Bank Tops 



urchins, and some worms living on and in 

 the glauconitic sediment. The benthic pop- 

 ulation is not dense, however, probably be- 

 cause the glauconitic sands are coarse and 

 very low in their content of organic matter. 

 Measurements show that the pH is typically 

 about 7.8 and the Eh about +300 mv, both 

 similar to the overlying sea water. Oxygen 

 dissolved in the overlying sea water is as 

 much as 90 per cent of saturation, depend- 

 ing on the water depth. These various facts 

 show that the general environment in which 

 glauconite is now found is one of oxidizing 

 conditions, in contrast to the common state- 

 ment that glauconite is restricted to anaero- 

 bic environments (Cloud, 1955; Lochman, 

 1957). Further chemical work may show 

 that glauconite forms in a microreducing 

 environment produced by decomposing 

 protoplasm within foraminiferal tests or in 

 faecal pellets, even though the interstitial 

 and overlying waters are oxidizing. 



It might be argued that the glauconite off 

 southern California is not forming now but 

 has been reworked from older sediments 

 that were deposited under anaerobic condi- 

 tions. In answer, it can be pointed out that 

 the glauconitic sands form a fairly uniform 

 blanket where present, that they are not 

 covered by a different kind of sediment, 

 that they overlie igneous and metamorphic 

 as well as sedimentary rocks, and that the 

 underlying sedimentary rocks belong to var- 

 ious parts of the Miocene and even to the 

 Pleistocene (Coronado Bank). Additional 

 evidence of age is provided by enclosing 

 foraminiferal tests which include in addition 

 to long-lived forms several of Pliocene to 

 Recent age: Angulogerina angulosa, A. car- 

 inata, Cassidulina californica, C. limbata, 

 C. lomitensis, C. tortuosa, C. translucens, 

 Cibicides mckannae, C. refulgens, Uvigerina 

 hollicki, U. peregrina, and U. senticosa (O. L. 

 Bandy, personal communication; Pratt, 

 1956). Some of these species are rare in 

 rocks older than Late Pliocene, and others 

 are rare in Recent sediments, perhaps sug- 

 gesting that they may really represent Pleis- 

 tocene deposits. Comparison of enclosing 

 Foraminifera with their normal depth range 

 shows that at least some of the glauconite 



213 



formed at the depths at which samples con- 

 taining it were taken. This is indicated by 

 the presence of glauconite within tests of 

 the shallow water species Cassidulina cali- 

 fornica, C. limbata, C. tortuosa, and Uvigerina 

 hollicki in samples from shelves and bank 

 tops and of the deep-water species Uvigerina 

 peregrina and U. senticosa at depths of 600 

 to 2000 meters on the continental slope off 

 Point Conception (Bandy, 1953a). 



Laboratory examination shows that the 

 glauconite grains occur in several forms. 

 Probably most of them are internal casts 

 still concealed within foraminiferal tests, a 

 type noted long ago by Murray and Renard 

 (1891) for five species of Foraminifera. At 

 least 17 species were found by Pratt (1956) 

 to contain glauconite; further work will 

 doubtlessly reveal many others. Casts are 

 far more abundant in Cassidulina than in 

 Globigerina, which has an aperture about 

 half as large as that of Cassidulina. Casts 

 also appear to be more common in weathered 

 opaque tests than in fresh-looking ones and 

 in larger specimens than smaller ones of a 

 given species. Casts have also been ob- 

 served in sponge spicules. A second mode 

 of occurrence, rare, is as replacements of the 

 calcium carbonate and silica of foraminiferal 

 tests, echinoid spines, and sponge spicules. 

 Thirdly, glauconite occurs as spheroidal, 

 ovidal, or discoidal pellets, perhaps copro- 

 lites. Finally, there are irregular grains, 

 lobed, cracked or layered, and smooth- to 

 rough-surfaced. The color ranges from 

 light green to dark green to black, some of 

 the darker ones having brown cracks or sur- 

 faces. The light-green grains are much 

 softer than the darker ones and have a lower 

 index of refraction and a lower specific 

 gravity. They also are mostly restricted to 

 internal casts still protected by a foraminiferal 

 test. 



The origin of the glauconite is still uncer- 

 tain, and very possibly several origins are 

 involved. Grains presenting a continuous 

 series from biotite to glauconite can be 

 selected in the manner of Galliher (1935), 

 probably indicating the formation of some 

 glauconite from biotite. However, it should 

 be pointed out that biotite is not as common 



