a silvery sheen on the surface apparently due to a development of sericite. These shales are quite soft. The sandstones are most pronounced well down in the section, a massive bed immediately overlying the conglomerate. These sand- stones are for the most part thiek-l)edded and highly quartzose. The cherts and limestones usually occur in beds from twenty to fifty feet in thickness interstratified with the shales and sandstones. The lime- stones are characteristically in thin bands, one to two inches thick, Avith alternate layers of black chert less than a half-inch in thickness. The limestones which are light gray in color, weather chiefly by solution so that thin layers of the black chert are left as prominent ridges on the weathered surface. Both the limestone and the chert bands are cut by numerous irregular veinlets of white caleite. The cherts are black when fresh, weathering to a brownish-black. In addition to occurring inter- banded with the limestones, the cherts occur as thick beds interstratified with limestone, sandstone or shale. The central portion of the series shows a strong development of hard siliceous shales. These shales are strongly banded, the alternate bands being black and either white or cream colored. The individual bands are usually fairly thin, grading downward to the thickness of a sheet of paper. On the weathered surface the light colored bands are fre- quently colored pinkish, reddish or yellowish. These silicious shales occur interbedded with beds composed of alternating bands of gray limestone and black chert, or with beds of pure quartz sandstone. The limestone-chert beds are probably the most characteristic phase of this portion of the series. The upper quarter of the series is composed chiefly of cream-colored silicious limestones. These limestones are quite hard and under the microscope, show some development of pyroxenes and plagioclases. This partial recrystallization is probably referable to the intrusion of quartz monzonite. The limestones are usually thick bedded and mas- sive although locally alternations occur with thin liands of black chert. The beds exposed south of the Garlock Fault are composed chiefly of black chert, either with or without alternating bands of gray lime- stone. These beds most strongly resemble beds of the central portion of the series to the north of the fault. The total thickness of strata represented in the series is believed to be between 15,000 and 16,000 feet. While these figures make no allowance for repetition of strata by faulting, it is not ])elieved that any important repetition has occurred, no prominent faults having been observed within the area occupied by this series. — 33 — Mode of Accumulation. Such a thick and well-graded series of sediments as those here repre- sented could hardly have accumulated under any but marine conditions. Excludinear to jwssess a normal granitic texture. At numei'ous places in this area intrusive contact zones occur between the Atolia (piartz monzonite and the Rand schist, in Avhich abundant flat angular fragments of the schist are scattered through the granitic rock. Such zones with their included fragments are believed to be the forebears of the orbicular rocks of the region, the orbieles being only later stages of the inclusions whose shapes have been modified by resorbtion and whose composition has been modified by diffusion. The last stage of the process would be the complete assimilation of the orbieles and their total disappearance in the granitic matrix. In the field the writer observed all gradations from included angular fragments of schists possessing their original composition and texture, through angular fragments showing only a trace of a schistose texture and apparently modified in composition, to rounded orbieles with granitic texture and the composition of a quartz diorite and finally to places where only the faintest concentration of dark minerals remained to suggest the former presence of an orbicle. One highly angular fragment found isolated in the quartz monzonite only a short distance from the contact west of the highway just southeast of Randsburg showed sharp edges and appeared to be bounded in part by what had been a schistose PLATE 11. A. THIN SECTION OE ORBICLE. P = plagioclase ; B = biotite ; H = hornblende. 30 dia. Ordinary light. B. QUARTZ-EPIDOTE ROCK. From veinlet in Quartz monzonite. Br— epidote; Q = quartz. 50 dia. Ordinary light. 37841 — facing p. 36. Ii — 37 — cleavage. Traces of the former schistosity could ho made out iu the mass of the rock. Under the microscope the composition and texture were found to be quite similar to those of the rounded orbicles, differing only in the feldspar being oligo- clase instead of andesine, in the presence; of alnmdant green hornblende and tlie absence of augite. The texture was fine-grained, granitic. Relation to Adjacent Formations. The Atolia quartz iiionzonite is intrusive into tlie Rand schist and the nnditferentiated l^deozoie series of the El Paso \M()nntains. This batholithic invasion liad no ctTecl in cnntrollin^- or modifyino- the types of rocks found in the fJohannesl)urg' gneiss or tlie Rand schist groups, except near the contact, and its Jieneral effects in modifying the Paleo- zoic series appear to have been very slight. The contact between the Rand schist and the southern area of quartz monzonite is believed to be in ])art a fault contact. Locally, however, this contact is certainly one of intrusion. One-half mile west of the Rand C'ontact Mine, near the ])ower line, the limestone adjacent to the contact shows a development of a deep-green mineral, apparently epidote. Still further west on this contact, in the southwestern portion of the Hand ^lountains, the (piartz monzonite cuts across to the north side of the mountains. The mai)ping in this portion of the area clearly brings out the intrusive nature of the (jnartz monzonite, while at a number of places contact minerals are found develo])ed in the adjoining rocks. The schists are not much affected adjacent to the intrusive, hardening and the loss of the schistose cleavage being the chief altera- tions observed. The limestones, however, commoiily show a megascopic development of large crystals of dark-colored hornblende and reddish- brown garnet. In the region of the El Paso Peaks, in the northern part of the quad- rangle, strong contact zones are developed. The El Paso Peaks them- selves are capped with a heavy dark greenish-lilack rock which is composed chiefly of dark-green epidote and brown garnet, witli lesser (piantities of coarsely crystalline calcite, quartz and lead-gray metallic plates of specular hematite which at times ai)pear ruby-red by trans- mitted light. This I'ock is coarsely crystalline and quite massive. Occasional masses consisting almost entirely of epidote are found, the epidote occurring either as porous aggregates of ititerlocking crystals or as large compact radiating crystalline masses several inches in length. Further south, coarsely crystalline rocks, composed of augite, horn- blende, calcite, pyrite and more or less epidote, were observed. Locally, garnet was developed. The workings of the Hummer Mine are princi- pally in rocks of this character. Along most of this contact, however, the Paleozoic limestones offer little or no visual evidence that they have suffered from an igneous intrusion. Contact zones are well developed a half-mile southeast of Randsburg, both along the contact and in the' enclosed xenoliths (possibly Johannes- burg gneiss xenoliths in part) within the quartz monzonite. Here the limestones have been altered to rocks composed of dark reddish-brown garnets and dark green diopside. These rocks are usually massive, but in a few exposures are coarsely banded, the banding resulting from the development of alternate layers of green diopside and brown — 38 — srariiet. Microscopically abundant plagioclase (labradorite) and some calcitef are fonnd to occui- intergroAvn Avith the ^rarnet and diopside. T^nlike the other contact rocks observed, the garnets of these banded varieties are well formed crystals about an eighth of an inch in diameter. These rocks weather chiefly by solution of all the minerals except the garnets, so that on a weathered surface the garnets stand out as small spheroidal bodies, many of which show crystal faces. After Effects of the Batholithic Invasion. Aplite and pegmatite dikes are occasionally met with, but nowhere within the confines of the quadrangle do they show a strong develop- ment. Just outside of the confines of the quadrangle, in the prominent hill north of the Kandsburg-Mojave road, pegmatite dikes become locally abundant. They are characteristically developed as coarsely crystalline graphic intergrowths of cream to flesh-colored orthoclase and quartz, with sporadic flakes of dark-colored mica. Scattered through the rock and quite commonly imbedded in the orthoclase crystals are abundant minute crystals of wine-colored garnet. Most of the dikes observed were less than three feet in width. Epidote is strongly developed as a late magmatic mineral in all portions of the quartz monzonite terraine, possibly excepting the small stock-like mass to the south of Randsburg. It is most characteristically developed as a fine-grained intergrowth with quartz, occurring in small dikes or veins cutting the quartz monzonite'. These dikes or veins vary in thickness from about two inches down to a small fraction of an inch. In many parts of the area heavy, finely crystalline, light gi'cen-colored fragments derived from these veins or dikes can be picked up on the surface. Epidote was also observed in some of the granitic rocks, both from the northern and southern portions of the quadrangle, associated with horiiblende and biotite and apparently a primary mineral. The occurrence of epidote in the El Paso Peaks contact zone has already been described. Large bunches of white 'bull' quartz up to fifty feet or more in diameter are commonly met Avith in the field, either cutting the quartz monzonite. or any of the older formations, being especialh' common in the Rand schist. These masses probably represent faulted segments of veins in part. While usually quite barren of any mineral other than the massive glassy quartz, locally these veins carry abundant coarsely crystalline and commonly radiating light-green epidotes, and more rarely large crystals of pink orthoclase and flakes of either light or dark micas. These veins Avere formed under conditions of high temperature and high pressure,^ possibly in part approaching conditions under which the pegmatites form, and hence are probably closely related to the invasion of the quartz monzonite. These veins are described in this portion of the paper because they are not known to possess any economic value. Correlation of the Three Areas of Quartz Monzonite. The correlation of the three areas of granitic rocks as being parts of the same batholithic invasion is made for the following reasons. (1) 'According to Lindgren's classification, tliese veins would be termed hypothermal. W. Lindgren. A Suggestion for the Terminology of Certain Mineral Deposits. Econ. Geol., Vol. 17. (1921), pp. 292-294. — 39 — The areas are similar in i^eneral eomposition though locally somewhat variable iu the vicinity of the intrusive contacts. (2) At least two of the areas, those in the northern and southern parts of the quadrangle, possess similar peculiarities — namely, the strong development of late magmatic epidote. Epidote is also found in the quartz veins believed to be associated with and located not far distant from the central area of granitic rocks. (3) The granitic rocks are intrusive into all rocks in the quadrangle of Paleozoic age or older. The southern area of quartz monzonite continues to the southea.st for fifteen miles where in the vicinity of the Silver Dome Mine it is found to intrude a series of sedimentary strata not unlike those of the El Paso Mountains and pre- sumably of Paleozoic as^e. (4) There is an entire absence of any evidence which would even suggest the presence in this quadrangle of an older batholithic invasion. The three areas of quartz monzonite as mapped are believed to be actually connected with each other beneath the later cover of Tertiary sediments and lavas present in the eastern portion of the quadrangle. The contact between the Rand schist and the southern area of quartz monzonite was traced approximately bj' means of prospect shafts for a distance of nearly a mile beyond the place where it passes beneath the cover of alluvium. The contact maintains a course of about N. 25° E. The Chicken Hawk shaft and the Big Six shaft lie to the southeast of the quartz monzonite-schist contact, both having etitered the quartz monzonite at moderate depths. The Navajo (Grady No. 1) shaft is in the schist. The last known point on the' contact between the schist and the southern part of the small central area of quartz monzonite is in the crosscut from the Fox Lease shaft where the contact was cut at a distance of 250 feet southeast of the shaft. Thus, at the last known points the two contacts are less than a mile apart and are approaching each other at an angle of about sixty degrees. Similarly the east end of the northern contact of the central area of quartz monzonite is swinging as though to join with the quartz mon- zonite of the northern part of the Lava Mountains, six miles distant. Figure 3 shoAvs the probable extension of the quartz monzonite beneath the cover of alluvium. Tertiary sediments and volcanics. Mechanics of Intrusion. Evidence that the batholithic invasion occurred both by assimilation of the invaded rocks and by injection was found in studying the Atolia quartz monzonite. The former process was the major one. The* evidence of injection is chiefly found in the broken and distorted condition of portions of the Rand schist surrounding the intrusive in the central part of the quadrangle. This portion of the intrusive repre- sents a cupola of the main batholith, and the injection which is a feature of this mass is believed to have had only a local importance. Evidence of assimilation is more widespread. One of the major lines of evidence, the orbicular diorites, has already been described. A second line of evidence is found in the observation that the amount of limestones found on the contacts with the quartz monzonite is wholly out of proportion to the quantity of limestones present in the adjacent rocks. And wherever a limestone and some other type of rock occur together in contact with the quartz monzonite, the limestone will usually — 40 — project out into the quartz monzonite far beyond the other rock. In addition it may be pointed out that the inclusions found in the quartz monzonite consist of limestone much more commonly than would be expected. i III Figure 3. Shaded area represents portion of quadrangle in which the Atolia quartz monzonite would outcrop if the Tertiary and later deposits were removed. Limestones compose only a small portion of the Rand schi.st, certainly less than five per cent of the whole and probal)ly only two or three per cent. And yet limestones occur along from one-fourth to one-third of the exposed portion of the contact with the southern area of quartz monzonite. i — 41 — In the vicinity of Randsbiirg three prominent reentrants of the limestones into the' quartz raonzonite are brought out in the mapping:. All three show on the geological map, one on the south contact about PLATE 12. A. ORBICUL.VR QU.\.RTZ DIORITE. Exposed three-fourths of a mile southeast of Randsburg. ■•.-^!fe»^ %f*'/ J^'S B. WB.\THERING IN THE ROSAMOND SERIES. Exposure north of Bedrock Spring. ;i mile fi-om Kandsburg and just west of the highway. The second just out of l\andsl)urg on the north contact a quarter of a mile east of the highway ; the third on the same contact a quarter of a mile further east. * — 42 — This superabundance of the limestones along the contacts and as inclusions, and the tondencj^ of the limestones to project into the quartz nionzonite are believed to be due to the limestone being less susceptible to assimilation than the other rock types present. Age. Within the quadrangle the quartz monzonite is known to be younger than the series of Paleozoic sediments which may be Carboniferous in part. And they unconformably underlie a series of sediments which date in age back to the middle Miocene. To the north and northwest, however, more or less continuous expos- ures of plutonic rocks occur which finally pass into the quartz mon- zonites of the Inyo Mountains and the quartz monzonites, granites and related rocks of the Sierra Nevada. The plutonic rocks of both these areas are known to be late Jurassic or possibly early Cretaceous^ in age. The nearest exposures in the Sierra Nevada are less than fifteen miles distant from the region of the El Paso Peaks. There can be little doubt but that the Atolia quartz monzonite is the correlative of the plutonic rocks of the Sierra Nevada and of the Inyo Mountains and is itself probably late Jurassic in age. THE ROSAMOND SERIES. The Rosamond series, continental in origin, and consisting chiefly of stratified conglomerates, feldspathic sandstones and clays, either out- crops or underlies later deposits over about one-third of the area of the quadrangle, chiefly in the east-central portion. The largest exposures occur about the base of Red Mountain and in the north-central portion of the Lava Mountains. Beds of the Rosa- mond series presumably underlie the whole area covered by the lavas of Red Mountain and the Lava Mountains. Passing out of the area mapped, northeast from the exposures in the Lava Mountains, large areas of Rosamond strata are found which are not covered by later lava flows. Other prominent occurrences are found in the vicinity of Summit Diggings and in the region half-way between Summit Diggings and Johannesburg, with smaller exposures occurring on the summit of the El Paso IMountains just south of the Hummer ]Mine and in the valley on the northwest side of the El Paso Mountains. This valley is pre- sumably underlain by Rosamond beds which continue on and constitute the main mass of Black ]\Iountain. five miles to the west. The area covered by alluvium south and southeast of Summit Diggings is likewise considered to be largely underlain by Rosamond strata of which occasional small outcrops are found. Evidence of former extensions of the series is found in the occurrence of two small blocks of feldspathic sandstones lithologically similar to sandstones occurring in the Rosamond elsewhere in the quadrangle. One of these blocks was found faulted in the schist a mile southeast of Randsburg just west of the forks in the highway. The second occur- rence which was also a fault block, was found near the tongue of quartz monzonite in the west end of the Rand Mountains. Neither occurrence was large enough to map. 'A. Knopf and E. Kirk. U. S. G. S. Prof. Paper 110. I 1 — 43 — These two occurrences, in conjunction with the small area on the summit of the El Paso Mountains would sufrprest that at one time the Rosamond series may have covered much of the quadrangle. Lithology. As has already been indicated, the Rosamond series consists largely of stratified deposits of conglomerates, feldspathic sandstones and clays, with some admixed volcanic material, most of which have been but poorly consolidated. Lesser quantities of other types of rocks occur. The sandstones are by far the most abundant as well as the most characteristic beds represented. They vary greatly in appearance, size of material and composition of the individual grains, but possess the common feature of being composed of poorly Aveathered materials. Practically all the minerals found in the quartz monzonite and schists of the region are represented in these sandstones. Quartz as might be expected is the most abundant constituent but may be approached in quantity by the feldspars. The feldspars are white or faint pink in color and quite commonly show fresh lustrous cleavage faces. They are most conmionly plagioclases. Dark colored flakes of maea are usually moderately abundant while occasional grains of amphiboles or pyroxenes are observed. Many of the larger grains are composed of rock of the various types found in this region. The sands are usually but poorly sorted, fine sands, coarse sands and pebbles usually occurring together in the same bed. In shape the individual grains are characteristically subangular to highly angular although in many cases well-rounded grains may be seen. Over most of the area the beds are porous and show little induration and practically no cementation, so that the m/aterial may be crumbled between one's fingers. Locally however the beds have been highly silicified by hydrothermal agencies. (See page 50.) The sandstones of different horizons differ greatly in color. The most conmion colors represented include greenish-gray, cream, buff and deep red or maroon. These colors are independent of weathering on the present surface since mJatorial removed from depths of several hundred feet in shafts in tlie vicinity of Osdiek is highly colored. Locally stray pebbles and boulders may be found in any of the sandstones. By an increased admixture of these coarser phases the sandstones pass over into conglomeritic sandstones and finally into sandy conglomerates. In no case observed did the coarser portion of the conglomerate exceed fifty per cent of the whole, the balance being sands of varying degrees of fineness. The size of material present in these sandy conglomerates varies greatly from place to place. Pebbles and boulders of all sizes up to two feet in diameter may be found. Nor does such coarse material appear to be confined to any particular horizon. In the saddle just west of the hill in the center of the map which is mapped as rhyolite, a heavy sandy conglomerate in the basal portion of the Rosamond is exposed which is composed of boulders of quartz monzonite up to and possibly exceeding two feet in diameter. Immediately underlying the lava flows on the south end of Red Mountain and at a horizon apparently — 44 — well up in the Rosamond series, a conglomeritic sandstone occurs con- taining boulders over a foot in diameter. In tlie lower portion of the series the ])oulders and pebbles observed consist solely of granitic rocks and of schists. The schists of these fragments appear to be identical in character with those now exposed in the Rand ^Mountains. In the upper horizons boulders and pebbles of granitic rocks and schist fragments are equally common but in addition numerous pebbles and boulders of light-colored porphyritie rhyolite and dark-colored diabase appear. AVhile some of these pebbles- are moderately well rounded, most of them are distinctly angular with rounded edges. These detrital rhyolite and diabase fragments appear to be identical in appearance, composition and texture wdth the rhyo- lites and diabases known to be intrusive into the lo\Aier portion of the Rosamond series. The absence of these fragments in the lower horizons of the Rosamond makes it certain that they have, in the vicinity of Red Mountain, been derived from the erosion of the intrusives in question. In addition, at a number of places well up in the series subangular pebbles of a reddish-brown porphyritie rock with a glassy groundmass were found which strongly resembled types found in the Red ^Mountain andesites. At various horizons, beds of clay, which may be in part admixed with tuffaceous material occur interstratified with the sandstones. These beds are quite subordinate in amount as compared with the sandstones. They vary in color almost as widely as do the sandstones, grayish-green, cream, ])uff and pure white beintf common colors. Tlie clays proper are quite fine in grain but usually a few rounded sand grains or occasionally even a larger fragment may be present. At times traces of stratifica- tion may become apparent through these sand grains being arranged in layers. These clays are usually (|uite soft and on the surface, porous. Locally however as wath the sandstones they have been highly silicified by hydrothermal agencies. In these cases the resulting rock is not unlike a light colored acid lava in appearance. They can usually be distinguished however by the presence of occasional rounded sand grains. The tufFaceous phases are ([uite similar in general appearance and coloi- to the j)ure clays, hut can he icUMitified ]\v tlie presence of scattered shai'i)-edged angular fragments of dark-colored lavas. The tuffaceous horizons observed by the ^^^iter were all well towards the top of the series. The clay beds are quite commonly clmracterized by scattered flakes and fi-agments of gy psiun. Or the gypsum may occasionally be developed in a thin bed or seam. Locally round limy concretions up to several inches in diameter occur, as on the lower slope of Red Mountain, in the canyon east of the Big Four sliaft, while at times thin seams or beds of crystalline ealcite may be found. In the central part of the Lava Mountains erosion has cut through the base of the lavas exposing the underlying Rosamond beds in a small moon-shaped area. In this locality a bed of gray limestone some two or three feet in thickness is exposed. In the northern part of the Lava ]\Iountains. about two miles from the eastern border of the nuip, a single bed of light-brown translucent — 45 — chert was observed interbedded in sandstones. Tlie bed was some three feet in thickness and sliowod a well-developed banding. Although depositional banding may commonly be observed, especially in the sandstones*, many of the beds, including practically all the days and heavier conglomerates, are quite massive and offer little evidence of the structure. Weathering. The general softness of the beds makes them easily eroded by the intermittent streams of the region. The action of the streams has been chiefly downeutting with the resulting development of canyons bordered by steep cliffs which have been more or less modified in form by rain wa.sh and wind action. Hence in many parts of the quadrangle excellent exposures of Rosamond strata are obtainable. Such exposures are especially well developed in the northern part of the Lava Moun- tains. Where stream action has been less intensive the softness of the beds has resulted in but poorly developed exposures. The lower slopes of Red ^Mountain show such poor exposures, but higher up the slope, immediately below the contact with the lavas, sapping action has resulted in many good outcrops which are scattered on all sides of the mountain. This same sapping action aided by the steepness of the upper slopes of the mountain has resulted in covering much of the lower slopes with a coating of lava boidders whicli in places are so thick as to sti'ongly resemble lavas in place. Structure and Thickness. The poor exposures over much of the area where the Rosamond beds outcrop, aided by the lava talus which in many places covers the sur- face and by the massive nature of many of the beds of the Rosamond series, makes any exact statement of the structure impossible. It is known that the strata are rather highly folded into somewhat complex folds and probably more or less faulted. In the northern part of the Lava ^Mountains the beds overlie and dip away from the quartz monzonite at angles of about twenty-five degrees. In a short distance they pass into comiplex folds in which the beds may possess any attitude from horizontal to vertical. A mile west of Summit Diggings a small overturned synclinal fold was observed in which the strata passed from a dip of twenty degrees north to an overturned dip of about eighty-five degrees to the north \nthin a distance of less than 200 feet. In the vicinity of Red ^fountain the structure appears to be equally complex. In the region from a mile to two miles east and southeast of Johannesburg the strata dip to the southwest at an angle of twenty degrees. A mile further south they lie flat. A mile south of Johannes- burg the strata dip to the northeast at angles of from ten to twenty i degrees. Seemingly there is a closed synclinal basin developed here which centers about two miles southeast of Johannesburg. At the time of tliis \^Titing the Big Four shaft which appears to lie near the center of this depression had reached a depth of 1100 feet, all in beds of the Rosamond series. At the bottom of the shaft what appeared to be I lines of stratification showed a dip to the west of fifty -five degrees. I — 46 — Whether this basin is the result of folding alone, or whether faulting has been an important factor in its development is not known. ^i "While post-Rosamond folding has certainly occurred, there is evidence™ that folding and faulting were in progress during the deposition of the strata. It has already been stated that rhyolitic and diabasic rocks which are known to be intrusive into the lower portion of the series are found at higher horizons incorporated in the Rosamond as detrital material. This result could only have come about through contem- poraneous folding which elevated material which had just been deposited, allowing its erosion and re-incorporation in strata still being deposited. Furthermore the silver mineralization is knowji to have occurred during the deposition of the Rosamond. And yet, the silver deposits are cut by inter-mineralization faults which must also have effected the Rosamond beds. Manifestly where the conditions of deposition are so involved and where the exposures are so poor no exact figures can be given for the thickness. It seelns probable however that the maximum thickness of the Rosamond strata within the quadrangle does not exceed, although it probably approaches, 1000 feet. The depth of material already found in the Big Four shaft is largely the result of sinking diagonally through the highly inclined strata. Relations to Adjacent Formations. The Rosamond series is distinctly younger than and overlies all of the formations previously described. It is contemporaneous in part with certain rhyolitic and diabasic intrusives. And it is older than the Red Mountain andesites. The material composing the Rosamond appears to have been derived very largely from the quartz monzonites and schists found within the region. The relation between the Rosa mond series and the Red Mountain andesites is, in general, one of angular unconformity. This is especially true in the northern portion of the Lava Mountains, where the sedimentary beds may at times stand vertically beneath the lava flows. Immediately beneath the contact with the lavas on the' northwest side of Red Mountain, the Rosamond beds dip twenty degrees to the south or southwest, while the contact dips five degrees to the east. About two miles further south on this same contact, however, the Rosamond strata are flat and appear to grade upward through tuffaceous phases into the overlying lava series. Thus over most of the region there appears to be a definite bre'ak between the two groups with locally a gradation. This can be explained only by contemporaneous folding accompanied by erosion of the crests of the folds, the eroded material being deposited in the adjacent troughs of the folds, and by the absence of any time bre'ak between the Rosa- mond series and the Red Mountain andesites. The unconsolidated nature of the Rosamond beds would lend itself readily to an action such as that postulated, so readily in fact, that the upper surface of the Rosamond during its later history might be expected to approach the condition of a peneplained surface without any break in sedimentation of the down-folded (or do\vn-faulted) troughs having occurred. Such a surface is found off the map to the east and northeast, where the — 47 — upper surface of the Rosamond, which thou<:li ohler tlian the lava flows has here never been covered with them, is quite flat though now tilted. Conditions of Accumulation. The types of sediments occurring in the Rosamond series within this quadrangle as well as the structural features, permit of only one mode of origin — namely, accumulation in intermittent lake basins in an arid climate. The deposition may have been in part subaerial. The freshness and angularity of the feldspars and the presence of biotite and ferromagnesian minerals in these sediments, testify to the slight degree of weathering, the short transportation undergone and to the rapid deposition. The slight alteration observed in such easily- altered minerals could only result from erosion in an arid climate such as that which now exists in the region. The presence of the gypsum is best explained by precipitation in shallow waters as the result of evaporation. The gypsum is further evidence of the aridity of the Rosamond climate. The fairly well-sorted and well-bedded nature of certain of the strata is indicative of aqueous deposition, while the presence of beds of clay, chert and limestone, bears evidence that still water conditions existed such as -those which might be expected in a lake. In contrast to these are those horizons which show a massive heterogeneous mixture of boulders, gravels and sands. Such horizons may be subaerial deposits in part, the result of accumulations of flood waters. During the period intervening between the intrusion of the quartz monzonite and the deposition of the basal beds of the Rosamond, this i-egion appears to have possessed a Avell-developed drainage system, for during this interval not only was the land surface reduced to one of low relief, but all the products of erosion (which must have repre- sented a tremendous quantity of resulting sediments) were completely removed from the region. The lakes in which the Rosamond strata were deposited were the residt of faulting and w\irping wliich destroyed the pre-existing drain- age system and created enclosed ])asins in which sedimentation began. These movements did not cease with the formation of the basins and the inception of the Rosamond sedimentation, but continued for a long period. This is shown by the incorporation within the Rosamond strata of fragments of contemporaneous intrusive igneous rocks — the rhj^olites and diabases. It is further shown by the presence of detrital gold in the tungsten placers near Atolia overlying the quartz monzonite and in the basal beds of the Rosamond series of that part of the area. This gold was without doubt, derived from the region a few miles to the north where it is known to be later than the basal Rosamond. Seem- ingly, the basal Rosamond beds near Atolia are much later than the basal beds exposed south of Johannesburg. The localization of these basins of deposition is indicated by the derivation of much of the material from the immediate vicinity. Age and Correlation with the Rosamond Series of Adjacent Regions. No fossils were found within the Rosamond series in the Randsburg quadrangle. On the basis of correlation with strata in adjacent parts of the desert, however, the age is determined to be Upper Miocene. — 48 — Til the type section near Rosamond, 1650 feet of strata occur con- sistinji" chiefly of p-ranitic detritus but with much rhyolitic tuff at certain horizons. Two rhyolite flows occur interbedded in the series, one 100 feet thick, 750 feet from the base, the other 300 feet thick, 1250 feet from the base. Recurrent outcrops of these' lavas occur to the westward, finally passing' beneath Pliocene sediments near the west end of Antelope Valley.^ North of Barstow, in the Barstow Syncline, arkosic and tuffaceous sediments occur overl3ang an acid basalt or a basic andesite, fragments of which occur in the overlying sediments.- These beds are known from their fossil content to be Upper Miocene in age.'' The Ricardo beds, exposed in Redrock Canyon which cuts across the El Paso Mountains eighteen miles west of Randsburg, consist of arkosic and tuffaceous sediments which are very late Miocene to early Pliocene.* The beds in j^art, overlie intrusions of quartz porphyry (rhyolite?) and diorite porphyry (diabase?). Two basalt flows separated by fifty feet of sediments, occur near the center of the' series, while at Black Mountain, five miles west of the northwestern corner of the Randsburg Qiiadrangle, the series is unconformably overlain by thin flows of olivine basalt.^ It seems probable that the igneous sequence' during the Tertiary has been similar over a limited portion of this part of the Mojave Desert. Thus the rhyolites of the type section at Rosamond are believed to be equivalent to the rhyolitic intrusions near Randsburg, Avhile the basalt underlying the Barstow syncline may be the equivalent of the diabase of this area. Similarly the quartz porphyry and diorite porphyry which underlie the Ricardo section, may be the equivalent of these same rocks. The basalt flows of Black Mountain are correlated with the basic intrusives of the northern part of this quadrangle. These are uncon- formably younger than the Red oMountain andesites. Hence the andes- ites which are of a basic type, may be the equivalents of the basalts which occur interbedded in the Ricardo section. If this reasoning be true, the Rosamond series represented in the Randsburg quadrangle, would be somewhat older than the Ricardo beds and more or less equivalent to the beds at Rosamond and those of the Barstow Syncline, or in other words, their age would be Upper Miocene. RHYOLITE-LATITE SERIES OF INTRUSIVES. Shallow intrusive igneous rocks of acid composition are of quite wide- spread occurrence in the Rand Mountains. The intrusions may take a variety of forms, including pipes, dikes and sills. These are known to intrude the lower beds of the Rosamond series, while detrital frag- ments of the same intrusives are found higher in thatseries. These 'C. L. Baker. Notes on the Later Cenozoic History of the Mojave Desert Region Univ. Calif. Bull. Dept. Geol., Vol. 6, (1911), pp. 333-383. =C. Li. Baker. Op. cit. 'J. C. Merriam. A collection of Mammalian Remains from Tertiary Beds in the Mojave Desert. Univ. Calif. Bull. Dept. Geol., Vol. 6 (1911), pp. 167-169. New Protohippine Horses from Tertiary Beds on the Western Border of the Mojave Desert. Univ. Calif. I'.uII. Dept. Geol., Vol. 7 (1912), pp. 435-441. ■•J. C. Merriam. Op. cit. C. L. Baker. Op. cit. =C. L. Baker. Physiography and Structure of the Western El Paso Range, and the Southern Sierra Nevada. Univ. Calif. Bull. Dept. Geol., Vol. 7 (1912), pp. 117-142. PLATE IS k:^f*'f: "^ A. ROSAMOXIJ SA.Xl )SI( ).\K. i^=: quartz. :!(( ilia. Ordinary light. ■^&m :w^ f£\m -ik^r - ^ i^ ^'r B. RHYOLITK. Q rrr quartz phenocrysts ; M = altered muscovite ; groundmass is a fine granular intergrowth of quartz and orthoclase. 50 dia. X nicols. 37841— facing p. 48. — 49 — igneous intrusives therefor are of the same age as the lowei* part of the Rosamond series. The occurrence of these intrusive rocks is practically confined to the Rand ^Mountains, althouirh certain rhyolites in the North Rand District north of Red Mountain (not mapped), probably belong to the' same group. There is also a suspicion that dikes of this series may occur in the vicinity of St. Elmo for although none were seen, abundant rhyolitic fragments, some of good size, occur in that neighborhood. Outside of this area, dikes possessing the same features and j^robably the correlatives of those here d&scribed, are known to occur at Fremont Peak and in the vicinity of the Silver Dome Mine, fifteen miles to the southeast. The largest of the many intrusive masses is a pipe which forms the major portion of the rugged hill a mile southeast of Johannesburg. The dikes Avithin a radius of three miles of this pipe show a rough radial arrangemejit with respect to it which suggests a very close relationship betweeii the dikes and the pipe. The dikes find their major development in the region within two miles of Randsburg. These dikes are of all sjzes. Many were observed which had a thickness of only a few inches. Others were from fifty to a hundred feet in thickness. Tlie majority would probably be between two and ten feet. The longest of the many dikes is about a mile and a half in length, ruiniing from the top of the first hill west of the pipe described above almost to the Yellow Aster Mine. This dike is about twenty feet in thickness. ]V[ost of the dikes mapped, however, are' only a few hundred feet to a few hundred yards in length. These short dikes are considoi'ed by the writer to be short faulted segments of what were originally longer continuous dikes. Great numbers of these short segments occur which were not mapped. Little distinction exists between the dikes and the sills except that the latter have been inti-nded along the planes of schistosity of the Rand schist. The mo.st prominent development of sills in the region was found three-fourths of a mile east of Sidney Peak. Both the dikes and sills show sharp and well-defined contacts with the intruded rocks. Chilled borders are' usually to be observed along the contacts of the intrusive. Fragments of the intruded rocks are occasionally found in the dikes and sills but are by no means common. Petrology. The common features of this group of rocks are the general porphy- ritic habit, the fine-grained to glassy groundmass and the liglit color. The phenocrysts while usually prominently developed are generally small. P'eldspars are the most characteristic phenocrysts, usually show- ing well-developed cleavage faces and good crystal outlines, although in some cases observed, they were somewhat rounded. For the most part they show no albite twinning. Quartz phenocrysts are quite character- istic of many of the rocks, either as i-ounded grains or as well-developed crystals with pyramidal terminations on either end. The quartz though usually colorless, is in some cases smoky. Some of the dikes show a rather strong development of biotite and some hornblende; in other occurrences the ferromagnesian minerals are entirely absent. It is only in specimens from the larger dikes that the groundmass can be resolved with a hand lens, in most cases being microervstalline to 4—37841 — 50 — aphanitic. More rarely the groiindmass may be glassy when it may show a well-developed flow structure. The color of the groundmass, and hence the color of the resulting rock, is usually a light gray, white or cream, though with an increase in the visible' content of biotite and hornblende the groundmass may become much darker in color. Where the groundmass is glassy the rocks appear darker in color than where the groundmass is crystalline. A mile and a half northeast of Osdick a small pipe of this series which is not over twenty feet in diameter, is intruded into the Rosamond strata. The rock of this pipe is a vitreous black glass which, however, is quite transparent even in moder- ately thick flakes. In composition, the rocks of this series were determined microscopi- cally to vary from rhyolites through trachytes and quartz latites to latites. A few of the dikes may even have the composition of andesites. The average composition would approximate that of an acid quartz latite. Microscopically the more acidic rocks were found to be practically free from magnetite or any of the ferromagnesian minerals. Many of the rhyolites were found to consist solely of orthoclase and quartz in varying proportions, possibly with traces of muscovite. These rocks are practically white in color. With the addition of plagio- clases a small quantity of ferromagnesian minerals aj^pear also. The only plagio- clase observed had the composition of an acid oligoclase, while the iron-bearing minerals included brown biotite, green hornblende, magnetite and pyrite. Apatite was also observed. An especially interesting rock type was found in the east end of the dike which terminates at the top of the first hill west of the large rhyolite pipe. This rock, pure white in color, appeared to be porphyritic in the hand specimen, phenoci'ysts of orthoclase and quartz of moderately small size being set in a finely crystalline groundmass. Under the microscope however the 'groundmass' was found to consist of orthoclase crystals of approximately the same size as the phenocrysts, each of these orthoclase crystals showing a well-developed vermicular intergrowth with quartz. The quartz intergrown in each orthoclase crystal possessed a common orientation ; that in adjacent crystals was oriented differently. The phenocrysts were quite clear and free from any trace of intergrowth. IMuscovite in traces was the only other mineral present. A number of intersections of dikes of this series were observed in thef field. Wherever these dikes intersected, it was observed that the more basic and darker-colored dike cut through the more acid and lighter-colored one. Seemingly, the intrusion of the dikes of this series was a slow enough process that one dike could be injected and solidify and new fractures be formed before the injection of the next later dikes, Seemingly ' also, the magmatic source of the material was becoming slightly more basic as time elapsed. Effects on Intruded Rocks. Intrusions of rocks of this rhyolite-latite series are known to occur in the Johannesburg gneiss. Rand schist, Atolia quartz monzonite and the Rosamond series. In the vast majority of cases, the intrusion has had no effect on the intruded rock so far as can be told. In a few cases, however, the effects on the intruded rocks are noteworthy. Strata of the Rosamond series surrounding the large rhyolite pipe lying southeast of Johannesburg have been highly silicified as the result of the intrusion, the silicification apparently being the result of hydro- thermal activity. On the north the silification continues out as far as the floor of the valley, while on the west all of the Rosamond strata PLATK 14. A. RHYOLITB WITH VERMICULAR INTERGROWTH. Q = quartz phenocrysts ; groundmass composed of crystals of orthoclase (black) showing a vermicular interarowtli of quartz (white). 50 dia. X nicols. <■■• -■'"•',' BASALT. From dike of L'pper Miocene age. A = augite ; H = horn- blende ; groundmass consists of lath-shaped crystals of labradorite set in a light brown glass. 50 dia. Ordinary light. 37841 — facing p. 50. — 51 — exposed are hiprhly silicified. Near the saddle on the contact between the Rosamond beds and the quartz nionzonito, the ([uartz monzonite itself has been lii.i2:hly altered apparently by the same hydrothermal activity. Where they have been hijihly silicified as they have he're, the arkosic sandstones of the Rosamond series become extremely hard so that they break throu- is quite rugjjed and prom- inent by reason of the rhyolite pipe composing' it. The dikes, especially in the elevated country (juite commonly follow alonp- ridp'e lines and appear to have been an i)nportant factor in controlliuii' the position of the ridg-es. Tn the flats, dikes are commoidy found to outcrop wheli other rocks show no exposure whatever. The dikes are eroded chiefly by the breaking out along joint planes of rather angular fragments, more or less rectangular in shape. These fragments commonly cover the slopes for some distance below the out- crop of a dike. The surface of the dikes is usually stained a yellow-brown by iron oxides. This same discoloration works in along the joint planes as well, staining the rock for a distance of a half an inch or more from each fracture. The' soil adjacent to the dikes is quite commoidy stained the same brown color, so that at times when no dike is exposed, its presence may be inferred by the discoloration of the soil. DIABASE— BASALT SERIES OF INTRUSIVE. Dark-colored basic dikes having the composition and texture of either a (lial)ase or a basalt are much more common in the Randsburg quadrangle than would be indicated by the mapping. Although only a few dikes were observed whicli were large enough to map ; dikes of this gi"ou]i were found underground in nine different mines of the region and are known to occui' in at least seven others, either through finding diabase fragments on tlie dumps of these mines or through second-hand information. Field Relations. The areal distribution of This series of dikes appears to be roughly the same as the distribution of the rhyolite-latite series. They were observed in various parts of the Rand ■Mountains. Fragments of diabase were picked up on the dump of the North Rand ^Mine, southeast of ^fountain Wells, and also on the dump of the I'nion Mine at Atolia. Outside of the quadrangle, similar dikes are known at Fremont Peak and at the Silver Dome ^line. In addition, certain strongly porphyritie dikes having the composi- tion of hornblende-hypersthene diabase which are found in the region southeast of the El Paso Peaks probably belong to this series. Because of lack of time in the field, the^e last dikes were not mapped. The dikes vary considerably in size. The largest known in the region is a mile and a half in length and from twenty to fifty feet in thickness. This is the dike passing through the Butte, Kenyon, Wedge and Little Butte mines. — 53 — The dikos observed imderjrround were usually small, varying in thickness from a few inches up to ten feet. For the most ]iart, thes(> smaller dikes tliou of rocks occurs in the quadrangle possessing the composition of basalts, it has been found most expedient in this report to adopt the term Red ^Mountain andesite for this group. Interbedded with the lava flows which constitute the major portion of the serie s are important beds of agglomerate and tuff. The agglomerates 'i: E. Spurr. U. S G. S. Bull. 208, p. 216. An aleutite Is a rOck having a com- position intei-mediate between an andesite and a basalt. — 58 — are composed of angular fragments of lavas similar to those already described which are imbedded in a tiiffaceoiis matrix. The fragments are of all sizes, many of those observed being over ten feet across. The agglomerates appear to be especially well developed near tlie base of the series. The tnffs are quite commonly white in color although some were observed wliich were pink or reddish-brown. They are quite tine in grain but contain small scattered f ragmelits of lava and crystals of feldspar, biotite and hornblende. Locally within the quadrangle, these tuffs have been mined for an abrasive. Occasional inclusions of pebbles from the Rosamond series are. to be found within the lava flows. One or two inclusions of schist, similar in all respects to schists of the Rand series, which were found by the writer on the east side of Red Mountain could easily be mistaken for evidence of underlying schistose rocks by one not familiar with the Rosamond series. Structure and Thickness. These lavas have been little effected by folding since they were orig- inally poured out on the' surface. The base of the lavas in Red Moun- tain is a flat even surface which dips to the east at an angle of five degrees. This dip is probably the result of slight folding in the region since the lavas were formed. The lavas have been effected by faulting, being involved in move- ments along the Garlock Fault. They seem to be effected by faulting also in the region north of Red Mountain. No detailed information was obtained concerning the position and extent of these latter faults. The lavas appear to reach their maximum thickness in Red Mountain where approximately 1400 feet of flows and pyroclastics exist. It is not believed that -erosion has reduced the original thickness to any appreciable extent. The maximum thickness in the Lava Mountains appears to be much less, being in the neighborhood of 850 feet, while over much of the area now occupied by flows the original thickness was probably only a few hundred feet. Age. The Red Mountain andesites are probably Pliocene in age, being limited by the Rosamond series of the quadrangle below, and by an unconformable relation to the Black Mountain basalt (IMiddle Pliocene to Pleistocene) above. On the basis of correlation with the basalt flows which occur interbedded in the Ricardo section, which would seem to be reasonable, the Red IMountain andesites would be very early Pliocene. BLACK MOUNTAIN BASALT. Basalts of this group, which includes both intrusive and extrusive types, are only found in this area in a few small occurrences in the north-central part of the quadrangle. Field Relations. The largest of these occurrences is an irregular intrusive pipe wliich has overflowed on the surface. The basalt is now exposed over an area roughly one-third by one-fourth mile which lies just east of the railroad PLATE 16. i ^m ^ '^S^-:^r.3JS/^SX ^. 1 iv^'f A. PYROXENE AXDESITK FROM RED MOUNTAIN. L - labradorite ; B tir biotite ; H = hornljlende ; A rzr augite. The groundmass is a light brown glass. 50 dia. Ordinary light. B. SAME .AS "A" BUT WITH CROSSED NICOLS. 37841— facing p. 58. — 59 — a half-mile north of Summit Dip:o:incrs. Good exposures are obtainable in a cut made in buildiiifr the railroad. The basalt in tliis pipe has intruded its way through Rosamond strata and Red Mountain andesites to the surface, and has overflowed part of the surrounding surfacef, covering it with a basalt layer about three feet in thickness. This flow was largely down the side of a small canyon which existed in the Red ^Mountain andesites previous to the basalt extrusion. The intrusive pipe appears to underlie most of the area mapped as basalt in this occurrence. A quarter of a mile to the northeast two other small occurrences of basalt were noted, tliougli from the poor exposures it could not be determined whether they were intrusive or outlying remnants of the flow above? described. In the region a half mile northeast of Summit Diggings, two small iiills, one on either side of the county road, possess a capping of thin basalt flows, identical in appearance and composition with that found along the railroad. The flow in the southernmost of these hills is only a few feet thick, the flow in the other occurrence being much thicker. Both overlie strata of the Rosamond series. It is believed that all of these occurrences are remnants of a flow derived from the intrusive' pipe adjacent to the railroad. They are now found in isolated patches due to erosion having removed the intervening portions of the flow. The second largest exposure of basalt is at the head of Hardcash Gulch, a mile and a half west of Summit Diggings. The mass, from its mapping and also from its effect on the surrounding beds of the Rosa- mond series is known to be an intrusive pipe. It transgresses the bed- ding of the Rosamond series and has highly silicified the sandstones and clays of that series so that they now exist as hard and resistant rocks. The silicified arkose sands with their fresh feldspars strongly re^semble granitic rocks. This silicification persists for some distance east from the pipe possibly indicating an unexposed extention of the intrusive. Eastward along the line of the Garlock Fault, almost to the border of the map, an occasional small exposure of basalt may be found. These appear in each case to be small pipe-like intrusive masses only a few feet across whose position has been controlled in some way by the faulting. Where these pipes cut beds of the Rosamond series, as for example, two and one-half miles north of Bedrock Spring, the' sands and clays of that series are silicified. The name Black IMountain basalt has been adopted for this group of rocks due to the widespread basalt flows present on Black Mountain, seven miles west of El Paso Peaks, where they occur unconformably overlying Tertiary sediments. These flows extend northeastward almost to the corner of this quadrangle'. There can be little doubt that the basaltic intrusives and extrusives of this quadrangle and the flows of Black Mountain are correlative. Basalt flows, similar in composition and appearance to those here described, occur at similar horizons in various parts of the nearby desert province, as for example, the flows capping Black Mountain (a different mountam from the one above), fourteen miles east of Fremont Peak. — 60 — Petrology. The rocks of this group may be classified as augite-hornblende basalts. They are characteristically dark colored, usually being black but some- times medium or dark gray. The groundmass is dense and stony in appearance and is practically always highly vesicular, even in the intrusive facies. The vesicles possess extremely irregular forms and are but seldom filled. At times a faint flow structure may be made out. Pheuocrysts, while usually present, are neither abundant nor conspicu- ous. Plagioclase feldspars showing albite twinning and occasional prisms of hornblende and pyroxene can be identified with the hand lens. Microseopioally the groundmass was found to be composed of minute lath-shaped feldspars and abundant scattered grains of magnetite. The feldspar laths which have the composition of labradorite always appear to be arranged with their long axes parallel, developing a flow structure. Brown glass in moderate amounts was sometimes present. The phenocrysts were found to consist of well-developed plagioclase crystals, showing both albite twinning and zoning. They varied in composition from labra- dorite to bytownite. Brown hornblende, present in moderate amount, usually shoiwed strong reaction zones of magnetite. The pyroxene observed was identified as augite. It was usually subordinate in quantity to the hornblende. No hypersthene was recognized. In two cases quartz was present, apparently as a primary mineral. Inclusions which have without doubt been derived from the under- lying Rosamond series are occasionally found in the basalts. The writer observed fragments of both quartz and schists. These rocks are very little weathered. At the most, they show only a weatherstained .surface, so that fresh fragments are easily obtained. Due to jointing, the basalt breaks out in platey or prismatic blocks which are strewn over the surface of most of the e'xposures. Age. The age of the Black Mountain basalts may be anything from Middle Pliocene to Pleistocene. They are unconformably later than the Red ^lountain andesites. as is shown by the flow on the side of the ancient canyon on the surface of the andesites, and by the basalt flows over- lapping on the surface of the Rosamond series. The basalts on the other hand are earlier than much of the movement along the Garlock Fault. Boulders of basalt are incorporated in alluvium which has been elevated several hundred feet by the faulting. The freshness of the basalts and the persistence of portions of flows which were originally only a few feet thick, argue for the recency of their formation. tSo that it seems probable that the Black Mountain basalts are very late Pliocene or early Pleistocene in age. ALLUVIUM. All unconsolidated deposits of Quaternary age occurring within the quadrangle have been grouped together as alluvium. And only those portions of the alluvium were mapped which covered some con- siderable area aiid which were present in sufficient thickness to effec- tually mask the underlying formations. Thus the amount of alluvium mapped is necessarily incomplete and approximate and materials of different types and different modes of origin have been grouped together. PLATE 17. A. BLACK MOUNTAIN BASALT. From intrusive pipe near Summit Diggings. B = bytownite plienocryst ; groundmass consists of lath shaped feldspars and fine magnetite crystals surrounded by brown glass. 50 dia. Ordinary light. B. SAME AS -A" BUT WITH XICOLS CROSSED. 37841 — facing p. GO. — 61 — Ancient Gravels. The relative afre of tlie various phases of the alluvium is usually indetermiuate, hut in a few cases at least unconsolidated dejiosits are known to antedate much of the movements on the (iarlock Fault. At various points noi-th of the fault, as for instance ovei-lyiii