FRANCISCAN GROUP 



29 



with the analyzed graywacke, column 7, and black 

 siltstone, column 9. However, they contain more total 

 iron, which is also more oxidized, and they contain 

 less aluminum and magnesium. Unlike many of the 

 other rocks of the Franciscan group, the shales occur- 

 ring with chert have a potash-soda ratio of more 

 than 1. 



FOSSllS 



Although (he Radiolaria of the chert in some places 

 are remarkably well preserved, they are not suf- 

 ficiently diagnostic to date the Franciscan group more 

 closely than Jurassic or Cretaceous. Hinde (1894, p. 

 236), who studied the Radiolaria from Angel Island 

 in San Francisco Bay and Buri-Buri Ridge in San 

 Mateo County, reported as follows: 



The majority of them evidently belong to simple spheroidal 

 and ellipsoidal forms, included in Haeckel's suborders Sphae- 

 roidia and Prunoidia * * * The most distinctive feature * * * 

 [is] the number and variety of forms of the genus Dictyo- 

 mitra present. 



Some of the better preserved Radiolaria found in the 

 New Almaden district are shown in figure 19. 



Origin 



The interesting problem of the origin of the chert 

 in the Franciscan group has been dealt with by several 

 of the geologists most familiar with the rocks of the 

 California Coast Ranges. It has not been particularly 

 studied during this investigation, but a summary of 

 the various suggested origins is given here, along with 

 some pertinent data gathered in the course of our 

 work, for the benefit of those who may not wish to 

 read the extensive literature on this subject. 



Although many of the early workers described ob- 

 servations bearing on the origin of the chert, or sug- 

 gested possible sources for the silica, the first to make 

 a thorough field and laboratory study of the problem 

 was Davis (1918, p. 353-408). He concluded, after 

 dismissing many other possibilities, that the silica 

 came from submarine siliceous springs of magmatic 

 origin, that most of the silica was chemically precipi- 

 tated and the Radiolaria were only incidental fossils, 

 and that the rhythmic bedding and wedging out of 

 beds were best explained by colloidal segregation of 

 silica from the intermixed shaly material. Taliaferro 

 (1933. p. 51, 54) believed that the source of the silica 

 must lie in siliceous water that accompanied the out- 

 pourings of volcanic material. At first he suggested 

 that the greater part of the silica was supplied by 

 magmatic water, but some silicic acid may have been 

 liberated by the interaction of hot basic lavas with 

 sea water. In a later report Taliaferro (1943b, p. 147- 

 148) stated, "a considerable amount [of the silica] 



might have resulted from the interaction of hot lava 

 and sea water." Trask and others (1943, p. 58) and 

 Trask and Pierce (1950, p. 219-221) in their reports 

 on the manganese deposits of California gave evidence 

 suggesting that the chert was deposited in local basins, 

 but reached no conclusion regarding the source of the 

 silica. Bramlette ( 1946, p. 55) , who studied intensively 

 the cherts of the diatomaceous Monterey formation in 

 California, suggested that possibly the solution of the 

 thinner shelled siliceous organisms may have provided 

 the silica for the chert during diagenetic changes or 

 low-grade metamorphism of the Franciscan rocks. 

 Another suggestion formerly entertained, that the 

 cherts result entirely from the accumulation of radio- 

 larian tests under abyssal conditions, does not seem 

 to fit observations made in the Franciscan rocks, and 

 it has been generally abandoned. 



Most of our observations having a bearing on the 

 origin of the chert consist of relationships between 

 the chert and greenstone. These can be seen on the 

 geologic map of the district, which shows that chert 

 is most abundant in areas containing greenstone, and 

 that more than three-fourths of the chert lenses are 

 either enclosed in greenstone or border a mass of 

 greenstone. Probably equally significant is the fact 

 that nearly all the chert in contact with greenstone 

 lies on the upper side of the greenstone masses. Some 

 chert lenses, however, are apparently enveloped by 

 clastic sedimentary rocks, and although some of the 

 lenses are stratigraphically only a few hundred feet 

 above the greenstone masses, others would seem to be 

 truly far removed from any igneous rocks. The map 

 also indicates that the ratio of chert to greenstone is 

 about 1 : 250, which is pertinent if one considers the 

 possibility of the silica being derived by reaction of 

 sea water with hot lava. Whether or not this ratio is 

 representative of the Franciscan group cannot now 

 be determined because of the lack of detailed maps of 

 much of the extensive area occupied by Franciscan 

 rocks. 



The association of the chert and greenstone strongly 

 suggests that the cherts in the Franciscan group are 

 in some way genetically related to the greenstone. The 

 writers believe it likely that the silica and iron of the 

 chert and accompanying shale was derived chiefly, if 

 not entirely, by reaction of the hot mafic lava with 

 sea water. At shallow depth sea water, even though 

 heated nearly to its boiling point, is incapable of dis- 

 solving much more silica than can be retained on cool- 

 ing to normal sea temperature; but, under pressures 

 that exist at a depth of 10,000 feet or more, many 

 times as much silica could be dissolved. Hot silica- 

 ladened waters so formed would rise and be cooled to 



