Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/geologyofpalenmo36hopp ^ STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES GEOLOGY OF THE PALEN MOUNTAINS GYPSUM DEPOSIT RIVERSIDE COUNTY, CALIFORNIA SPECIAL REPORT 36 DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO y SPECIAL REPORTS ISSUED BY THE DIVISION OF MINES l-A. IB. 2. 3. 7-A. 7-B. 8. 9. 10- A. 10-B. 10-C. 11 12. 13. 14. 15. 16. Sierra Blanca limestone in Santa Barbara County, California, by George W. Walker. 1950. 5 pp., 1 pi. Price 25 0. The Calera limestone, San Mateo and Santa Clara Counties, California, by George W. Walker. 1950. 8 pp., 1 pi., 6 figs. Price 250. Geology of part of the Delta-Mendota Canal near Tracy, Cali- fornia, by Parry Reiche. 1950. 12 pp., 5 figs. Price 250. Commercial "black granite" of San Diego County, California, by Richard A. Hoppin and L. A. Norman, Jr. 1950. 19 pp.. 18 figs. Price 25 0. Geology of the San Dieguito pyrophyllite area, San Diego County, California, by Richard H. Jahns and John F. Lance. 1950. 32 pp., 2 pis., 21 figs. Price 50*. Geology of the Jurupa Mountains, San Bernardino and River- side Counties, California, by Edward M. MacKevett. 1951. 14 pp., 1 pi., 14 figs. Price 250. Geology of Bitterwater Creek area, Kern County, California, by Henry H. Heikkila and George M. MacLeod. 1951. 21 pp., 2 pis., 15 figs. Price 35 0. Gem- and lithium-bearing pegmatites of the Pala district, San Diego County, California, by Richard H. Jahns and Lauren A. Wright. 1951. 72 pp., 13 pis., 35 figs. Price $2.50. Economic geology of the Rincon pegmatites, San Diego County, California, by John B. Hanley. 1951. 24 pp., 1 pi., 5 figs. Price 35 ■g 3 s * ■° 41 » _ _a OJ 53 ^- . " 9> a 5 a" lis- 3 ,£ o E a £ M «- A -a •a eg I s &"si ^"•■°as 2 £ _a * -a £ •"S'8 ■aS.f^-8 j- = » a o> C * e. = ■3 5 - ^ o '5 =- cs go o " 2 so << I C3 a ■ o a .a « u o : A Sgl a A 5 -a •S a a. o v 2 0,5 GEOLOGY OF THE PALEN MOUNTAINS GYPSUM DEPOSIT RIVERSIDE COUNTY, CALIFORNIA By Richard A. Hoppin * OUTLINE OF REPORT Page Abstract 3 Introduction 3 Descriptive geology 5 , Stratigraphy and petrography 8 Maria formation 8 Stratigraphic succession 8 Petrography 8 Environment of sedimentation 15 McCoy Mountains formation 15 Meta-igneous rocks 16 Gravels 17 Structure 17 Metasomatism 19 Contact metamorphism 19 Feldspathization 20 Other mineralization 21 Geologic history 21 Correlation 22 Economic geology 24 References 24 Illustrations Page Plate 1. Geologic map of the Palen Mountains gypsum deposit In pocket Frontispiece. Aerial photo showing gypsum deposit 2 Figure 1. Aerial photo of the southern gypsum-bearing series 4 2. Index map showing location of Palen Mountains gypsum deposit 6 3. Sections across Palen Mountains gypsum deposit 7 4. Photo of stratified Recent gravels overlying meta- igneous rock 8 5. Photo of intrusive contact between porphyroblastic meta-igneous rock and west-dipping marble and gypsum beds 8 6. Photo of marble and siliceous marble on gypsum 9 7. Photomicrograph of cataclastic deformation in mar- ble of the Maria formation 9 8. Photomicrograph showing flowage folds 9 9. Photomicrograph of fine-grained gypsum, Maria for- mation 12 10. Photomicrograph of fine-grained gypsum, Maria for- mation 12 11. Photomicrograph of predazzite in Maria formation 13 12. Photomicrograph of fine-grained, granular marble of Maria formation 13 13. Photomicrograph of feldspathic quartzite, Maria for- mation 14 14. Photomicrograph of pink quartzite 14 15. Photomicrograph of lime-silicate marble, Maria for- mation 14 16. Photomicrograph of actinolite-feldspathic grit 14 17. Photomicrograph of quartz grit, McCoy Mountains (?) formation 15 18. Photomicrograph of porphyritic quartz latite, McCoy Mountains (?) formation 15 19. Photomicrograph of zoisite-epidote-garnet quartzite, McCoy Mountains (?) formation 16 20. Photomicrograph of conglomerate, McCoy Mountains formation 16 21. Photomicrograph of meta-igneous rock 16 22. Photomicrograph of porphyroblastic meta-igneous rock 17 23. Photomicrograph of porphyroblastic meta-igneous rock 17 24. Photomicrograph of meta-igneous rock^ 17 25. Photomicrograph of feldspathized quartzite 18 26. Photomicrograph of skarn 18 27. Photomicrograph of tremolite-marble 19 * Department of Geology, State University of Iowa, Iowa City. Por- tion of a thesis submitted for the degree Doctor of Philosophy, California Institute of Technology, June 1951. Manuscript sub- mitted for publication May 1951. Page 28. Photomicrograph of intrusive rock at contact with tremolite-marble 19 29. Photomicrograph of contact zone in marble xenolith . 20 30. Photomicrograph of tremolite-magnetite rock 20 31. Photomicrograph of fluorite in muscovite-quartz schist 21 32. Photomicrograph of kyanite crystals in quartz-seri- cite schist 21 ABSTRACT A deposit of gypsum 5 square miles in area is at the north end of the Palen Mountains. Interbedded gypsum, marble, quartzite, felds- pathic quartzite, and lime silicate rocks, together with metamor- phosed intrusives, are arranged in east-trending bands. The rocks are intensely deformed and metamorphosed. Folding, faulting, brec- ciation, and shearing occurred as a result of strong deformation. The tectonic structures range in magnitude from large mappable units down to those microscopic in size. The area has also been subjected to regional, contact, and metasomatic and hydrothermal metamorphism in that order. The gypsum occurs as massive beds of finely crystalline material of very high grade interbedded with marble or as thinly laminated gypsiferous epidotic schists. Little anhydrite is found at the sur- face and its presence at depth cannot be ascertained without drill- ing. Although the gypsum is of high quality, its value is lessened by fragments of marble in the gypsum beds that will increase the cost of mining. The deposits have been variously dated as pre-Cambrian and as Paleozoic. The lack of fossils and the small amount of geologic work done in the southeastern Mojave desert make dating and cor- relation difficult. The deposits are similar to those in the Little Maria and Maria Mountains to the east with an Upper Paleozoic age designation probably the better alternative. Possibly the gyp- sum, marble, schist, and quartzite beds are the deformed and meta- morphosed equivalents of the gypsum, limestone, shale and sand- stone of the Kaibab and Moenkopi (Permian and Triassic) forma- tions in southern Nevada. INTRODUCTION The Palen Mountains gypsum deposit covers 5 square miles of the interior of one of the typical isolated moun- tain ranges of the Basin Ranges province. The study of the area had as one objective the investigation of the internal structures of a part of one of these ranges. The gypsum deposit is unusual in that it is associated with intensely deformed metamorphic rocks and is cut by one large, irregularly shaped igneous mass and by several small, sill-like igneous intrusions. The latest topographic map available is the Palen Mountains quadrangle made by the United States Army Corps of Engineers during World War II, scale 1 : 48,- 000. To facilitate the geologic mapping, the gypsum area was photographed at a scale of 800 feet to the inch and a semicontrolled mosaic constructed at a scale of 400 feet to the inch by Pacific Air Industries of Long Beach, California. The contours were constructed by use of the stereoscope. Two United States Geological Survey bench- marks, check points obtained with a Paulin altimeter, and elevations of prominent peaks on the quadrangle map were used to provide vertical control. A total of 35 days was spent mapping the deposit in the spring, fall, and winter months of 1950. The field work was supplemented by laboratory examination of 122 thin sections of significant rock samples. Previous Geologic Work. Little detailed work has been done in the area and age determinations of the (3) Special Report 36 — 3 O ts — D s ;3 5 a ad *~ a; 2 5S O g o '5 >oi C3 Palen Mountains Gypsum Deposit carious pre-Tertiary rocks have been only tentative. The me report on the gypsum deposits of the Palen Moun- iins is that of Harder (1909)*. His study did not cover letails of structure and metamorphism. Similar studies lave been made on the gypsum deposits in the Little [aria Mountains to the east by Surr (1911). Brief i (descriptions of these deposits and also of those in the I Maria Mountains can be found in Hess (1920), Tucker .and Sampson (1929), Jenkins, et al (1937)*. Much more (detailed work has been done in the Eagle Mountains 25 miles to the west by Harder (1912) and by Hadley I I [(1948). The rocks in which the iron ore occurs are simi- j lar in a broad way to those in the Palen Mountains gyp- ffeum deposit. Very general reports in which the geology l of the southeastern Mojave region is discussed can be found in Darton (1907), Brown (1923), Hazzard, et al. (1938), and Miller (1944). Location. The Palen Mountains gypsum deposit is in northern Riverside County, 30 miles northeast of Desert (Center and about 50 miles northwest of Blythe. The de- posit is accessible by a dirt road branching eastward I (from the Parker highway 17 miles north of Desert Cen- ter. This road, when not in regular use, is likely to be I (blocked by drifted sand. The location of the deposit is Ishown on the index map (fig. 2). Climate and Vegetation. The climate is hot and dry. [[n the summer daytime temperatures are often above 120°F and night temperatures seldom drop below 60°F. During the winter the climate is ideal, the temperature iseldom going below freezing or above 80°F. Strong |winds prevail for several days at a time during and after frontal passages, but the winds in this area are nowhere as severe or of as long duration as in the desert farther north. The rainfall is light and sporadic. Con- nective showers occur during the summer from July through September with less frequent frontal showers occurring spasmodically in the winter. The annual rain- Ifall varies between 1 and 5 inches, rarely exceeding the latter figure. The rain, when it occurs, comes as very strong showers and creates great torrents in the washes. [During these showers, owing to the lack of vegetation jand of good soil, erosion is extremely rapid and great quantities of debris are moved in a very short time. The area is devoid of trees except along the bordering outwash aprons, where locally there are small stunted growths of ironwood and paloverde. The rest of the area is covered by scattered clumps of creosote bush, ocotillo, cactus, and a few varieties of sagebrush. The dry washes also contain catsclaw, porcupine bush and other bushes, and, in favorable spots, a few trees. Topography. The Palen Mountains, which have a general north trend, are about 15 miles long and vary considerably in width. Except at the north where they merge with the Granite Mountains, they rise precipi- tously from the surrounding broad, flat desert areas. Al- though older accounts of the area include the Granite Mountains with the Palen Mountains, current usage limits the name Palen to the mountains south of the low • Editor's Note. Since this was written. Gypsum in California, by W. E. Ver Planck, has been published as Bulletin 163 by the California Division of Mines. The Palen Mountains deposit is discussed on pages 21-24 ; Little Maria Mountains are discussed on pages 13-20. pediment pass between the gypsum deposits and the Granite Mountains. The Palen Mountains have a maximum altitude of about 4000 feet; the Granite Mountains to the north rise to about 4500 feet. The gypsum area ranges in alti- tude from about 1250 feet to a maximum of 2000 feet. The regions to the north and south of the deposits are extremely rugged and are very difficult of access. Within the gypsum area the relief is much more subdued except where recent gullying has opened deep, straight-sided cuts as much as 20 or 30 feet in depth. Acknowledgments. The author wishes to express his appreciation to Remington Stone, consulting mining en- gineer, who first brought the deposit to his attention ; to John Webb and Fleetwood Lawton, owners of the prop- erty, for their cooperation ; to Arthur Kintano and Har- mon Speaker for their aid during the times the property was visited; to Miss Ellen Powelson for drafting the index map ; and to Ian Campbell of the California Insti- tute of Technology whose guidance and encouragement have been a great source of inspiration. DESCRIPTIVE GEOLOGY Although the Palen Mountains gypsum deposit is com- plex in detail, the broader geologic features may be simply described. Interbedded gypsum and marble, quartzite, feldspathic quartzite, and lime-silicate rocks, together with meta- morphosed intrusive rocks, are arranged roughly in bands trending east. Dips are predominantly to the north at moderate angles except where there may be reversals near minor warps. Miller (1944, p. 25) proposed the name Maria for this gypsum-bearing metasedimentary formation. The main deposits of gypsum are in the southern part of the area and are isolated from the smaller, discon- nected northeastern and northwestern gypsum-bearing sedimentary masses by metamorphosed igneous rock. This dark gray rock, probably once a quartz diorite, was intruded as a very irregular sill. The footwall, or south contact, dips northward and is fairly regular, though modified by later faulting, but the hanging wall is ex- tremely complex. In fact, the whole north half of the area appears to be the cupola-roof pendant zone of the intrusive. The intrusive varies considerably in texture and appearance throughout its extent. Some portions of the intrusive are very small irregular masses 1 or 2 feet in diameter which were punched into the metasediments, and others were intruded as narrow, ramifying dikes or thin sills an inch or even less in width. The intrusive- metasedimentary relations become so complex in the northwestern part of the area that a portion was mapped only as a meta-igneous and metasedimentary complex. Within the large meta-igneous masses themselves, small blocks of contact-metamorphic marbles and quartzite, and irregular patches of gypsum and gypsiferous schist are scattered. Trending east across the center of the area is a series of sharp-pointed peaks consisting mainly of dense, fine- grained lime-silicate rock and quartzite. The beds do not, however, strike east. Each peak is a rotated or offset fault block in which the beds strike northeast and dip 45° to 50° northwest. The contact between these rocks Special Report 36 o K) 0] o a s c a — d — ' o Ml e o a a X V a IN U P o Palen Mountains Gypsum Deposit w. %r w luA',1 'Z' /cv. rx&sa '/' »"> 0> o E -3 1°: Q. n k- o E ■o boh" 1 3 D 0) =1 O ' (0 c _ - — o - E >> °* • ** o* #» E jTi" .- o — O" fc o> 10 0.0.0.0.0. LU \&$£ \X^ \ s'V , \ ■ CD 'SI V //, •■ -"--h V; ..... 9k <':-P •**« Figure 5. Intrusive contact between porphyroblastic meta-igneous rock on the left and west-dipping marble and gypsum beds. Height from base of picture to top of hill is 50 feet. of the others. It is assumed, therefore, that these are all different units. It -is doubtful that these represent all the original units of the formation, but it is difficult even to hazard a guess as to the thickness of the stratigraphic section that is missing because of intrusion and faulting. Furthermore, the thicknesses of the lithologic units vary considerably from place to place. The gypsum beds, and to a lesser extent the marbles, have been so contorted that the beds now visible probably bear little resemblance to the original series. The gypsum pinches and swells rapidly over short distances. A thick bed may pinch out completely within a distance of a few hundred feet. In a few spots, however, there are blocks in which the bed- ding and laminae of epidotic material are still visible. Marble lenses and individual large blocks and smaller fragments appear to be present everywhere in the gyp- sum. Although the gypsum is of high grade, difficulty will probably be encountered in mining because of these impurities. Three geologic sections have been constructed. Section A-A' includes the south-central portion of the area, sec- tion B-B' crosses the northeastern gypsum-bearing series, and section C-C intersects the northwestern gypsiferous group. The sections do not include all of the rock units present in the area owing to intrusion, faulting, and lateral changes in facies. In the construction and interpretation of the cross- sections the oldest beds are assumed to lie to the south, thus requiring that the section be right side up. Al- though no sedimentary criteria were noted that would be of aid in proving normal succession, two lines of evi- dence support the assertion that the beds are not over- turned. First, the attitudes of the drag folds are those that would be expected along the limbs of a normal an- ticline. Second, the roof zone of the meta-igneous sill-like intrusive is located in the northern part of the area where contact metamorphic effects are much stronger than in xenoliths farther south in the lower part of the mass. Dip and strike determinations are often only approxi- mate because bedding planes are highly distorted or obliterated ; the attitudes shown are mainly those of lithologic boundaries. These boundaries, in turn, are usually very irregular, commonly blocky. Petrography The textures and mineralogical compositions of the rock types comprising the Maria formation represent the effects of several periods and types of metamorphism on the original sedimentary series. Five kinds of metamor- phism are distinguished: 1) regional, 2) contact, 3) metasomatic, 4) hydrothermal, and 5) dynamic. Regional metamorphism was the earliest and most widespread and took place prior to the instrusion of the igneous body. The regionally metamorphosed rocks were later locally contact metamorphosed during intrusion. A second, less intense regional metamorphism may have occurred after intrusion. Later metasomatic and hydro- thermal activity further modified the rocks. Dynamic metamorphism, represented by local cataclastic struc- tures and textures, recurred throughout the entire in- terval from the onset of the first period of deformation. Some minerals or mineral groups were formed during two, or perhaps three, of the different metamorphic Palen Mountains Gypsum Deposit epochs. Therefore in some of the rocks it is difficult to determine both the relative importance of the various metamorphic types and the paragenesis of the minerals making up these rocks. The rock types of the Maria for- mation can be divided into five groups: 1) gypsum and gypsiferous schist and quartzite, 2) marble, 3) quartz- ite, 4) lime-silicate marble, and 5) grit. Figure 6. Intensely folded beds of laminated marble and siliceous marble lying on gypsum. Height from base of picture to top of hill is about 100 feet. Gypsum. The gypsum occurs as massive beds of white, finely crystalline rock of very high grade inter- bedded with marble or in thinly laminated gypsiferous epidotic schist and quartzite. The average diameter of the anhedral gypsum grains is 0.2 mm. Scattered grains of calcite and epidote are present in almost all samples. Little anhydrite has been found and its presence at depth cannot be ascertained as no drilling has been done. Marble occurs in the gypsum in masses of all sizes ranging from small fragments an inch or two in diameter up to large isolated blocks and lenses several tens of feet in thickness. Undoubtedly the original sedimentary series contained zones in which limestone and gypsum (or an- hydrite) interfingered or had beds of gypsum (or anhy- drite) enclosing thin beds or lenses of limestone; several Figure 7. Cataclastic deformation in marble of the Maria forma- tion. Broken calcite fragments of all sizes down to the dark, ex- tremely fine-grained, crushed matrix. PlJfne polarized light. X30. areas on the map suggest such possibilities. However, deformation has been so intense that much of the wedg- ing out or pinching and swelling of the gypsum beds is of tectonic origin. Furthermore, the large marble blocks not only are chaotically distributed throughout the gyp- sum beds, but also are randomly oriented as indicated by the sharp difference in attitude of bedding from block to block. Many of the small fragments of marble are in- tensely sheared ; coarse calcite grains are elongated and quartz grains are stretched and fractured and show waxy extinction. These blocks and fragments of marble are called "tectonic" impurities. Figure 8. Flowage folds in a band of fine-grained actinolite con- taining quartz, chlorite, and green biotite. The needles flow around the bends of the folds. The top part of the photograph shows granular epidote and quartz. Maria formation, crossed nicols. X30. Schist and quartzite laminae — forming thin streaks in the gypsum or closely spaced layers separated by a thin bed of gypsum an inch or two thick or any amount between — are green in color and consist of fine-grained epidote, green biotite, chlorite, and a variable amount of quartz. The washing away of gypsum causes the beds of gypsiferous schist and quartzite to slump and leaves a slope covered by a lag gravel of flat, green fragments. The bedded deposits of gypsum in the Palen Moun- tains were formed through sedimentary processes of dep- osition. Almost all geologists agree that the large bedded deposits of calcium sulphate were formed by precipita- tion from sea water during evaporation, but the question of which sulphate, gypsum or anhydrite, was deposited originally is most perplexing. Posnjak (1938, 1940) restudied the relations between anhydrite and gypsum and showed that the earlier work of Van't Hoff and his associates was somewhat in error. He determined that saturated solutions of NaCl are not necessary in order to allow anhydrite to form. Above 42 °C anhydrite only can form even in the absence of any other salt and in very dilute solutions. Posnjak writes as follows : At 30°C the solubility of gypsum and anhydrite first increases rapidly in the presence of increasing amounts of sea salts, goes through a maximum of about twice the usual salinity of sea water, and then gradually decreases. However, the decrease is more rapid for anhydrite and an intersection of the two curves 10 Special Report 36 01 — N a » ^a 01 u o co OI Q O © o o u 6C u V OS 0> — C3 S a E o T3 3 S3 V o 3 O o> a ed 81 3 o> st a a a 3 it a a 3 CO ft t>i M 13 ba c © o> a 3 C 3 V a 05 o 3 .2 "S si 2 XI bl a « is 2 s CS a a 3 CO a O X 03 a; to c a o i — 3 "= -m 3 fc- a C3 T3 0> s '3 bfl A a * 3 'ft -o a St J £ GO o I a "C St o> X! a s co ft >> Mi oi J* xl hi o & 0) T3 m a 3 « — o> I- O « *• a & . o o -a a a 03 St -m "3 t, X o> at o a .9 * O CO ?3 3 2 a I o> t< «M '8 o ft S 52 st 3 » °" ■w ° 3 *S o o *ft CO o .5*3 St 01 «l o si 2 A t, a St cc 3 =4-1 oT O 6C 5 a; a> 'I » 3-° if -2 ""2 a 83 01 01 t», O t. tH Oi o ! a - ! 3 . CO CO I ft — . £» ft : m 3 l **-! 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'53 ft >> be J3 V •3 3 3 O 3 O "3 3 o 3 ft_ CO ai *3 01 % " H ""© .lft 3 o> ft Oi © « 3 3" a 0) a .2 o 3 3* I ■ft © © o t- 1ft CO O N M 3 ft O ■*-» bl 3 a 3 3 a 1 C 3 u -bl «4H o 3 o a 3 CO 3. o> '« -w bi '£ -3 tc U> OI 3 ■3 01 o> <^ '3 bi 2 u i> -w be ±~ 4— 3 3 O" -3 3 3 3 a -3 o> y. be -3 3 3 ■*-* >i 3 !S ^ fe bi o O 3_0 3 -3 ■3 3 3 .8 ft o bl 3 3 C ■3 8 3 3 3" - U o> 3 a 3 ■3 8 3 CP 3 0) 3 3 a 3 © C<5 lft -4-> 'B © '5 3 CO CO fc CO ZJ *j CO ■bl o _"3 o •r. 3 -t-i 'ft o 01 8 EH 8 CO. 2 ■2 be S .s CO ■" «♦-( 5 ' 3 3 O o U o -3 * (-i EC a S o) ^s 3 3 _a "u 3 01 a 3 CO ft >, be oi 5 3 O 02 12 Special Report 36 Figure 9. Fine-grained gypsum, Maria formation. Calcite grains in upper left and an aggregate of granular epidote in lower right. Plane polarized light. X30. takes place at about 4.8 times the usual salinity, the point at which anhydrite becomes the stable phase. Sea water is unsaturated with respect to CaSd and only after its salt content has increased by evaporation to 3.35 times the usual salinity can deposition take place. Between this concen- tration and the one required for stable deposition of anhydrite nearly one half the total CaSOi in sea water will be deposited at 30° as gypsum. Since at a somewhat lower temperature at which evaporation of a marine basin may be assumed to have taken place the conditions in all probability will not be greatly modified, a large proportion of CaS0 4 may always be expected to be deposited as gypsum. Sedimentary marine deposits of pure anhydrite must therefore either be at least partly derived from originally deposited gypsum or have been formed close to or above 42°C, the transition point of the two minerals. These results indicate that the first-formed product will always be gypsum because most evaporites probably were deposited at temperatures below 42 °C. Only when the volume of the sea water is reduced to about one-fifth the initial volume is direct precipitation of anhydrite possible. Gypsum is currently being deposited in the Gulf of Karabugas on the eastern side of the Caspian Sea and in the Great Bitter lake of Suez. Because most gypsum deposits grade into anhydrite at depth, most geologists regard the anhydrite as the original material to be precipitated. Anhydrite is found at depth in the Maria formation in the Little Maria Mountains just east of the Palen Mountains. It is prob- able that drilling in the Palen deposits will also reach anhydrite. Thus, the research of Posnjak and the current deposi- tion of gypsum from sea water are convincing argu- ments for primary deposition of gypsum; on the other hand, the gradation of gypsum to anhydrite at depth suggests that the original material was anhydrite. In order to reconcile these views it is necessary to show that anhydrite can be derived from gypsum. The anhydrite is then reconverted to gypsum by ordinary weathering. The United States Bureau of Mines (Farnsworth, 1924), which conducted some experiments on the sta- bility relations of gypsum and anhydrite, found that anhydrite placed in pyrex glass bombs with water was Figure 10. Same field as figure 9, with crossed nicols. unchanged under a temperature of 210° C and a pres- sure of 19 atmospheres. Gypsum similarly treated changed over to anhydrite at a temperature of about 160° C. It was concluded that anhydrite is the stable form of CaS0 4 under conditions of high temperature accompanied by pressure. Posnjak (1938) was able to form a small amount of anhydrite by prolonged heating of gypsum at less than 200° C. The rate of formation of anhydrite gradually increased with increasing temperature. He made no ref- erence to the possible effect of pressure. Bowles and Farnsworth (1925) pointed out that the combined volume of anhydrite and water is more than the volume of the gypsum ; therefore, unless the water is immediately removed from the system, gypsum sub- jected to uniform pressure will remain gypsum. How- ever, they indicate that under conditions of high tem- perature and pressure, temperature is the controlling factor, overcoming entirely the retarding effect of pres- sure. Extrapolating from a surface temperature of 20 °C using geothermal gradient of 1°C per 30 meters, a gypsum bed would have to be at a depth of 5400 meters in order to reach a temperature of 200° C. The effective temperature in the transformation of gypsum to anhy- drite will probably be slightly lower as the time factor increases. Much more experimental work on this prob- lem is needed, especially with respect to the effect of stress. Unfortunately, geologic evidence supporting the conversion of gypsum to anhydrite is lacking.* With regard to the Palen Mountains deposit, the ex- treme deformation and absence of drilling data make it difficult to determine whether gypsum or anhydrite was the primary mineral. The sample of gypsum pic- tured in figure 10 shows a fair preferred orientation, but, in view of Fairbairn's (1949) studies the signifi- cance of orientation in gypsum must be viewed with caution. Fairbairn found an identical orientation in each of three diagrams prepared from mutually perpendicu- lar sections of a foliated gypsum rock and suggested the possibility of the reorientation of gypsum grains during * Two recent papers cite evidence for this conversion (Goldman, 1952 ; Stewart, 1953). Palen Mountains Gypsum Deposit 13 the preparation of thin sections. Megascopic examina- tion of the gypsum beds in the Palen deposit reveals that deformation fabrics do exist. It is improbable that a tectonite fabric in anhydrite would be maintained in gypsum formed by hydration because of the large in- crease in volume, and therefore the gypsum itself must have been deformed. Furthermore, it seems more likely that blocks and fragments of marble now occurring as isolated masses could be jostled about more easily in gypsum than in the more competent anhydrite. The Maria formation may be compared with the Cas- tile formation in west Texas and New Mexico (King, 1947, pp. 470-477). The Castile formation consists of laminated anhydrite averaging 1,250 feet in thickness, halite locally up to 900 feet in thickness, and a minor proportion of calcite in the form of small lenses. The presence of halite indicates that the high salinity re- quired for the precipitation of anhydrite must have been reached. Also, the laminae are undisturbed. There is no doubt, therefore, that here is a good example of initial anhydrite deposition. This thick, almost continuous sec- tion of anhydrite with some halite contrasts greatly with the interbedded marble and gypsum of the Maria formation. It does not seem likely that water from which CaC0 3 and CaS0 4 were being precipitated alternately would reach the necessary concentration for deposition of anhydrite. Furthermore, the Palen deposits contain no halite, nor are there any chloride minerals present that could have been derived from halite through metamorphism. Taking into account the compelling evidence for the initial deposition of CaS0 4 as gypsum and the strong probability that anhydrite occurs at depth, the follow- ing history of the gypsum is suggested : (In order of decreasing age) 1. Deposition of gypsum. 2. Deep burial allowing anhydrite to form under conditions of high temperature. (The period of pre-intrusion regional metamorphism and de- formation). 3. Erosion prior to the deposition of the McCoy Mountains (?) formation. As the anhydrite was now in the zone of weathering, some gyp- sum again formed. 4. Burial followed by igneous instrusion. It is difficult to de- termine whether this burial was deep enough to allow a second conversion of anhydrite to gypsum. The low degree of regional metamorphism of the McCoy Mountains beds suggests that the conversion may not have taken place. Furthermore, the strong deformation now visible in the gypsum probably dates from the beginning of the post- intrusion period of deformation rather than being limited to only the very recent faulting. There was probably some local dehydration next to intrusive contacts. 5. Present period of erosion. Any anhydrite near the intrusive has been rehydrated. This hypothesis requires that most of the gypsum of this deposit, though secondary, is older than the intru- sive rock, or pre-Nevadan, as indicated by table 5. Marble. The marbles are predominantly white- or buff-colored, finely crystalline rocks. A smaller propor- tion are pink, dark brown, or gray. There are no dolo- mitic phases. The calcite grains are anhedral in shape and generally have an interlocking texture. The grain size averages 0.5 mm in diameter. Minor impurities are quartz and tremolite. In the northeastern gypsum-bearing area thermal metamorphism was superimposed upon the already re- Figure 11. Predazzite in Maria formation. White blades are bru- cite ; gray, twinned grains are calcite. Plane polarized light. X30. Figure 12. Fine-grained, granular marble of Maria formation. The aggregate of white blades and the scattered small blades are a white chlorite (leuchtenbergite?) . One or two of the blades is white mica. gionally metamorphosed rocks. Marble containing mag- nesium was changed to periclase marble. By hydration of the periclase, the rocks were changed to brucite mar- ble, or predazzite (fig. 11). Marble with potash, silica, magnesia, and alumina was recrystallized to granular calcite and white mica. The white mica is being replaced by a white chlorite (fig. 12) which is probably the iron- free variety, leuchtenbergite. In those marbles contain- ing a white mica with some iron, the replacing chlorite is green. An incompetently folded, laminated series of marble and siliceous marble occurs in the southern gyp- sum belt. The laminae, which are only an inch or two thick, consist of marble, tremolite marble, quartzose marble, and calcareous quartzite. Lime-silicate Marble. Lime-silicate marble, dense, fine-grained, white rocks (fig. 15) which lie directly above the meta-igneous rocks, form the series of sharp 14 Special Report 36 Figure 13. Green, fine-grained epidote-tremolite feldspathic quart- zite, Maria formation. Tremolite (gray blades) shows good pre- ferred orientation. Dark granules are epidote. White matrix con- sists of granular quartz and feldspar (albite or orthoclase). Plane polarized light. X30. Figure 14. Pink quartzite with white mica. Contains scattered euhedra of hematite which give the rock its characteristic pink color. Crossed nicols. X30. peaks across the middle of the area. They are strongly thermally metamorphosed. Their texture is granoblastic and the grains average 0.1 mm in diameter. Calcite with smaller amounts of wollastonite and diopside are the principal minerals, while a minor amount of clear, gran- ular alkali feldspar is present. Locally, scattered aggre- gates of pale yellowish green epidote, 1-2 mm in diam- eter, are developed. Quartzite. Quartzite occurs as thick beds, as laminae several inches thick interlayered with marble, and as narrow partings in gypsiferous beds. It is pink, white, and green in color and is fine-grained, the average grain size ranging from 0.1 mm to 0.5 mm in diameter. Epi- dote-tremolite feldspathic quartzite (fig. 13) is but one example of the many mineralogical varieties found. Fig- ure 14 is a microphotograph of a relatively pure quartz variety containing white mica and scattered euhedra of hematite. These samples are from the two distinctive quartzite beds lying stratigraphically above the lime- silicate marble series (section B-B', fig. 3). In contrast to the laminated marble-siliceous marble series, the thick beds of quartzite are cut by closely spaced joints that divide the rock into rectangular blocks whose greatest dimensions are seldom more than 3 inches. The quartzites generally show a planar structure due to the parallel orientation of mica plates and tremolite- actiholite blades or fibers. Grit. This term is used to distinguish a group of rocks which are characterized by a marked hiatus in grain size. Grains of feldspar and quartz 0.2-0.3 mm in diameter are surrounded by an extremely fine-grained matrix of actinolite, biotite, sericite, epidote, sphene, Figure 15. Lime-silicate marble, Maria formation. Very fine- grained calcite (gray), with granular wollastonite (dark) and diop- side. Plane polarized light. X30. Figure 1G. Actinolite feldspathic grit. Microcline, albite, and quartz in an extremely fine-grained matrix of these same minerals. The actinolite and green biotite, epidote, and sphene are interstitial to the larger, subrounded grains. Crossed nicols. X30. Palen Mountains Gypsum Deposit 15 feldspar, and quartz (fig. 36). The larger, subrounded feldspar and quartz grains are etched to a slight extent by the matrix minerals. There are all gradations between typical grit and equigranular quartzite. Minor Rock Types. No large beds of paraschist are present in the Maria formation. The main occurrence of schist is as thin laminae in gypsiferous sections. Within the marble are scattered, irregular lenses of tremolite-actinolite amphibolite. It is not known whether these are sedimentary or igneous in origin. The unusual contact metamorphic types are discussed in the section on contact metamorphism. Environment of Sedimentation The original sedimentary section consisted mainly of limestone, gypsum, gypsiferous shale and sandstone, sandstone, feldspathic sandstone, and argillaceous, feld- spathic sandstone (grit). Some of the limestone was dolomitic ; however, dolomite does not seem to have been an abundant mineral in this area. The laminated mar- ble and siliceous marble probably is the metamorphosed equivalent of a slightly dolomitic thinly layered lime- stone and chert sequence. Such a group of sediments was probably deposited under arid or semiarid climatic conditions in a broad, shallow basin on a continental platform. The rate of evaporation must have been great enough to allow the precipitation of gypsum at times when the basin was partially or wholly isolated. From time to time the salt concentration of the basin water was lowered enough by fresh supplies of sea water so that limestone was de- posited instead of gypsum. The presence of some detrital material indicates that there were periods when the nor- mally dry streams entering the basin were filled with sediment-bearing water. The sediments deposited in such an environment belong to the "Foreland," or "Plat- form," facies (Petti John, 1949) consisting of the aerobic and saline subfacies. Many geologists have attempted to define a process by which thick deposits of CaS0 4 were formed, in spite of the fact that the salt comprises only 3.6 percent of the total salts in the sea. For an excellent summary of the numerous ingenious theories the reader is referred to Pettijohn (1949, pp. 354-362). Intensity of Metamorphism. An evalution of the met- amorphic rocks in terms of the facies concept is difficult because the area has been subjected to several periods and kinds of metamorphism. Furthermore, the area was not uniformly influenced. For example, the southern gypsum belt was little, if at all, affected by the intru- sion of the igneous rock. The mineralogical and physical characters of these rocks reflect the period (or possibly periods) of regional metamorphism with some modifica- tion by late hydrothermal activity. On the other hand, the rocks in the hanging-wall zone of the intrusion were completely reconstituted by thermal metamorphism. The feldspathic quartzites and grits of the southern gypsum series are probably the best indicators of the grade of regional metamorphism. These rocks have an assemblage consisting of quartz-albite-microcline-biotite- epidote-actinolite-chlorite. This assemblage belongs to the biotite-chlorite subfacies of the greenschist facies (Turner, 1948, p. 94). To this subfacies belong the rocks \.* Figure 17. Quartz grit, McCoy Mountains (?) formation. Sub- rounded, medium-grained quartz in a matrix of very fine-grained quartz, pale green sericite, and epidote with magnetite, chlorite, muscovite, and apatite. Crossed nicols. X30. Figure 18. Porphyritic quartz latite, McCoy Mountains ( ?) for- mation. Embayed quartz (q), altered plagioclase oligoclase? (p), and slightly altered orthoclase (o) in a microcystalline ground- mass of sericite, biotite, epidote, calcite, and sphene. Crossed nicols. X30. of the biotite zone as defined for pelitic schists. This is low-grade regional metamorphism. The lime-silicate marble is an excellent example of a contact metamorphic rock. The calcite-wollastonite- diopside assemblage is stable in high-temperature facies (pyroxene hornfels facies) in general, including the high-temperature subfacies of the amphibolite facies (Turner, 1948). McCoy Mountains (?) Formation The McCoy Mountains (?) formation, which consti- tutes the entire area of the Palen Mountains south of the gypsum deposits, was examined only in the imme- diate vicinity of its contact with the Maria formation. Grit (fig. 17), phyllite, argillite, conglomerate, quartzite, and porphyritic quartz latite (fig. 18) that have been 16 Special Report 36 Figure 19. Zoisite-epidote-garnet quartzite, McCoy Mountains (?) formation. Veinlet consists of calcite (gray), quartz (white), and epidote (gray, high relief). Plane polarized light. X30. subjected to slight regional metamorphism are highly- altered, probably by hydrothermal solutions which found easy access in the highly sheared and broken rock typical of the contact zone. Two examples of these altered rocks are shown in figures 19 and 20. Because these rocks are so strongly altered hydro- thermally it is difficult to compare their degree of meta- morphism with that of similar rocks in the Maria for- mation. The general impression gained was that the McCoy formation shows a lower grade of regional meta- morphism. The pebbles in the conglomerates are unde- formed (although quartzite pebbles can probably survive a fairly high grade of metamorphism) and a thin-sec- tion of phyllite showed a very early stage of recrystal- lization from the original shale. The age relations of the McCoy Mountains formation and the Maria formation are not clearly established in this area. For this reason the beds south of the gypsum deposit are designated as questionable McCoy Mountains formation. Meta-lgneous Rocks Both the Maria formation and the McCoy Mountains (?) formation were intruded by an igneous complex. Only the igneous-Maria intrusive relations are present in the area mapped, but a traverse at the east end of the south border ridge south of the map area revealed that variably porphyroblastic feldspathic meta-igneous rock like that in the Maria formation cuts into and sur- rounds irregular masses of volcanic rock and con- glomerate. These intrusive rocks are now strongly foliated, fine- grained, biotite-epidote-albite-quartz schists (fig. 21) that retain few of their original igneous features except a few relict crystals of zoned intermediate feldspar (fig. 23 ) . Standing athwart the schistosity in some samples of schist are aggregates of extremely fine-grained epidote surrounded by granular quartz and albite. These may represent completely recrystallized, somewhat calcic plagioclase feldspar. The rather high quartz content (25- 30 percent), considerable biotite, and the relict andesine suggest that the original igneous rock was a diorite or Figure 20. Conglomerate. Quartzite pebble in a fine-grained matrix of garnet and epidote. Some replacement is beginning to take place along the outer edge of the pebble. Note the small fibers of actinolite. Crossed nicols. X30. quartz diorite, or possibly as acidic a variety as a grano- diorite. The foliation dips north in general. A question arises concerning the origin of the foliation in the meta- igneous rocks. As they are strongly schistose over their whole extent and show little trace of their igneous origin, the first assumption is that the intrusive action occurred before the onset of the regional deformation which so profoundly affected the Maria formation. How- ever, the folds and much of the faulting in the meta- sediments end abruptly at the intrusive contacts; simi- larly, the meta-igneous rock is not intensely deformed throughout its whole mass as are the metasediments. As the early regional metamorphism was terminated by in- tense deformation, the intrusion must have taken place somewhat later. Also, the highly amphibolitic rocks in the contact zones have well-developed decussate textures which could hardly have been maintained during such severe deformation. The fact remains, nevertheless, that the original igneous rock has been metamorphosed. The Figure 21. Meta-igneous rocks. Biotite-epidote-albite-quartz schist. Plane polarized light. X30. Palen Mountains Gypsum Deposit 17 schist probably represents a low to medium grade of regional metamorphism (Harker, 1939, pp. 287-289). The amphibolitie rocks, because they represent a facies which is commonly a product of medium- and high-grade regional metamorphism, may have been able to resist any significant recrystallization in a lower intensity of meta- morphism. There must have been, therefore, a second period of regional metamorphism ; and it may well rep- resent the first effect produced by the deep seated igne- ous activity that later culminated in the emplacement of the granite comprising the Granite Mountains. Metasomatic activity that followed the second period of regional metamorphism has largely obliterated the schistosity of large parts of the intrusive and introduced abundant porphyroblasts of alkaline feldspar. Feld- spathization is most pronounced along the south border of the intrusive and decreases northward. Figure 22. Porphyroblastie meta-igneous rocks. Large perthite porphyroblast with quartz (q) inclusions is being replaced by albite (a), and is cut by an irregular veinlet of clear, granular orthoclase and quartz (left center). The large albite fragment is also cut by orthoclase (o). Crossed nicols. X30. Gravels Approximately a quarter of the area is covered by talus or by a red, calcareous, stratified gravel varying from a few inches to over 30 feet in thickness. Recent gullying has in some places exposed the gravel-bedrock contact which is sharp and is essentially a level surface with little relief (fig. 4). One or two terraces are present along some of the washes. In the middle and west portions of Palen Pass the gravels lie upon an older series of white, calcareous, stratified gravels. These lower gravels are cut by north- west-trending steep faults dipping either to the north or the south. The displacements along these faults are mainly of the normal type. The white gravels have an average dip of about 15° to the north, but in some of the fault blocks the dips are very steep or may be toward the south. The contact between the two gravel series is sharp. The elevation of the contact is not consistent from one spot to another and in several places the upper gravels are missing. A part of this difference in eleva- tion is due to the very recent faulting which has dis- placed the contact several inches or even several feet. Figure 23. Porphyroblastie meta-igneous rocks. Relict plagioclase in perthite. Core (Na-andesine) highly altered to sericite and epi- dote ; clear, corroded border is Na-oligoclase. Crossed nicols. X30. Some of these recent breaks are renewals of earlier movements along faults in the lower gravels ; others are new and show the same amount of displacement in both gravel series. No attempt is made here to date these gravels except to suggest that the upper red gravels are probably Re- cent and that the lower gravels are Pleistocene or per- haps Tertiary in age. STRUCTURE The deformation of the rocks of the Maria formation has been profound. Evidence of folding, faulting, crush- ing, brecciation, shearing, and jointing is present, rang- ing in scale from the large, mappable features down to those microscopic in size. Only the most significant struc- tural features are included in the map. Most, if not all, of the lithologic boundaries mark loci of movement ; very few bedding planes and depositional contacts remain. Figure 24. Meta-igneous rocks. The dark aggregate (right center) is very fine-grained epidote, surrounded by granular albite and quartz (white). This may represent a completely recrystallized, zoned plagioclase. Plane polarized light. X30. 18 Special Report 36 Five principal bands that extend roughly east across the range are distinguishable. From north to south these are: (1) Metasedimentary rock, principally in the northwest part of the area. The northwestern gypsum-bearing series is in the eastern part of this band. (2) Meta-igneous rock, extending about two-thirds of the way across the range from west to east. (3) Metasedimentary rock. The northeastern gypsum-bearing series is in the eastern end of this band, while lime-silicate marbles occur along the south contact with meta-igneous rock. In the western part of this band the intrusive-meta- sedimentary relations are so complex that a portion was mapped as a meta-igneous and metasedimentary complex. (4) Meta-igneous rock, mostly porphyroblastic. (5) Metasedimentary rock, containing the southern gypsum- bearing series in the eastern part. As in band 3 the struc- tural complexity increases to the west, and a portion was mapped as a highly deformed complex. Deformation Before the Quartz Diorite Intrusion. All of the folding, most of the internal deformation, and a good share of the faulting in the Maria formation took place before the period of intrusion. These folds and faults are truncated by the metamorphosed quartz di- orite (fig. 5). Folding has modified the north-dipping east trend of the lithologic bands. The gypsum and marble area in the northwest corner of the deposit is a northwest-plunging anticline complicated by small cross warps and by elon- gate furrows. Similar small scale warping is present at the southeast end of the deposit. The reconstruction of these folds is based on the attitudes of lithologic con- tacts. Deformation within the various rock types is so severe that bedding is obscured. The marbles are intensely folded, broken, and sheared. Folds within a series of marbles have amplitudes ranging from 20 feet down to fractions of a foot. These folds are tight, many are isoclinal, and the axial planes vary in dip from vertical to almost horizontal. These planes dip to the north in general although in some places they too have been cross folded. This incompetent, plastic folding is strikingly revealed in the thinly laminated marble-siliceous marble series which makes up the north side of the high east- west ridge in the southeast part of the deposit. Figure 6 shows highly contorted beds of this series lying on gyp- sum in the western part of the area. Probably closely associated in time with the folding and internal deformation was considerable faulting along bedding planes along with higher-angle thrusting. The bedding plane movements and thrusting developed zones of brecciation (fig. 7) and drag folding. The con- torted marble-siliceous marble series may thus represent drag due to faulting rather than simple drag folding of an incompetent bed due to minor adjustments and slip- ping of competent strata during folding. Two thrusts can be definitely mapped. The first is in- dicated at the southeast border of the area. This fault strikes N. 22° W. and dips 30° to the west. The sole of the thrust is marked by a tectonic breccia containing broken blocks of marble as much as three feet in diame- ter. The rocks to the east of the thrust consist only of marble and gypsum in contrast to the much more varied section to the west. The thrust is terminated at its north end by the intrusive and at the south end by a steep reverse fault. The direction of movement of the upper plate was probably north to south as indicated by the east west, north-dipping imbricate faults. North-south compression is further suggested by steep, north-south tear faults and by steep NE-SW and NW-SE strike-slip faults. The irregular north-trending trace, therefore, is not the frontal margin of the thrust plate but rather is the eroded east end of the overthrust. The westerly dip probably is part of an undulation on a thrust surface dipping to the north. It could not be determined whether the undulation developed during thrusting or represents later warping of the thrust surface. The frontal margin of the thrust was cut out during a later period of defor- mation by the steep fault to the southeast. Another thrust is indicated on section B-B' drawn across the northeast part of the area. The base of the thrust is marked by a zone of coarse, granulated marble (fig. 7). The thrust dips approximately 30° to the north. Just to the southwest of the cross section an imbricate fault in the upper plate dips 50° to the north. Unfortunately these thrusts are traceable only over a very short distance because they are truncated by the Figure 25. Feldspathized quartzite. Microcline grains replacing quartz. This rock is typical of the many irregular masses occurring in the Maria formation. Crossed nicols. X30. Figure 26. Skarn. Garnet (dark gray) cut by calcite veinlets (light gray). Some granular quartz (white). Plane polarized light. X30. Palen Mountains Gypsum Deposit 19 Figure 27. Tremolite-marble. Specimen taken at contact with in- trusive. Tremolite (t), calcite (c). Plane polarized light. X30. later intrusive rock. As nearly as can be ascertained, these thrust traces have not been offset, suggesting that thrusting culminated this period of the deformation. Deformation after the Quartz Diorite Intrusion. Faulting apparently has taken place repeatedly since intrusion of the quartz diorite. Folding does not seem to have recurred since the early period of deformation. The south contact of the intrusive, which dips north at angles of from 45° to nearly 90°, has been consider- ably modified by faulting. All along its south boundary the porphyroblastic rock is intensely sheared and broken to distances of from 20 to as much as 500 feet away from the contact. Along the west end of the contact, zones of intensely crushed porphyroblastic rock as much as 12 feet wide are strongly stained red by iron oxide. In at least three places cross faults have offset the con- tact with displacements of 100 to 200 feet as measured along the fault traces. In the northeast section of the area east-striking faults are cut by several north- and northwest-striking cross faults. On one northwest-striking fault the strike slip displacement of the meta-igneous contact is 500 feet. The surface of this fault, which is exposed at the northwest end, dips 45° to the north. Ill-formed, horizontal grooves that are still visible in the gypsum hanging wall indicate that the resultant relative movement was almost wholly strike-slip in nature. The fault at the southeast border of the area separat- ing the Maria formation and the McCoy Mountains (?) formation dips steeply to the northwest. The marble along the fault is intensely brecciated. In several places the marble appears to be dragged down near the fault zone indicating a reverse component of movement. This fault is offset by later faulting for short distances at sev- eral places. The extensions of known faults into the meta-igneous rock are difficult to follow, although there is no doubt that many such faults exist within the meta-igneous series, as indicated by the many shear zones and by the sudden changes in attitude of foliation. The nature of the forces causing this period of defor- mation is less readily understood but it appears that they were vertically, rather than horizontally, directed forces. A good share of the deformation may be the re- sult of upward and outward directed components of forces due to the emplacement of igneous magma below and mainly to the north of the gypsum deposits. Displacement in the Recent gravels in Palen Pass in- dicates that some of the faulting is very recent. The entire western half of the southern gypsum belt was mapped as a highly deformed complex of gypsum, marble, laminated marble, siliceous marble, and gypsif- erous schists. This area is so highly deformed that trends can no longer be traced without extremely detailed work and even then a coherent picture may not be attained. The northerly dip is still visible, however. Many of the fractures are marked by quartz veins. Jasper breccias in several places give evidence that move- ment has occurred more than once in these zones. Deformation has produced well-developed shear folia- tion and granulation in the marbles. Crush breccias, flaser structure, and mylonites are also present. These structures were formed during all the periods of defor- mation. Figure 28. Hornblende (h), epidote (dark, granular grains), sphene (s), and granular bytownite (white). Specimen taken in intrusive rock at contact with tremolite-marble. Plane polarized light. X30. METASOMATISM Contact Metamorphism The intrusion of the quartz diorite produced marked contact metamorphism upon the already regionally meta- morphosed and deformed Maria sediments. These effects were most pronounced in the roof zone of the intrusive where the volatile constituents were concentrated. This upper contact zone is exceedingly complex and is marked by very irregular and intimate intrusion of small dikes, sills, and tiny apophyses of igneous material. This en- vironment was highly favorable for strong chemical ac- tivity and resulted in transfer of material between the intrusive and the country rock. The lime-silicate marbles and other marbles have been described in a previous section. Skarns of garnet (fig. 26), epidote, and amphibole are common in the invaded rocks. The border phase of the intrusive commonly contains an abundance of calcic min- 20 Special Report 36 Figure 29. Granular magnetite and calcite. Specimen from 6-inch contact zone in marble xenolith. Plane polarized light. X30. erals such as hornblende, epidote and granular zoisite, calcic plagioclase feldspar, and, finally, euhedral sphene (fig. 28). Undoubtedly there was some assimilation of the country rock. Contact zones in xenoliths of marble lower in the in- trusive generally are narrow, being only a few inches or a few feet wide. Some zones have tremolite bands par- allel to the contact (fig. 27). In one sample the tremolite occurs as radial aggregates up to a half inch in diameter. The calcite merely recrystallized to a granular aggre- gate of anhedral grains. In several places the evidence suggests that some iron may have been introduced by the intrusion as shown by amphibole-iron ore masses (fig. 30) in marble xenoliths and by narrow siderite-magnetite granulites (fig. 29) in the marble next to the intrusive. The effect of intrusion on the gypsum is difficult to determine because few clean contacts are found on the surface. Several that could be studied were found to be fault contacts. Probably the only effect would be dehy- dration and this would be wiped out as soon as the ma- terial reached the zone of weathering. The contact action of the very small, isolated in- trusions scattered throughout the area appears to have been negligible. Rather, the intrusions themselves were strongly modified and now are high in such calcic min- erals as hornblende, epidote, and sphene. Feldspathization Subsequent to the intrusion and regional metamor- phism of the quartz diorite, the whole area, including the McCoy Mountains beds, was subjected to fairly in- tense metasomatic activity. The effects are particularly evident in the meta-igneous rock. The southern third of this rock contains porphyroblasts of alkaline feldspar. The feldspars vary in soda content from place to place so that they may be microcline, perthite, or antiperthite. The porphyroblasts are best developed along the south border of the intrusion and gradually decrease in size an abundance toward the north. At one point along the south contact of the meta-igneous series the lower 6 inches of the rock is almost completely sericitized ; above Figure 30. Tremolite-magnetic rock. Contains some calcite and chlorite. Material occurs as irregular masses in some of the marble xenoliths. Plane polarized light. X30. it is a foot thick, massive, granitic-appearing zone. The original intrusive contact of meta-igneous rock appar- ently was modified by faulting and this fault zone un- doubtedly was the avenue up which the feldspathizing solutions rose. The feldspars are not always large and well-developed within the porphyroblastic phase of the meta-igneous rock. One interesting locality exposes a massive, faintly foliated non-porphyroblastic rock which grades abruptly into porphyroblastic rock with schist "inclusions." The former contains abundant fine-grained albite and micro- cline with a few ill-formed coarser feldspars. It is pos- sible that the formation of the large porphyroblasts has forced much of the biotite to concentrate in aggregates resembling inclusions and thus may represent a process of metamorphic differentiation. The micas in the "in- clusions" still parallel the old foliation and the long dimension of the aggregates trend roughly in the same direction. Within non-porphyroblastic phases of the meta-igneous rock, there are zones where incipient feldspathization is visible. These zones may be marked by regularly distrib- uted, small irregular feldspar grains or by bands of poorly developed feldspar grains parallel to the foliation. These bands do not have sharp boundaries but rather grade into the schist. These bands may be only thin seams or may be several inches wide. A paragenetic sequence for five different ages of feld- spar was determined. Starting with the oldest, these feldspars are: 1) relicts of zoned intermediate plagio- clase; 2) recrystallized, fine-grained, granular albite; 3) perthite porphyroblasts; 4) albite, with myrmekite; and 5) fine-grained, granular orthoclase. Figure 22 shows a large perthite being cut by albite and both in turn being replaced by clear, granular orthoclase. In the north-central part of the map an area of highly feldspathized and recrystallized meta-igneous rock is present. This area seems to be limited on all sides by zones of intense shearing and granulation. The rocks show complicated recrystallization textures and as meta- somatism increases, the amount of biotite decreases and Palen Mountains Gypsum Deposit 21 " . r yV>^ rJr * ■*: J fr~ 1 ' Jl " fc -v » < Figure 31. Fluorite (f) introduced into an intensely sheared muscovite-quartz schist. Plane polarized light. X30. potash feldspar, muscovite, iron ore increase, and even rutile appears. The same direction of foliation is found in this material as in nearby meta-igneous rock. The general shape of this area and the distribution of steep shear zones around its perimeter and inside suggest that this section may lie above a protuberance of a deep seated granitic mass. As the granite punched its way upward into the meta-igneous rocks, it created zones of intensive fracturing overhead through which vapors and fluids emanating from the magma found easy access. This deep-seated granite is probably genetically related to the granite of Granite Mountain to the north. Crosscutting these feldspathic zones are aplitic and pegmatitic dikes and sills, generally less than a foot in width. At first glance these appear to have sharp con- tacts, but upon closer inspection many have hazy bound- aries and have thin stringers of feldspathization extend- ing out across and along foliation planes of the schist. The meta-igneous rock is commonly slightly more feld- spathic next to these dikes. Also, relict biotite in the aplites maintain their original orientation. Nevertheless, some aplites and most of the truly pegmatitic bodies have very sharp contacts. It is a problem to decide where replacement by permeating feldspathic solutions ends and true intrusion by granitic magma begins. No doubt the borders of true magmatic intrusions will show grani- tization phenomena. Similarly, the meta-igneous rock along the northeast side of the area becomes increasingly more feldspathic and recrystallized as the Granite Mountains are ap- proached. In the very southeast end of this range the same general relations exist as in the gypsum area, namely, meta-igneous rock in various stages of feld- spathization and recrystallization with later crosscutting sills and dikes of aplite and pegmatite. Small feldspathic zones (fig. 25) are present in the meta-sediments but whether they are replacements, or fillings, or intrusions cannot be ascertained at this time, because no where 'was it possible to view their contact relations. The sequence of events suggested by the above obser- vations is as follows: Emanations from a deep seated Figure 32. Kyanite crystals in quartz-sericite schist. A few small, dark grains of magnetite. 1'laue Dolarized light. X30. magma rose along previous fault zones, fractures, joints, and foliation planes causing varying degrees of feld- spathization and recrystallization. As the magma rose more fractures developed, especially over high points, with the result that one large area and many smaller areas of intense recrystallization were created. Finally, a small amount of the granitic magma itself reached the levels now exposed and is present as small bodies of aplite and pegmatite. In this area, at least, no bodies of granite are present at the surface. Other Mineralization Veins containing one or more minerals such as epi- dote, calcite, quartz, magnetite, hematite, pyrite, musco- vite, and serpentine are found throughout the whole area, including the McCoy Mountains (?) formation. Green copper stains and small quantities of copper sul- phides are present in a few quartz veins. A small amount of gold has been found by prospectors. Irregular stringers and pods of white quartz are found in all the rocks. Thin jasper veinlets are present in the marble in the northwest part of the area. Hydrothermal fluorite (fig. 31), kyanite (fig. 32), and apatite were found in a few shear zones. The veins and shear zone-type mineralization cut feldspathized rock, aplites, and pegmatites and thus represent the last stage of mineralization. GEOLOGIC HISTORY Table 5 is a reconstruction of the chronology of the geologic events which have been involved in the history of the Palen Mountains gypsum deposit. Dating is tenu- ous and even the relative positions of several of the events are debatable. However, the study of the deposit leads the author to think that the succession of events shown below is the one most nearly compatible with the facts so far available. Events two and three were the beginning and end of what was undoubtedly one continuous period of defor- mation. The early stresses, along with the heat and pressure occasioned by deep burial, brought about re- crystallization and the development of new minerals. With the passage of time, the stresses intensified until 22 Special Report 36 they culminated in thrusting accompanied by both brec- ciation and incompetent folding. Events five, six, and seven may well represent another long period of intrusion and deformation climaxed by the emplacement of the granite to the north. CORRELATION Deposits of gypsum in the Little Maria Mountains (Ver Planck, 1950, p. 227; Surr, 1911, p. 787-790) are believed to belong to the same formation as the gyp- siferous section a few miles to the west in the Palen Mountains. The beds form part of a series of slightly metamorphosed sediments that cross the range east-west. The sediments are bordered on the north by granitic rocks and on the south by gneiss. The rocks of the gyp- sum belt are quartzite, crystalline limestone, and quartz- albite-mica schist, all dipping 50 to 80 degrees to the northwest. The gypsum occurs in limestone as persistent beds up to 50 feet thick and in the schist as lenticular bodies which have a more limited extent along the strike. The gypsum is a coarse-grained snow white aggregate of transparent grains. In many places the gypsum con- tains thin layers and lenses of schist. Schist is also found in the gypsum interbedded with limestone. An- hydrite is found at depth. The alteration of anhydrite to gypsum apparently is controlled by fractures and other openings. The iron ore deposits of the Eagle Mountains (Had- ley, 1948, p. 4) occur in contact metamorphosed sedi- ments, which have been folded, faulted, invaded by ir- regular sill-like bodies of quartz monzonite, and cut by dikes of fine-grained igneous rock. Table 2 is a reproduction of the chart by Hadley (1948, p. 5) of the rock units in the eastern part of the Eagle Mountains iron district. A comparison of his section with that of the Palen Mountains shows certain broad similarities. The vitreous quartzite does not ap- pear to be represented in the gypsum deposit. The feldspathic quartzite possibly can be correlated with the feldspathic beds in the south part of the A-A' cross section. The sporadic bodies of quartizite in the meta- igneous rock which lie stratigraphically below the lime- silicate rocks (southeast end of section B-B') cropping out along the middle of the gypsum claims in the Palen Mountains may be the equivalents of the upper quartz- ite beds of Hadley. The lime-silicate rocks which he de- scribes may well be contemporaneous with the lime- silicate beds in the gypsum area. The large amounts of marble, gypsum, and meta-igneous intrusives do not seem to be represented in the Eagle Mountains.. The occurrence of iron ore in small quantities in the Palen Mountains under very similar conditions as those in the Eagle Mountains certainly suggests that these iron ore deposits may be of the same age. Hewett (1931, p. 87) reports beds of gypsum in the red shaly sandstone at the top of the Supai formation and between the two limestone members of the Kaibab limestone. Both these formations are Permian in age. The lowest Triassic, the Moenkopi (p. 32) consists of basal conglomerate with sandstone, limestone, and red and green shales and tuff beds. He describes a greenish epidotic alteration of pebbles, the alteration preceding the rounding of the pebbles during transportation. Simi- lar pebbles are found in the congromerate of the McCoy Mountains ( ?) formation along the south border of the Palen Mountains gypsum deposit. Table 2. Rock units in the eastern part of the Eagle Mountains iron district, after Hadley * Rock units in Approximate order of age Lithology thickness, feet Slope wash Coarse sand and gravel, eom- and alluvium monly cemented by caliche. Lo- cally contains abundant boulders of iron ore. to 100 + Dike rocks Syenite porphyry, diabase, gran- ite. Quartz monzonite Conglomerate Lime-silicate rocks Upper-ore bed Quartzite Coarse-grained and porphyritic quartz monzonite with biotite and hornblende. Metamorphosed limestone con- glomerate. Contact-metamorphic rocks com- posed of mica, actinolite, diop- side, feldspar, and quartz. Iron ore with lenses composed dominantly of lime-magnesia sil- icates. 400 + 100 30 to 300 White to dark-gray glassy quartzite ; sporadic bodies of iron ore. 200 to 300 Lower ore bed Iron ore with lenses composed dominantly of lime-magnesia silicates. Feldspathic quartzite Vitreous quartzite Coarse- to fine-grained feld- spathic quartzite and schist. Pure, coarsely recrystallized quartzite. Total average thickness of metamorphic rocks 40 to 140 50 to 150 150+ 1250 * Hadley, J. B., Iron ore deposits in the eastern part of the Eagle Mountains, Riverside County, California : California Div. Mines Bull. 129, p. 5, 1948. Longwell (1949, p. 929) reports that the Moenkopi formation in the Lake Mead region of Nevada is ap- proximately 1500 feet thick, consisting of an upper con- tinental member of shale, largely chocolate brown, in part gypsiferous, with interbedded sandstone, and a lower marine member consisting of interbedded lime- stone, shale, sandstone, and gypsum. The Kaibab lime- stone is 600 to 800 feet thick and consists of an upper and a lower limestone member separated by gypsum. Beneath the Kaibab are about 2,000 feet of red beds con- sisting of brick red and mottled sandstone with sub- ordinate sandy shale. Harder (1909, p. 409) gives no age for the Palen gyp- sum deposit. In appearance, texture, and metamorphism, he states, the rocks resemble others of the southeastern Mojave Desert, which have been generally considered pre-Cambrian. He also remarks in his paper on the Eagle Mountains (1912, p. 503) that the later intrusives and flows are considered to be of Mesozoic and later age. Table 3 is a reproduction of his general section of rocks in the southeastern Mojave and Colorado Deserts (1912, p. 19). His section was based on observations as he traveled from one iron ore area to another. No fossils were found so the ages are unknown and only the gen- eral relations are given. He also notes that the rocks are distributed through the ranges, some of which con- tain most of the formations in the series, but others con- Palen Mountains Gypsum Deposit 23 Table 3. General section of rocks in the southeastern Mojave and Colorado Deserts, after Harder* 10. Basalt. 9. Unconsolidated desert deposits. 8. Basalt; slightly tilted. 7. Partly consolidated shale, sandstone, and conglomerate ; hori- zontal. 6. Trachytic, andesitic, and rhyolitic flows; tilted and broken; probably Tertiary. 5. Red and brown sandstone, shale and conglomerate ; tilted. 4. Intrusive granite, syenite, monzonite, and diorite and their porphyritic phases in sills, dikes, and irreglar batholiths ; prob- ably Mesozoic. 3. Quartzites, crystalline limestone and dolomite and conglom- erates ; age unknown. 2. Purple and gray slates, shales, sandstones, and quartzites; age unknown. 1. Schists, crystalline limestone and dolomite, gneiss, and granite ; probably pre-Cambrian. Recent • Harder, E. C, Iron ore deposits of the Eagle Mountains : U. S. Geol. Survey Bull. 503, p. 19, 1912. sist almost entirely of one or two formations, so that it is difficult in many places to tell the relation between various rocks in the succession. Darton (1907, pp. 470-475) describes a series of sedi- mentary rocks in an area 50 miles north of the Palen Mountains on the Santa Fe Railroad which are Cam- brian in age. He also mentions a section of sedimentary rocks which might possibly be of Carboniferous age. No fossils were found. These later sediments are only slightly metamorphosed quartzites and crystalline lime- stone. The metamorphism was apparently caused by in- trusions of coarse granite. Harder (1909, p. 409) sug- gests that these rocks might be the same age as those in the Palen Mountains although the rocks in the Palen Range were much more metamorphosed by heat and pressure during the intrusion of the granite to the north. Table 4- Succession of geologic events in Eagle Mountain region, after Harder * 9. Erosion exposing all the rock formations, accompanied by the sculpturing of mountains and followed by the development of great outwash aprons around the mountains. 8. Doming of the sediments and intrusives, accompanied by great faulting. 7. During the later part of the intrusion, or shortly after it, iron ores and metamorphic minerals were introduced by deep-seated solutions replacing the dolomite and to a slight extent the quartzite. 6. The heat and pressure accompanying the intrusion recrystal- lized and consolidated the sediments and perhaps locally de- veloped metamorphic minerals. 5. Intrusion of quartz monzonite in two main sills, one in the vitreous quartzite below the dolomite lenses and the other in the quartzite conglomerate beds above the dolomite lenses. The first is discontinuous, though locally of great thickness ; the second is very thick and is continuous throughout the extent of the iron-ore belt. 4. Erosion interval followed by submergence and deposition of a great thickness of quartz sandstone ; then the deposition of arkosic sandstone, followed by the formation of beds and lenses of dolomite and quartz sandstone, and, lastly, of beds of sand- stone and conglomerate. 3. Great dynamic metamorphism, resulting in the alteration of granite porphyry to augen gneiss and the sediments to schists and crystalline limestone. 2. Intrusion into the sediments of porphyritic granite. 1. Deposition of sandstone, siliceous shale, and dolomite. Pleistocene and or Tertiary? Laramide? Nevadan? Triassic? Upper Paleozoic? Table 5. 11. Faulting of Recent gravels. 10. Deposition of gravels. 9. Small scale normal and reverse faulting. 8. Erosion; deposition of Tertiary (?) or Ple- istocene gravels. (Probably some faulting here.) 7. Intense metasomatic activity including feld- spathization and recrystallization followed by intrusion of aplites and pegmatites and finally by hydrothermal veins. 6. Faulting and perhaps very slight regional metamorphism. 5. Intrusion of quartz diorite into the Maria and McCoy Mountains beds. 4. Erosion followed by deposition and burial of McCoy Mountains ( ?) formation. 3. Intense cataclastic deformation including folding, faulting, breeciation, and crushing. 2. Deep burial followed by the onset of stresses responsible for the regional metamorphism of the sediments. 1. Deposition of the Maria sedimentary series of limestones, gypsum, sandstone, arkose, and some shale. * Harder, E. C, Iron ore deposits of the Eagle Mountains : U. S. Geol. Survey Bull. 503, p. 27, 1912. Miller (1944, p. 28) discovered crinoidal remains in the crystalline limestone associated with gypsum beds in the southeastern part of the large area of the Maria for- mation in the Maria Mountains. Although accurate age determination of these fragmental crinoidal remains was impossible, they were tentatively dated as post-Cam- brian, possibly Silurian. Miller concluded that a post- Cambrian Paleozoic age of the Maria formation was rea- sonably established. The McMoy Mountains formation, according to Miller, makes up the southern part of the Palen Mountains and the Coxcomb Mountains. This formation is dated as probably Paleozoic or Triassic by Miller (1944, p. 52). Harder (1912) dates these same rocks as two on his general section, the gypsum series as three, the quartz diorite as four, and the granites to the north as either one or four (see table 3). Brown (1923, p. 43) in his interpretation of Harder 's general section states that the first three series in the section are probably pre-Cambrian. His opinion is based mainly on the facts that these rocks are clearly the oldest in the region, having suffered the greatest metamorphism and being intruded or overlain by all the other series ; that they are similar to the pre-Cambrian rocks of the Grand Canyon and other parts of Arizona ; that although vast thicknesses of the sedimentary beds are exposed in places no fossils have ever been found within them. He considers it likely that the granite in Granite Mountain belongs to the oldest series. The next two series, he con- tinues, are closely associated with the first and make up the remaining masses of the Maria, McCoy, Palen, Cox- comb, and Chuckwalla Mountains. Except for some gra- nitic, dioritic, and porphyritic intrusives, they are prob- ably of Mesozoic age. He identified no rocks of Paleozoic age, but admitted that further studies might indicate that some formations described as probably pre-Cam- brian are probably younger. 24 Special Report 36 Lee (1908, p. 15) regarded the metamorphosed sedi- ments consisting of quartzite, argillite, and limestone in northwestern Arizona as pre-Cambrian. Bancroft (1911, p. 23) lists as pre-Cambrian granite, gneiss, schist, quartzite, limestone, dolomite and argillite, all of which are cut by intrusives of diabase, aplite and pegmatite of different ages. Mesozoic (?) granite cutting the pre- Cambrian was dated as such only because others in the Pacific area are so dated. No attempts have been made to correlate these rocks with those in the gypsum deposits 50 miles to the west. Harder (1912, p. 27) discusses the succession of events in the Eagle Mountains. His table is reproduced here. Obviously, little agreement exists among the various workers concerning both the actual and the relative ages of the rocks. The Maria formation is considered by some to be pre-Cambrian in age and by others, Paleozoic. The relative ages of the Maria formation and the McCoy Mountains ( ? ) formation are difficult to decipher in the Palen Mountains. As the present study in this area was limited to the gypsum deposit and the narrow region bordering the deposit on the south, a final answer was not determined. It has already been indicated that the green and dark green schist, argillite, shale, and con- glomerate found at the southwest border of the area dip northward beneath the gypsum marble series. If there is no fault between the two formations, the McCoy Moun- tains (?) formation is the older. However, if a fault separates the two formations, the alternative accepted here, then the McCoy Mountains beds could be either stratigraphically above or below the Maria formation. By postulating a reverse component of movement along such a fault as is suggested along the southeast border, it would be likely that the Maria beds were the older. Also, it was pointed out in a previous section that the McCoy Mountains beds appear to be less metamorphosed than the rocks of the Maria formation and, therefore, must be the younger of the two groups. It is possible that the gypsum, marble, schist, and quartzite of the Maria formation are the deformed and metamorphosed equivalents of the gypsum, limestone, shale, and sandstone of the Kaibab and the Moenkopi formations. Apparently diastrophism has occurred more often and more severely in the southeastern Mojave region, along with greater intrusive activity. - The gypsum deposits of the Palen, Little Maria, and Maria Mountains all belong to the Maria formation. In addition, it seems probable that these metasediments are the same age as those in the Eagle Mountains even though no gypsum and no large thicknesses of marble occur in the Eagle Mountains. The metasediments con- taining the iron ore deposits of the Eagle Mountains probably are a part of the same formation. The age of the gypsum deposits cannot definitely be given, but it is likely that the Maria formation of the southeastern Mojave Desert represents highly deformed and metamorphosed equivalents of the gypsum-bearing Permo-Triassic formations of southern Nevada. With this in mind, the Maria formation is designated as Up- per Paleozoic (?) age. ECONOMIC GEOLOGY The gypsum reserves in the Palen Mountains deposit amount to several hundred million tons. The calculation of a fairly accurate estimate of the reserves would be an extremely difficult job, because the gypsum is cut out in many places by intrusives and faults, or wedges out between massive beds of marble. Although the gypsum is of high quality, its value is lessened by the presence of large and small fragments of marble that are "float- ing" in the gypsum beds. The difficulty of predicting the presence of these impurities will increase the cost of mining. For example, several places gave promise of being free of impurities, but in almost every case bull- dozing uncovered irregular masses of marble mixed with the gypsum. It is evident that the planning of a long- range mining program will be more difficult than for most undeformed, bedded deposits. In order to carry out a large-scale program of exploi- tation, a much more detailed mapping and sampling of the three gypsum areas must be made, supplemented by a drilling schedule to determine the depth to anhydrite and to provide more information on the sub-surface structure. While these detailed studies are being made to determine the feasibility of a large-scale development, some mining could be begun, provided close supervision of removal operations is maintained. Probably the best site for preliminary mining is in the east-central part of the southern gypsum belt where a large area of gypsum has been exposed at the surface by the stripping off of much of the overlying marble. REFERENCES Bancroft, Howland, 1011, Ore deposits in northern Yuma County, Arizona : U. S. Geol. Survey Bull. 451. Bowles, O., and Farnsworth, M., 1925, Physical chemistry of the calcium sulphates, and gvpsum reserves : Econ. Geology, vol. 20, pp. 738-745. Brown, John S., 1923, The Salton Sea region, California: U. S. Geol. Survey Water-Supply Paper 497. Darton, N. H., 1907, Discovery of Cambrian rocks in southeastern California : Jour. Geology, vol. 15, pp. 470-475. Fairbairn, H. W., 1949, Structural petrology of deformed rocks : pp. i-ix, 1-344, Addison- Wesley Press, Inc., Cambridge, Mass. Farnsworth, M., 1924, Effects of temperature and pressure on gypsum anhydrite : U. S. Bur. Mines Rept. Inv. 2654. Goldman, Marcus I., 1952, Deformation, metamorphism and miner- alization in gypsum — anhydrite cap rock : Geol. Soc. America Mem. 50. Hadley, J. B., 1948, Iron ore deposits in the eastern part of the Eagle Mountains, Riverside County, California : California Div. Mines Bull. 129, pp. 1-24. Harder, E. C, 1909, The gypsum deposits of the Palen Mountains, Riverside County, California: U. S. Geol. Survey Bull. 430, pp. 407-416. Harder, E. C, 1912, Iron ore deposits of the Eagle Mountains: U. S. Geol. Survey Bull. 503. Harker, A., 1939, Metamorphism, pp. i-ix, 1-362, Methuen and Co. Ltd., London. Hazzard, J. C, Gardner, D. L., and Mason, J. F., 1938, Mesozoic ( ?) metavolcanic and sedimentary rocks in San Bernardino and Riverside Counties, California : Geol. Soc. America Proc, 1937, p. 279. Hess, F. L., 1920, Gvpsum deposits of the United States: U. S. Geol. Survey Bull. 697, pp. 78-79. Hewett, D. F., 1931, Geology and ore deposits of the Goodsprings quadrangle, Nevada : U. S. Geol. Survey Prof. Paper 162. Jenkins, O. P., 1937, Source data of the geological map of Cali- fornia : California Jour. Mines and Geology, vol. 33, pp. 9-27. Jenkins, O. P., and others, 1950, Mineral commodities of Califor- nia : California Div. Mines, Bull. 156. King, R. H., 1927, Sedimentation in Permian Castile sea : Am. Assoc. Petroleum Geologists Bull., vol. 31, pp. 470-477. Palen Mountains Gypsum Deposit 25 Lee, \V. T., 1008, A geological reconnaissance of a part of western Arizona : U. S. Geol. Survey Bull. 352. Longwell, C. R., 1049, Structure of the northern Muddy Mountain area, Nevada : Geol. Soc. America Bull., vol. 60, pp. 923-968. Longwell, C. R., 1928, Geology of the Muddy Mountains, Nevada, with a section through the Virgin Range to the Grand Wash Cliffs, Arizona : U. S. Geol. Survey Bull. 798. Miller, W. J., 1944, Geology of Palm Springs— Blythe Strip, Riverside County, California : California Jour. Mines and Geol- ogy, vol. 40, pp. 11-72. Pettijohn, F. J., 1949, Sedimentary rocks : pp. i-xv, 1-526, Harper Brothers, New York. Posnjak, E., 1938, The system CaSCK-ILO : Am. Jour. Sci., vol. 235, pp. 247-272. Posnjak, E., 1940, Deposition of calcium sulphate from sea water : Am. Jour. Sci., vol. 238, pp. 559-568. Stewart, F. H., 1953, Early gypsum in the Permian evaporites of northeastern England : Geologists Assoc. I'roc, vol. 64, pp. 33-39. Suit, Gordon, 1911, Gypsum deposits in the Maria Mountains of California : Min. World, vol. 34, pp. 787-790. Thompson, D. G., 1929, The Mohave Desert region, California : H. S. Geol. Survey Water-Supply Paper 578. Tucker, W. B., and Sampson, R. J., 1929, Riverside County : Cali- fornia Div. Mines Rept. 25, pp. 510-514. Turner, F. J., 1948, Mineralogical and structural evolution of metamorphic rocks: Geol. Soc. America Mem. 30. Ver Planck, William E., Jr., 1950, Gvpsum : California Div. Mines Bull. 156, pp. 223-230. Ver Planck, William E., 1952, Gypsum in California : California Div. Mines Bull. 163. fainted in California state printing office 87720 10-53 2M