GEOLOGY OF THE OTAY BENTONITE DEPOSIT SAN DIEGO COUNTY, CALIFORNIA By GEORGE B. CLEVELAND Mining Geologist, California Division of Mines f, Special Report 64 CALIFORNIA DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO, 1960 STATE OF CALIFORNIA EDMUND G. BROWN, Governor DEPARTMENT OF NATURAL RESOURCES DeVVITT NELSON, Director DIVISION OF MINES IAN CAMPBELL, Chief Special Report 64 Price $1.00 GEOLOGY OF THE OTAY BENTONITE DEPOSIT, SAN DIEGO COUNTY, CALIFORNIA By George B. Cleveland * OUTLINE OF REPORT Page Abstract 3 Introduction 5 Geology 5 San Diego formation 7 Sweitzer formation 9 Terrace deposits and alluvium 9 Bentonite 10 Physical and chemical properties 10 Origin 11 Reserves 11 Suggestions for prospecting 11 Mining operations, treatment, and uses 13 Selected references 16 Illustrations Plate 1. Geologic map and cross section of the Otay area, San Diego County, California In pocket Figure 1. Block diagram and index map of Otay area, San Diego County, California 3 2. Block diagram of Otay area, San Diego County 4 3. Differential thermal analysis curve of the Otay ben- tonite showing typical montmorillonite thermal characteristics 10 4. Diagrammatic sections showing the origin of the Otay bentonite deposit 12 5. Map showing distribution of bentonitic sandstone in southwestern San Diego County 14 Photo 1. General view of the lower Otay Valley from the north 6 2. Mound topography on Otay mesa 3 miles east of the Otay area G 3. Contact between the light-gray sandstones of the mid- dle or late Pliocene San Diego formation below, and the Plio-Pleistocene conglomeratic sandstone of the Sweitzer formation above 8 4. Minor bentonite beds in road cut along Otay Valley road near the bottom of Chester Grade 9 5. View across Otay Valley toward the north 15 6. Main pit on the Standard Oil Company property 16 Table 1. Chemical analyses of Otay bentonite 10 2. Staining reaction, Otay bentonite 11 3. Spectographic analyses of Otay bentonite 11 ABSTRACT Bentonite occurs locally in Pliocene sedimentary rocks in the coastal region of southwestern San Diego County. The lower Otay Valley contains a high-grade deposit of bentonite from which as much as 83,000 tons of clay, valued at about $670,000, has been obtained from the * Mining geologist, California Division of Mines. two principal mines, the General Petroleum Company mine and the Standard Oil Company mine. The bento- nite is intercalated in the flat-lying sandstone and arkose beds of the middle or late Pliocene San Diego formation. In the Otay area this formation and the overlying Plio- Pleistocene Sweitzer formation have a combined thick- ness of about 320 feet. In late Cenozoic time marine wave-cut terraces were formed on these and older rocks in the San Diego region. In the Otay area the Otay, Sub-Otay and Avondale terraces are most prominent. Bentonite occurs in six or more beds which range in thickness from less than 1 foot to 4 feet or more. The high-grade clay underlies an area of 1.3 square miles and contains reserves of more than 8.5 million tons of clay. Outside the mapped area bentonitic sandstone was found at 25 additional localities. Because of its high base exchange capacity the Otay bentonite has been used almost entirely as a decolorizer in refining petroleum products. The bentonite was derived from the alteration of a volcanic ash which may be related to the basic ex- trusive and pyroclastic rocks of the Salada formation in northwestern Baja California Norte, Mexico. SCALE IN MILES SAN YSIDRO QUADRANGLE california. -VexTco TIJUANA Figure 1. Index map of Otay area, San Diego County. (3) California Division op Mines [Special Report 64 Figure 2. Block diagram of the Otay area, San Diego County. 1960] Otay Bentonite Deposit, San Diego County INTRODUCTION The Pliocene San Diego formation, which underlies much of southwestern San Diego County, contains a bentonite-bearing zone that has been mined commerci- ally and contains recoverable reserves. The Otay ben- tonite deposit, which was developed in this zone, is the largest known occurrence of high-grade clay in this re- gion and the only one that has been productive. It is near the Mexican border, 11 miles south of San Diego, in sections 17, 18, 19, 20, 29 and 30, T. 18 S., R. 1 W., S.B.M. and in section 25, T. 18 S., R. 2 W., S.B.M. (fi- gure 1). The clay-bearing zone, which comprises several layers, is best exposed along the north and south slopes of the Otay Valley. This valley is 3 miles north of the Mexican border and is one of the large drainage courses in the southern part of the county. It extends westerly for 12 miles from an abrupt mountain front to the Paci- fic Ocean, and is cut in flat-lying sedimentary rocks. Along the sides of the valley the clay zone lies a few tens of feet below the surface of Otay Mesa; one of the extensive marine terraces in this region. The clay can be traced from the central part of Otay Valley 6 miles to the east where the coastal sediments overlap the crystal- line rocks of the southern Peninsular Ranges. East of section 20 in the mapped area, however, the clay is gen- erally impure and the clay layers are thin. Impure de- posits of clay were noted north of the area, but they appeared to be of local distribution. The clay also occurs several miles south of the Mexican border, along Mexico Highway 1 toward Rosarito Beach. Bentonite has been mined at several localities in the Otay Valley, but two mines have yielded nearly all of the production. These, the General Petroleum Company mine on the south side of Otay Valley and the Standard Oil Company mine on the north side, have a total re- corded output of about 23,000 tons of clay, valued at about $80,000. The size of the workings and the values estimated from combined production records indicate that these two mines yielded as much as 83,000 tons of clay valued at $670,000. The Otay deposit is easily accessible to rapid trans- portation and is near the large population centers of San Diego and Los Angeles. The larger mines lie within a half mile of Otay Valley Road, a paved highway that connects with the San Diego and Arizona Eastern Rail- road at Palm City, 5 miles to the west. Secondary roads lead to the workings. An annual rainfall of only about 10 inches supports a sparse cover of low brush, grass, cactus, and a few trees which grow in the side canyons. Vegetation is par- ticularly sparse in the vicinity of the clay deposits. Livestock raising and truck farming are the chief land uses in the area. The Otay clay was first described by Goodyear (1890, p. 139) who saw it during a geologic reconnaissance of San Diego County. He identified the clay as montmoril- lonite and briefly commented on its high moisture con- tent and wide distribution. A preliminary geologic map of western San Diego County was prepared by Ellis and Lee (1919) as part of a water-supply report which also included several well logs. These logs show clay beds ranging from 3 to 54 feet in thickness. Hertlein and Grant (1939, p. 61) noted the occurrence of bentonite in the San Diego formation and suggested a correlation with the ash beds (Salada formation) near Tijuana, Mexico. They mentioned a bentonite locality in La Choyas Canyon, near the center of the city of San Diego and the occurrence of four bentonite beds, each 50 to 60 feet thick, logged from a well drilled a few miles east of the town of Otay. In a later paper Hertlein and Grant (1944, p. 53) also described a bentonite bed en- countered in a well in Balboa Park, San Diego. Among the workers who have investigated the mineralogy and properties of the Otay clay are Ross and Shannon (1926), Grim (1942), Nutting (1943), Ross and Hen- dricks (1945), Sped, and others (1945), Rowland (1948) Bradley and Grim (1951), and Osthaus (1955). The Otay clay was studied as part of American Petro- ueum Institute Project 49 which was initiated to provide clay mineralogists with basic analytical data useful in comparative studies of clay minerals, and to establish certain clays as standards. This work included differ- ential thermal, x-ray, chemical, microscopic and spec- trograph^ analyses and a study of infrared absorption, pH, exchange capacity, electron micrography, magnetic susceptibility, particle size, and dye absorption. Thirty- three clays from various localities in the United States were selected. Of these, two montmorillonitic clays, the Otay bentonite in San Diego County, and the unique clay mineral hectorite in San Bernardino County, were chosen from California (Kerr and Kulp, 1949, Introduc- tion). The writer is indebted to the late George Clark Gester, formerly Chief Geologist of the Standard Oil Company of California, for providing information on the early history of the Otay mining operations. Dr. Paul F. Kerr, Director of American Petroleum Institute Project 49, has kindly allowed analytical data published in the A.P.I, reports to be reproduced here. GEOLOGY The coastal region of southwestern San Diego County consists of a series of marine wave-cut terraces on which San Diego, La Jolla and other coastal cities are built. The terraces are nearly flat and form broad benches that trend northwest, parallel to the present coastline. Only small remnants of the higher terrace surfaces re- main. They have been dissected by several streams which flow westward from the Peninsular Ranges which border the coastal plain on the east. Two series of terraces are recognized in this region (Ellis and Lee, 1919; Hertlein and Grant, 1939, 1954). One series lies north of the San Diego River and it comprises, in order of decreasing elevation, the Poway, the Linda Vista and the La Jolla terraces. South of the river, seven terraces have been de- scribed ; they are the Otay, the Sub-Otay, the Avondale, the Chula Vista, the Nestor, the Tijuana and the present coastal flats (Hertlein and Grant, 1954). The Poway ter- race, which is the highest, is as much as 1200 feet above sea level. The La Jolla, Chula Vista, Nestor and Tijuana terraces are generally better preserved, but are of smaller lateral extent than the higher terraces. Through- out most of the region the terraces are cut into sedimen- tary rocks that range in age from Cretaceous to Pleisto- cene. The Poway terrace is developed, in part, on pre- Tertiary metamorphic and igneous rocks. Terracing and emergence of the land surface probably began in 6 California Division of Mines [Special Report 64 Pliocene time and continued into the Pleistocene (Hert- lein and Grant, 1939, p. 59). The Otay Valley is a broad alluviated valley with moderate slopes that rise to steep bluffs near the rim of Otay Mesa (photo 1). Most of the valley is cut in a In the mapped area the San Diego and Sweitzer formations are generally undeformed. Only two north- trending faults were mapped, and they are normal and of small displacement. They are exposed at the east end of the main pit on the Standard Oil Company property. Photo 1. General view of the lower Otay Valley from the north. Otay Mesa is on the horizon and the General Petroleum Company mine workings are just below the rim of the mesa. Bentonite occurs in the middle or late Pliocene San Diego formation. monotonous sequence of pale gray sandstones and arkoses and brown mudstones of the upper part of the San Diego formation of middle or late Pliocene age. In the Otay area these rocks and the overlying Sweitzer formation, which is composed principally of conglomer- ate, are nearly flat-lying. Elsewhere they have a re- gional dip of 5 or 6 degrees south to southwest. South of the Tia Juana River, which parallels the Mexican border, they dip about 9 degrees west (Hertlein and Grant, 1939, p. 70). Another normal fault of small displacement possibly is hidden beneath the alluvium of Otay Valley. This is suggested by the extensive outcrops of the Sweitzer formation on the south side of the valley, compared with much less abundant outcrops of the formation north of the valley. Furthermore, the thickest clay bed, which appears to occupy the same stratigraphic position on both sides of the valley, is from 25 to 50 feet lower in elevation on the south side than it is on the north side. - 2* ■I Ph<_ 2. Mound topography on Otay Mesa 3 miles east of the Otay area. Peninsular Ranges taceou letamorphic and igneous rocks. View northeast. in background are composed of pre-Cre- 19601 Otay Bentonite Deposit, San Diego County Both the Otay and Sub-Otay terraces are well de- veloped and form the principal topographic features in the Otay area. On the north side of the Otay Valley, near the east end of the mapped area, a broad terrace lies at an elevation between 200 and 275 feet and corresponds to the Avondale terrace which is more extensively de- veloped on the outer coast. Remnants of other surfaces also can be observed along the valley slopes, but these are of little areal extent. All terraces lower in elevation than the Sub-Otay have been cut into the San Diego formation below the principal clay layer. Part of the mapped area is covered with numerous mounds. These are nearly symmetrical features that rise 1 or 2 feet above the surrounding surface and range in diameter from a few feet to about 25 feet. They are closely spaced and generally covered with a thin veneer of small rock fragments from the Sweitzer formation. These structures are best developed on the Otay Mesa south and east of the mapped area (photo 2). They were also noted in the broad area north of Otay toward Chula Vista and at several localities on the terraces along Otay Valley. San Diego Formation The San Diego formation, which is middle or late Plio- cene in age, extends south from Mission Valley for at least 18 miles, and east from the coast for about 12 miles to a steep mountain front composed mainly of porphy- ritic volcanic rocks. The formation is composed prin- cipally of pale brown, buff or bluish-gray, fine-grained sandstone and thin beds of pebble conglomerate. Calcite and brown biotite are common in the sandstone. Lenses of bentonite occur in the upper part of the formation. The formation has a maximum known thickness of 1250 feet southeast of San Diego, but thins markedly to the east. From place to place, it unconformably overlies the Rose Canyon shale member of the La Jolla formation, the Poway conglomerate, both Eocene age, and the Black Mountain volcanics of probable early Mesozoic age. It is overlain unconformably by the late Pliocene or early Pleistocene Sweitzer formation, by Pleistocene terrace deposits and bv Recent alluvium (Hertlein and Grant, 1939). The San Diego formation is the most widespread rock unit in the Otay area where it consists of poorly con- solidated gray and buff micaceous sandstone and arkose, thin beds of marl, brown mudstones and intercalated beds of brown, pink and white bentonite. In the Otay area only the upper 300 feet of the formation is exposed. The generally incompetent nature of these rocks has al- lowed considerable slumping, and a gently rolling topog- raphy is common. The slopes are steep and the hill crests are flattopped where marine or stream terraces have formed or where the San Diego formation is capped by the more resistant Sweitzer formation. Throughout the area the principal beds within the formation appear to be relatively persistent both in thickness and lithology. The sandstone and arkose beds are generally light gray on fresh surfaces, but are bleached to nearly white where exposed to weathering. They are normally from 2 to 15 feet thick and moderately well-bedded, although the bedding is generally obscured by slumping. The rock is composed of fine-grained, well-sorted, angular fragments of quartz, black to golden biotite, white altered feldspar, magnetite, and zircon, poorly cemented by a clayey matrix. Some of the thinner beds are well-cemented and resistant to Aveathering. Intercalated within the gray sandstone sequence is a layer of pale tan to dark buff colored coarse-grained sandstone. This unit is about 10 feet thick and is composed of limonite-cemented quartz fragments and angular clasts of pebble- to cobble-sized volcanic rocks. It is moderately well bedded and locally resistant to weathering. The base of the San Diego formation is nowhere ex- posed in the Otay area, but well logs indicate that the formation extends to a considerable depth. Three miles west of the Otay area and 100 feet lower in elevation, the San Diego Gas and Petroleum Company Well No. 1 in sec. 32, T. 18 S., R. 2 W., S.B.M., penetrated about 2600 feet of the San Diego formation before encountering the Eocene Rose Canyon shale. In 1931 the Itasca Petroleum Company drilled a well on Otay Mesa in sec. 33, T. 18 S., R. 1 W., S.B.M., about a mile east of the area. It bot- tomed in Pliocene (?) rocks at a depth of 1,548 feet (Jennings and Hart, 1956, p. 71). The well data suggest the San Diego formation may extend to nearly 2600 feet below the exposed section in the Otay area. On the north side of Otay Valley the contact between the Sweitzer formation and the San Diego formation is generally obscured, except near the eastern margin of the mapped area, by the slumping onto the San Diego forma- tion of Sweitzer cobbles (photo 3). On the south side of the valley the Sweitzer formation on the Otay terrace has been better preserved and is in relatively sharp con- tact with the San Diego formation. Bentonite occurs in the Otay area at six or more ho- rizons within the San Diego formation. The beds range in thickness from a few inches to 4 feet or more (photo 4). The thinner beds are poorly exposed and cannot be traced continuously for more than a few tens of feet, al- though one thin bed appears to crop out at nearly the same elevation at a number of localities throughout the area. The overburden above the thickest and purest bed ranges from about 25 feet to 150 feet in thickness and generally increases from west to east. Separate bentonite beds have been observed on the north side of Otay Valley at about the following elevations: 175, 215, 360, 375, 395, and 425 feet. In addition, bentonite is a major constitu- ent of many of the sandstones in the San Diego forma- tion and imparts to them, when moist, a plastic consist- ency and a brownish color. These properties are best observed after heavy rains as normally all of the sand- stone layers are pale gray in color and appear homo- geneous in composition. The known distribution of the high-grade clay in the Otay area appears to be confined to a lobe-shaped area about 2 miles long and 5 miles wide. From the southernmost boundary, which encom- passes the General Petroleum Company mine, the clay layer trends north across the Otay Valley to the Stand- ard Oil Company property and then broadens toward the northeast (plate 1). Within the mapped area the high- grade clay zone apparently underlies an area of approxi- mately 1.3 square miles. A brief reconnaissance on the north side of the Otay River beyond the mapped area toward the north and east indicates that the high-grade clay zone may underlie a considerably larger area. The principal bentonite bed occurs at nearly uniform elevations; between 350 and 375 feet on the north side California Division of Mines [Special Report 64 Son Di«go fm. i J ! <^*jW,> Hi \iA,*t -jB*' * Photo 3. Contact between the light-gray sandstones of the middle or late Pliocene San Diego formation below, and the Plio-Pleisto- cene conglomeratic sandstone of the Sweitzer formation above. View from Otay Valley road at Chester Grade a few hundred feet east of the Otay area. of Otay Valley, and between 325 and 350 feet on the south side. This bed averages about 3 feet in observed thickness, and in some exposures is 4 feet or more thick. Hertlein and Grant (1939, p. 78) reported bentonite beds in the Otay Valley area reached 9 or more feet in thickness. Tucker and Reed (1939, p. 38) give the thick- ness of the beds as 6 to 8 feet. In 1920, the late G. C. Gester, then geologist for the Standard Oil Company, directed a study of the clay deposits on the south side of Otay Valley. He stated that four individual beds were exposed and showed the following thicknesses : 21 inches ; 36 inches ; 30 inches ; and 36 inches (personal communication, 1957). The de- termination of thickness is often difficult because the clay expands when hydrated and the outcropping sur- face is greatly enlarged. Schroter and Campbell (1940, 11) have pointed out how this process can lead to us errors in the estimation of reserves. It may well ac nt for some of the discrepancies in thickness pre- via reported in the literature. The bed persists thro* out the area and although it crops out only where le slopes have been oversteepened by gullying, its pre nee elsewhere is indicated by the cracked and often spongy soil that overlies it. Though the clay com- monly occurs intercalated with a conspicuous marly zone, it apparently forms a single stratigraphic unit containing variously colored clay -bearing zones and thin impure sandy partings. The following section is typical of the clay zone on the north side of Otay Valley : Detailed Section of Clay Zone, Otay Bentonite 1 Description Thickness Cream to white, soft, slightly gritty waxy clay containing disseminated spots and coatings of manganese oxide ; which fractures on curved surfaces to thin plates 6 inches Dark pinkish brown, gritty, waxy clay, stained with man- ganese oxide; and contains granular fragments of cal- cium carbonate on fracture planes 7 inches Grayish-brown to grayish-green waxy clay 6 inches Light gray. hard, granular clay, uneven fracture ; con- tains flakes of biotite, small granular pods of calcite, and is stained by manganese inches Light grayish-brown, gritty clay 5 inches Light gray, impure, sandy clay. Contains abundant bio- tite, some garnet and ferro-magnesian minerals 12 inches Light grayish-brown, gritty clay 7 inches Total 4 feet, 4 inches 1 Section taken from NEiSWi of section 18. 1960] Otay Bentonite Deposit, San Diego County m I r Wf I %^i0.r ^jm ^UM^M9% - i_ i ■ ® ■$*>.;.»#%• WWW? - ""■' ' '--■■'-"-.--" _ •.?.'■• ft* , Photo 4. Several minor bentonite beds in road cut along Otay Valley road near the bottom of Chester Grade. Bentonite layers exhibit coral-like texture on exposed surface. Thick beds of mudstone in the San Diego formation are prominent along the Otay Valley Road, but were not observed on the south side of the valley. They are nor- mally dark brown in color but locally contain thin partings which are faintly red, purple or yellowish- brown. The rock is less resistant to erosion than the over- lying sandstone. The mudstone is composed principally of fine silt, clay, and iron oxide, but thin partings of sand are also present. Faint bedding is discernible in some exposures, but generally the beds are massive. A bed of impure brown bentonite 3 feet thick occurs near the top of the mudstone unit several feet below the Avondale terrace along the Otay Valley Road near the eastern limits of the mapped area. The San Diego formation is locally fossiliferous and contains abundant Pliocene mollusks, notably pectens. No fossils were found in the Otay area. Sweitzer Formation The general distribution of the Plio-Pleistocene ( ?) Sweitzer formation coincides with that of much of the underlying San Diego formation. It forms a thin and discontinuous veneer on the Otay terrace south of Mis- sion Valley. It is mostly a brownish -red conglomeratic sandstone cemented with ferruginous mud and is mod- erately resistant to erosion. In some localities near San Diego it grades into a much less resistant, light gray rock. The clasts consist principally of porphyritic vol- canic rocks, but granite, quartz, and quartzite are also common. The clasts are medium to well rounded and reach a maximum diameter of 4 inches. The formation has a maximum observed thickness of 65 feet, but it is generally less than 20 feet thick. It is older than the late Pleistocene marine terrace deposits and is believed to be between late Pliocene and early Pleistocene in age (Hert- lein and Grant, 1939, 1944, 1954; Goldstein, 1956). In the Otay area, the formation ranges in thickness from less than one foot, north of the Otay River, to 20 feet or more south of the river. It is characterized by a yellowish- to reddish-brown pebble and cobble con- glomerate, well to poorly cemented with a mixture of coarse-grained quartz sand and limonite. The cobbles are composed principally of gray, green, red, and black porphyritic volcanic rocks and light colored quartzite, all coated with yellowish brown limonite. The volcanic rocks in the San Diego area have been identified by Goldstein (1956, p. 35) as andesite, dacite, latite, rhyo- lite and quartz keratophyre. The clasts are rudely ori- ented parallel to the bedding. A three foot bed of yellowish-brown coarse-grained sandstone underlies the conglomerate over a small area in the northeast portion of the mapped area. Terrace Deposits and Alluvium Older Terrace Deposits. Deposits of Sweitzer-like sedimentary rocks lie on the Sub-Otay and Avondale terraces. As the deposits are generally much thinner and lack the sandy cementation that is characteristic of the Sweitzer formation on the Otay terrace, it is likely that they are material reworked from the Sweitzer formation of the Otay terrace. These deposits are particularly con- spicuous north of the Otay River. Younger Terrace Deposits. River-laid terrace de- posits form a wide band of dark grayish-brown soil on 10 California Division of Mines [Special Report 64 both sides of Otay Valley. They have a maximum width of several hundred feet and, because of their fertility, they are under cultivation. South of Permutter's Ranch, along the Otay Valley road, however, this layer is com- posed principally of cobbles derived from the Sweitzer formation or from older terrace material. The Younger terrace deposits can be distinguished from the Older de- posits mainly by their color. The Younger deposits are probably Recent in age and have a maximum thickness of about 50 feet. Alluvium. Alluvium mantles the floor of Otay Valley and Poggi Canyon. It is composed principally of pebble- to boulder-size clasts from the Sweitzer formation. Gravel and sand are locally abundant and are being mined for building material at two localities in the Otay Valley. Bentonite Bentonite is a rock term for a clay material com- posed principally of clay minerals of the montmoril- lonoid group (generally montmorillonite, less often beidellite). The general oxide formula of the montmoril- lonoids is (AI0O3 • Fe 2 3 ■ MgO) • 4Si0 2 ■ H 2 • nH 2 0. Bentonite is light in color, waxy or earthy in texture, has a dry hardness between 1 and 2, a specific gravity of about 2.5 and breaks with a roughly conchoidal fracture. Some bentonites, when placed in water, swell many times their dry volume whereas others show little change in volume. Certain non-swelling types have a natural ad- sorptive capacity and are known as fullers earth. An- other type known as activatable bentonite, of which the Otay clay is an example, can be acid-treated to develop this property. Most bentonites are an alteration product of volcanic ash. Physical and Chemical Properties The Otay bentonite ranges from a clayey-sand to a nearly monomineralic clay. The principal clay bed, in the vicinity of the main pit on the north side of the Otay Valley, is relatively pure. One sample contained only about 4 percent impurities, which include 1 to 2 percent sericite, 1 percent quartz, 0.5 to 1 percent orthoclase and traces of limonite and ferromagnesian minerals (Kerr, Main and Hamilton, 1950, p. 53). The highest-grade clay is mostly white, cream, or pale pink, but some of it ranges, from dark brown to a pale pinkish brown. The material is speckled with small, disseminated grains of manganese oxide and a minor amount of unaltered biotite. Some fracture surfaces are completely covered by manganese stain. West of the main pit on the Standard Oil Company property the clay contains granules of calcite which comprised several percent of the samples examined. Ross and Hendricks (1945, pi. 4A,B) have described relic volcanic-ash struc- tures such as unbroken glass bubbles and pumice shards in the Otay clay. These bogen or ash structures are •mon in the samples examined during this study. rly all the shards retained part of their original but have been completely altered to a crystalline eh. 'ineral. The clay has a soapy consistency when hycl,