JAMES W. HAWKINS Geological Research Division, Scripps Institution of Oceanography, La Jolla, California 92037 EDWIN C. ALLISON Geology Department, San Diego State College, and Research Associate, Scripps Institution of Oceanography, La Jolla, California 92037 DOUG MACDOUGALL Department ofSIO, University of California, San Diego, La Jolla, California 92037

Volcanic Petrology and Geologic History of Northeast Bank, Southern California Borderknd

ABSTRACT (see Fig. 1), is a flat-topped, steep-sided sub- marine hill rising about 900 m above the sur- Basaltic rocks, hyaloclastites and fossil rounding sea floor (Fig. 2). The bank top is fragments incorporated in volcanic material 17 km X 6 km within the 500 m depth contour were dredged from the flanks of Northeast which delineates the flattened summit. Maxi- Bank (lat 32°20' N., long. 119°40' W.) on the mum relief on the summit is less than 150 m Southern California Borderland. Unaltered, and local relief probably is only a few tens of fine-grained alkali basalt has 2.6 percent Ne and meters; the shallowest part of the bank is near 10.8 percent Ol in the norm. Trace element its south end where it rises to a depth of 357 m. abundances are typical of eastern Pacific basin The bank is 20 km east of the crest of the alkali basalts but Ba is enriched (880 ppm). 2700 m high Patton Escarpment which marks The fossil fragments, incorporated in breccia, the seaward edge of the Southern California hyaloclastite and agglomerate include a fauna Borderland (Emery, 1960). West of the Patton which lived in less than 50 m of water, as well as Escarpment are several seamounts ranging in forms from the intertidal zone. Bathymetry of height from 2000 to 3600 m. San Juan seamount the bank and the depth range of the dredged (3100 m high), the closest of these to the zone indicate that there has been at least 300 or escarpment, is encrusted with basaltic rocks as much as 500 m of subsidence since the (Emery and Shepard, 1945); it is believed to volcanism which incorporated the fauna in be a submarine volcano. East of the escarpment volcanic material. Isostatic adjustments due to the borderland comprises a series of north- crustal load of the bank can account for this westerly trending basins and ridges; in part, subsidence. these trends and the bathymetric features may A fission track date of unaltered basaltic glass be fault controlled. The basins are floored with gives an age of 4.5 ± 0.5 m.y.; a K/Ar date on Pliocene and younger silts and sands (Emery, unaltered holocrystalline basalt gave an age of 1960) while on the banks the rocks range in age 4.36 + 0.8 m.y.; the faunal assemblage is Plio- from probable Jurassic (Franciscan-type lithol- Pleistocene and manganese crust thicknesses ogies) to Pleistocene or Recent. Some of the are 0.2 to 1 cm. This is the youngest reported ridges rise high enough to form islands; these volcanism for the borderland. The magnetic islands are geologically similar to the nearby anomaly due to the bank lies on the projected coastal land areas. Some of the ridges or banks trend of anomaly 5c of the sea floor. It seems are covered with volcanic rocks and several, clear that this is fortuitous; there is no genetic such as Northeast Bank, have the morphologic relationship between these anomalies and these characteristics of submarine volcanoes. Krause data do not support the suggestions that (1965) reported widespread occurrences of Mason-Raff lineations can be traced through volcanic rock, some of it unaltered, young- the Borderland. looking basalt, in the borderland south of the region described here. North of the 32nd INTRODUCTION parallel young volcanic material is much less Northeast Bank, located on the continental abundant than in the area to the south discussed margin 230 km west of San Diego, California by Krause (1965). Northeast Bank is one of the

Geological Society of America Bulletin, v. 82, p. 219-228, 4 figs., January 1971 219

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PREPARED BY

U.S. BUREAU OF COMMERCIAL FISHERIES seUE AND UNIVERSITY OF CALIFORNIA INSTITUTE OF MARINE RESOURCES /! VI Figure 1. Generalized bathymetry of Southern California Borderland and location of Northeast Bank.

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32°30'N

8x Verticol Exaggeration

0 — • E 250 — I 500 — * ™ - ^-1000 — IhaSO — a 1500 —

10 20 km SCALE Contour interval 100 meters Figure 2. Bathymetry, sample location and profiles of Northeast Bank. Location of profiles shown by dashed lines. Bathymetry from Coast and Geodetic Survey Chart 1206 N-15, 1968.

localities at which volcanic rocks have been which has 0.5 cm glassy rinds, nearly all of it is recovered on the borderland; its shape suggests fresh and unaltered; (2) ragged blocks and that it may be a submerged volcano. In ad- slabs of volcanic breccia and hyaloclastite; dition to the flat-topped roughly conical form these are coated with manganese crusts which it is distinctive (and named for the fact) in that range from a few mm to 1 cm in thickness; (3) it is elongated N. 20° E., in contrast to the subrounded to rounded pebbles (1 to 4 cm northwesterly trending topographic and struc- diameter) of Mn-coated volcanic debris; (4) tural "grain" of the borderland region. How- an angular block of brownish-gray dolomite, a ever, the N. 20° E. trend is also apparent in the rounded dacite porphyry cobble, and a sub- orientation and elongation of features on the rounded graywacke cobble (the three clasts deep-sea floor. During a regional survey in total about 5 kg). The abundance of volcanic June 1969 by the R/V Alexander Agassiz, of material, as well as the angularity of volcanic the Scripps Institution of Oceanography, fragments, indicates that these rocks are volcanic rocks were recovered by dredging and typical of the zone sampled by the dredge. magnetic profiles were recorded by a towed They must represent either material broken magnetometer. The purpose of this report is to from outcrops or from essentially in situ talus describe the rocks recovered, to discuss their of a bank which is interpreted as being formed possible significance in terms of the regional of (covered with?) volcanic rock. The three geology, and to point out some features of rocks listed in category 4 are typical of the general geologic interest. banks on the borderland and they may be part of the platform which underlies the volcanic PETROLOGY OF THE SAMPLES rocks. Introduction Unaltered Basaltic Rock The probable track of the rock dredge A major part of the dredge haul consisted of during sampling operations is shown in Figure remarkably fresh aphanitic basaltic rock. 2. About 100 kg of rocks were recovered from Plagioclase (Anes-vo) is the dominant mineral; the inteival between 1280 and 550 m depth. it occurs as euhedral laths about 0.5 mm long The sample consists of: (1) angular blocks and with strong preferred orientation suggesting keystone shaped fragments of basalt, some of flow alignment. In a few samples there are

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plagioclase microphenocrysts (Anye). The pla- are probably an alkali feldspar. These patches gioclase laths enclose clusters of colorless to have a diffuse very small 2VT and resemble the faintly pink euhedral to subhedral clinopy- alkali feldspar termed potash-oligoclase (Mac- roxene (2VT = 60°); it probably is augite. Donald, 1942) or "anemousite" (Earth, 1931). Some grains have 2VT as low as 20° suggesting The diffuse character of the optic properties that subcalcic or pigeonitic augite may also be suggests that the positive axial sign may be due present. A few samples have euhedral micro- to overlapping of thin plates of alkali feldspar phenocrysts of augite. Olivine (Foss-g?) is the as discussed by MacDonald (1942). dominant microphenocryst (~8 percent of the One sample has an irregular cluster of rock) and grains with similar optical properties plagioclase grains (ABST) which was partly are also in the groundmass. The micropheno- melted on margins of the cluster and between crysts (0.6 to 1.5 mm) have well-formed individual grains and is spotted with glass. It crystal outlines but many are deeply resorbed; probably is a xenolith. No other evidence for all are fresh. A common form is remarkably contamination was found. elongated along crystallographic X. The A chemical analysis and molecular norm for a opaque minerals have X-ray characteristics of phenocryst poor sample is in Table 1. The magnetite; they are disseminated through the SiC>2, TiC>2 and KaO contents, the Ba, Sr, Y groundmass and less commonly are included by and Zr concentrations, and ratios such as olivine and pyroxene. Irreversible change of the Fe2O3/FeO, Na/K and K/Zr all are typical of magnetization on heating indicates the presence alkali basalts. The normative composition, with of maghemite (M. Ozima, 1970, written 2.6 percent Ne and 10.8 percent Ol would plot commun.). About 10 to 15 percent of the rock on the Ne side of the plane of critical silica is very finely crystalline and isotropic ground- undersaturation in the basalt tetrahedron mass occupying interstices between pyroxene (Yoder and Tilley, 1963). The composition of and plagioclase. Identifiable minerals include the Northeast Bank sample is also similar in plagioclase microlites, clinopyroxene (2VT = many respects to an average oceanic alkali 60°), olivine, apatite, and opaques. Scattered basalt described by Engel and others (1965; see irregular patches having low n and very low 8 Table 1). TABLE 1. CHEMICAL ANALYSES AND MOLECULAR NORMS OF ALKALI BASALTS

Major Elements Trace Elements (ppm) Molecular Norm

AG-D-2a Sample AG-D-2a (anhyd) EAB AG-D-2a EAB AG~D-2a EAB

SiO2 46.09 47.32 47.41 Ag 0.6 Cc 0.96 TiO2 3.18 3.27 2.87 B <20 Ap 0.32 1.96 AljOj 18.72 19.22 18.02 Ba 880 498 Or 10.10 9.99 Fe203 3.11 3,.19 4.17 Co 37 25 Ab 24.70 33.94 FeO 5.81 5,.97 5.80 Cr 170 67 An 31.43 26.85 MnO 0.17 0,.17 0.16 Cu 57 36 Ne 4.13 1.53 MgO 5.75 5,.90 4.79 Mn 1000 11 4.54 4.07 CaO 8.94 9,.18 8.65 Ni 230 51 Mt 3.33 4.44 Na20 3.44 3..53 3.99 Sc 32 26 Di 8.46 8.60 K2O 1.67 1.71 1.66 Sr 650 815 Fo 9.44 7.33 H20+ 1.21 0.79 U* 0.4 Fa 1.43 1.15 H2O~ 1.14 0.61 V 160 252 Fo/Fo + Fa 86.8 86.4 P20S 0.15 0..15 0.92 Y 52 54 An/An + Ab 59.9 44.2 CO2 0.37 0.37 Zr 240 333 D.I. 38.9 45.4 s tr. *U concentration determined by S.I. 29.1 23.5 fission trackstudy experiment

Total 99.75 100.00 99.84 Fe203/FeO 0.54 0.72 Na/K 3 2 K/Zr 59 41 K/Ba 16 28 K/Cr 84 206 Samples AG-D-2a Dredged from Northeast Bank (major elements analyzed by H. Onuki; spectrographic analysis for trace elements by E. Bingham). EAB Average oceanic alkali basalt (Engel and others, 1965)

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A minor amount of plagioclase-rich rock, crusted volcanic breccia and hyaloclastite. probably a basaltic andesite, may represent a Orange to buff colored hyaloclastite; irregularly differentiate of the basalts. The major min- angular to rounded, 1 to 2 cm diameter, eralogic differences for the plagioclase-rich orange-brown spotted glassy basalt fragments; rocks are: (1) they have abundant plagioclase and irregular clumps of gray silty ash are phenocrysts (Anes), these have oscillatory mixed in a porous aggregate. The dominant zoning; (2) they lack olivine; (3) augite mineral of the altered material, as indicated by phenocrysts are subrounded, many have both X-ray diffraction and glycolation of samples, is an opaque coating and hematite on their rims; montmorillonite; traces of calcic plagioclase (4) patches of sanidine are common and there and clinopyroxene also are present in the are a few irregular patches with large 2V7 hyaloclastite and the silty ash. One of the most (albite?). The groundmass is a dense micro- striking features of these samples is the crystalline aggregate; recognizable materials abundance of molds of fossil fragments (see include feldspar, opaques, glass and clinopy- Figs. 3 and 4). The intimate mixture of the roxene (?). Clusters of euhedral carbonate volcanic material and fossils, especially the (dolomite?) are enclosed by the groundmass. glassy debris which was molded around fossil These do not fill vesicles but have textures sug- fragments, suggests that fragmented and mixed gesting that they are an integral part of the organic remains were engulfed in freshly rock. Fibrous clusters of chalcedony fill the erupted lava and volcanic debris. The breccias rare vesicles. The hematite coating on grains and hyaloclastites are encrusted with layers of and in the groundmass suggests that these rocks manganese minerals. The outermost layers ap- crystallized in a more oxygen-rich environment than the basalts. Altered Basaltic Rocks A minor amount of volcanic rocks show effects of alteration. These effects were found only in diabasic textured basalts and are re- stricted to alteration of interstitial glass to chlorite, extensive alteration of olivine to iddingsite, formation of hematite from opaque minerals, and filling of vesicles and cracks with chlorite or carbonate, or both. Other major differences are: these rocks have purplish -- augite which partly encloses plagioclase giving ------a sub-ophitic texture; plagioclase micro------phenocrysts are Aneo and laths are zoned from An52 to Anw; all of the olivine is bright red- orange and completely altered to iddingsite; opaques are present as both plates and needles and are coated with hematite. They form about 5 to 7 percent of the rock in contrast to the fresh basalts which have about 3 to 5 percent. Patches of the alkali feldspar ("anemousite") are much more abundant than in the finer grained basalt and form 5 to 7 percent of the rock. Collectively, the data suggest that these samples cooled more slowly, the magma was enriched in alkaline components and probably was enriched in volatiles. The samples may be parts of pillow or flow interiors or may be dike fragments. The alteration probably is largely ------the result of late magmatic processes. ------Volcanic Breccia and Hyaloclastite Figure 3. Hyaloclastite and fossil assemblage molds from Northeast Bank. A and B. External molds of About one-third of the dredge haul was Mn- mollusks in hyaloclastite.

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Figure 4. Fossils from Northeast Bank. Shallow sublittoral assembkge (depth < 50 m); (A) Ostrea sp; (B) 'letraclita sp. Deep sublittoral assemblage (depth <150 m); (C) Patinopecten healeyi; (D) Patinopecten healeyi.

pear to be noncrystalline; they are X-ray with sea water. The poor sorting, faunal amorphous, form tiny rosettes, and give a mixing with inconsistent fragmentation, and nodular appearance to the surface of the crust. burial took place at about the same time as Inner layers are better crystallized and give the organisms lived. Paleontological evidence X-ray patterns typical of todorokite. Crust suggests a mixing of shells representative of thicknesses vary from a few millimeters to a depths both shallower and deeper than —50 maximum of 1 cm. m; this includes some intertidal forms (barnacle This rock assemblage indicates the inter- Tetraclita). Sedimentological evidence suggests action of liquid lava and hot volcanic debris deposition below surf base (about —10 m).

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PALEONTOLOGY OF THE BANK borderland. Emery (1960) believes that the borderland ridge-trough structure began to Some blocks of volcanic breccia and hy- form late in the Miocene. An older limit for the aloclastite contain abundant external molds of bank origin is probably late Miocene. mollusks and other benthonic marine in- Sedimentary rocks comparable in age to vertebrates. The fossils were identified by fossiliferous Northeast Bank volcanic breccias filling the molds with latex and using the casts may be represented in the non-volcanic Potato as models; examples are shown in Figure 4. The Harbor Formation on northeastern Santa Cruz fossils are in part fragments broken before Island which also contains Patinofecten healeyi burial and solution, and in part are fossils and other fossils indicative of water depths buried whole before separation of valves or similar to those interpreted from Northeast damage to delicate shells. At least two faunal Bank fossils (Weaver and Meyer, 1969, p. 99- associations are represented by Northeast Bank 103). The Santa Barbara Formation of the fossil assemblages. These could occur in con- nearby mainland and its correlatives on tiguous settings in that area during the northern Continental Borderland islands (Val- Pliocene or early Pleistocene epochs. A entine and Lipps, 1967) are younger than either moderate size oyster with finely crenulate inner the Potato Harbor Formation or the Northeast shell margins (Fig. 4A) and a barnacle with Bank rocks. large opercular plates (Fig. 4B) suggest Ostrea The dredged samples have been dated by palmula and Telraclita, respectively. These, three other independent methods. Thick- together with living counterparts of some of the nesses of manganese crusts suggest a wide range mollusks represented in the fossil assemblages, of ages depending on the accumulation rates now live in rocky warm eastern Pacific areas at used. Estimates of from 0.1 to 1 cm per m.y. depths no greater than 50 m. Patinofecten are obtained for deep sea samples. The dredged healeyi (Figs. 4C and D) and several other samples probably are greater than 1 m.y. and mollusks appear to represent slightly deeper less than 10 m.y. old using these rates. K/Ar and cooler marine conditions over a less rocky dates on submarine basalts are of questionable substratum. value because of the problems of retention of The paleontological age of the fossil as- mantle argon or exchange of K with sea water. semblages is Pliocene or early Pleistocene, based However, the sample used for a K/Ar date was principally on the occurrence of Patinofecten finely crystalline and, based on the depth healeyi. That fossil bivalve occurs in strata as interpretations discussed below, probably young as the Blancan land mammal age (1.7 to cooled in depths of about 50 m. Thus, it seems 3.5 m.y. according to potassium-argon analyses likely that the excess argon problem may have of Evernden and others, 1964; C. A. Repen- been avoided. The crystallinity and freshness ning, 1970, oral commun.) in the San Diego of the sample makes it seem likely that K Formation of coastal Southern California exchange with sea water was minimal. The (Allison, 1964). A lower range limit of P. sample gave an experimental age1 of 4.36 + healeyi may be represented by its occurrence in 0.8 m.y., or late Pliocene on the Holmes time the lower part of the Purisima Formation of scale. This agrees with K/Ar dates associated coastal Central California (Cummings and with other occurrences of Patinopecten healeyi. others, 1962). Glauconite in the same part of A fission track age determination on a glassy the Purisima Formation yields a potassium- rind gave an experimental age of 4.5 + 0.5 argon date of 6.7 ± 0.5 m.y. (unpub. deter- m.y. An appendix containing a discussion of mination by U. S. Geological Survey File No. this experiment is on deposit with NAPS2. The KA 396). Tetradtta suggests that the Northeast overlap of these four methods suggests a Bank fossil assemblage represents the younger probable age of from 4 to 4.5 m.y. or late part of this possible age range. Pliocene for the latest volcanic activity on the bank. REGIONAL AGE RELATIONSHIPS OF THE SAMPLES

The material recovered is representative only 1 K/Ar date by M. Ozima, University of Tokyo. of the outermost layers of the bank and we have * This material may be obtained from CCM Informa- no reliable way of determining the absolute age tion Corp.-NAPS, 909 Third Avenue, New York, New range of the whole edifice. The bank appears to York 10022. Request Document No. 01193, and enclose be superposed on the structural grain of the check to NAPS; $2 for microfiche, $5 for photocopy.

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HISTORY OF THE BANK m for the bank to reach isostatic adjustment would be required. Thus, the inferred sub- Assuming that the paleo-bathymetry in- sidence of about 500 m is easily explained by dicated by the fauna is correctly interpreted, a isostasy. net vertical displacement of the hyaloclastite of 500 m since its formation is indicated. Un- CONCLUSIONS like seamounts, Northeast Bank has a flattened The geologic record of the Southern Cali- summit (if larger and in deeper water it would fornia Borderland shows that volcanism was be called a guyot), suggesting that it was widespread and more or less continuous planed off by surf action. Thus, it once may throughout much of late Tertiary time. The have been an island and may have been eroded rocks of Northeast Bank are the youngest yet and submerged prior to the last eruption; reported in this region and are distinctive in alternatively it may never have reached above that they are alkalic basalts, in contrast to older surf base (—10m) and the flat summit may be volcanic rocks of the region, many of which are essentially a primary form resulting from typical of the calc-alkaline series (J. Hawkins, erosion and redistribution of volcanic material unpub. data). This may reflect a fundamental close to sea level. As the shallowest part of the change in the depth of magma generation in bank rises to within 357 m of the surface, it this region. could be argued that a minimum subsidence of Rocks of Northeast Bank give a pronounced 300 m was involved and the material recovered positive magnetic anomaly (unpub. SIO data). in the dredge haul has rolled downslope to its This high lies on the projected trend of sea- present location. The breccias, hyaloclastites floor anomaly 5C of the Mason-Raff lineations and some of the fossils are very fragile and show and, because of the coincidence, suggests that little evidence of abrasion; it seems highly un- the sea-floor trends continue through the likely that the breccias have moved very far borderland. If true, this poses formidable from their site of accumulation. Our inter- obstacles to the sea-floor spreading concept. pretation of the available data is that the fossil However, the Northeast Bank anomaly must fauna once capped the north end of the bank be only about 4.5 m.y. old and cannot have a when it was within 50 m of the surface. It was genetic relation to the 16 m.y. old anomaly engulfed in volcanic material during one of the 5C. Nothing in our data supports the idea that final eruptions and later was submerged some sea-floor magnetic trends continue through the 500 m. The flattened bank crest thus may in borderland. The magnetic data will be discussed part be depositional, built close to surf base, separately but preliminary results indicate the probably forming a cap on a flattened platform lack of a consistent regional magnetic pattern of erosional origin. The shallowest part of the in the borderland. bank may be either a remnant not removed by The subsidence of the bank as interpreted wave planation or it may be a constructional from paleontologic data was on the order of 500 form built below surf base. m in about 4.5 m.y. Although the borderland The net vertical displacement of between 300 is in a region where crustal dilation and, to 500 m in about 4 m.y. is of interest because presumably, vertical crustal movements have the borderland is part of a tectonically active been important in Quaternary time, it appears area, which is the site of present seismicity and that the subsidence of the bank could be due which, in terms of the new global tectonics, is at entirely to isostatic adjustments. the interface between the American and Pacific Four different dating techniques used on Plates. However, in spite of the proximity of these samples show close agreement. The latest the bank to basins interpreted to be down- volcanism (4,5 m.y. old) took place during late dropped or downwarped blocks, it appears that Pliocene time. the bank probably has subsided as a result of isostatic adjustments. The following assump- ACKNOWLEDGMENTS tions were used in calculating the amount of We thank E. L. Winterer, Chief Scientist of vertical adjustment due to a mass such as the Borderland Cruise for his assistance, for Northeast Bank. Bank density: 2.8 gm/cc; making the samples available for our study, and mantle density at the compensation depth: for his critical reading of the manuscript. The 3.25 gm/cc; depth of the sea floor prior to cooperation of Captain L. Davis and the crew subsidence: 1300 rr; height of bank prior to of the R/V Agassiz is acknowledged, and F. subsidence: 1250 m. Vertical movement of 900 Dixon is thanked for his supervision of the

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dredging operations. V. Zullo made barnacle sea floor off Southern California: Geol. Soc. identifications, M. Ozima made the K/Ar date Amer., Bull., Vol. 56, p. 431-478, 1945. and C. A. Repenning provided information on Engel, A. E. J.; Engel, C. G.; and Havens, R. G. radiometric dates and related mammal ages. Chemical characteristics of oceanic basalts and We thank D. Lai for initiating the fission track the upper mantle: Geol. Soc. Amer., Bull., Vol. 76, p. 719-734, 1965. work at SIO, and f. Hornbuckle for making the Evernden, J. F.; Savage, D. E.; Curtis, G. H.; and neutron irradiation. Partial financial support James, G. T. Potassium-argon dates and the for the study came from National Science Cenozoic mammalian chronology of North Foundation grants GA-1403 (to E. C. A.) and America: Amer. J. Sci., Vol. 262, p. 145-198, GA-16120 (to J. W. H.); ship operations were 1964. funded by National Science Foundation grant Krause, D. C. Tectonics, bathymetry, and geo- GA-1300. The track study was partly sup- magnetism of the southern continental border- ported through National Aeronautics and Space land west of Baja California, Mexico: Geol. Administration grant NASA NGL 05-009-002. Soc. Amer., Bull., Vol. 76, p. 617-650, 1965. MacDonald, G. A. Potash-oligoclase in Hawaiian REFERENCES CITED lavas: Amer. Mineral, Vol. 27, p. 793-800, 1942. Allison, E. C. Geology of areas bordering Gulf of Valentine, J. W.; and Lipps, J. H. Late Cenozoic California: in Marine geology of the Gulf of history of the southern California islands: in California (Tj. H. Van Andel, and G. G. Shor, Biology of the California Islands, Symp. Proc., Jr., eds.), Amer. Ass, Petrol. Geol., Mem. 3, Santa Barbara Botanic Garden, p. 21-35, 1967. p. 3-29, 1964. Weaver, D. E.; and Meyer, G. L. Stratigraphy of Barth, T. F. W., Mineralogical petrography of northeastern Santa Cruz Island: in Geology of Pacific lavas: Amer. J. Sci., 5th Ser., Vol. 21, the northern Channel Islands (D. E. Weaver, p. 377-405 and p. 491-530, 1931. ed.), Amer. Ass. Petrol. Geol., Pacific Sect., Cummings, J. C.; Touring, R. M.; and Brabb, Spec. Pub., p. 95-103, 1969. E. E. Geology of the northern Santa Cruz Yoder, H. S.; and Tilley, C. E. Origin of basalt Mountains, California: in Geologic guide to magmas: an experimental study of natural the gas and oil fields of Northern California and synthetic rock systems: }. Petrol., Vol. 3, (O. E. Bowen, Jr., ed.), Calif. Div. Mines p. 342-532, 1963. Geol., Bull. 181, p. 179-220, 1962. Emery, K. O. The sea off Southern California: John Wiley d Sons, Inc., 366 p., New York, 1960. MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST Emery, K. O.; and Shepard, F. P. Lithology of the 27, 1970

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