296 University of California Publications in Geology [Vol. 11 



3. In one case, described above, very small lenses of chert are 

 found within a stratum of sandstone, that, in its turn, is enclosed 

 within bedded cherts. The sandstone is soft, and very porous. The 

 lenses of chert are of the ordinary sort and exactly like the bedded 

 cherts that enclose the sandstone bed on either side. If these lenses 

 had been produced by the silicification of small lenticular masses of 

 diatomaeeous earth in the sandstone, the solutions must have perme- 

 ated the sandstone and filled the pore spaces in it. 



4. If one believes that these cherts were originally diatomaeeous 

 earths, then the remains of diatoms and radiolaria must have been 

 obliterated, during the alteration, since the cherts do not now show 

 them. Such an alteration is hard to understand, when one remembers 

 that the delicate shells of foraminifera are often found in a well pre- 

 served state in these cherts. However, the solutions may have been 

 able to dissolve silica but not to dissolve lime carbonate. It is also 

 possible that if opaline silica were introduced into a diatomaeeous 

 earth, the filling of the pore spaces with material of about the same 

 refractive index, would cause the organisms to become invisible. 



5. A further fact, that is incomprehensible on the assumption of 

 silicification, is the occurrence of white diatomaeeous shale, chocolate 

 bituminous shale, black flinty chert, black bituminous shale, white and 

 gray bedded cherts and terrigenous shales, interbedded with one an- 

 other, and all exposed in the same section. On the idea of original 

 differences in deposition, such a section presents no special difficulties. 



6. In many instances the cherts appear to be confined to definite 

 horizons, while the siliceous shales are confined to other horizons. 

 Arnold, for example, found that the lower part of the Monterey shale 

 in the Santa Maria district was characterized by cherts, while in the 

 upper part the organic shales were the predominant type. Lawson 30 

 found in the Concord quadrangle that there were three distinct hori- 

 zons of siliceous shale in the Monterey group, all separated by sand- 

 stones. Of these the lower horizon is predominantly cherty, while the 

 other two are characterized by siliceous shale and do not contain chert, 

 except in small amounts. In view of this it seems difficult to accept 

 the notion that the cherts occur in those parts of the formation which 

 are most contorted. The upper horizons in the Concord area are 

 about as much disturbed as the lower and yet they contain little 

 chert, and the general relations do not bear out the idea of chert 

 formation in regions of greater disturbance. In many places the 



so San Francisco Folio, op. cit. 



