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University of California Publications in Geology [Vol. 11 



them and this, together with the field distribution of the areas of 

 chert, led to somewhat different ideas concerning the mode of origin 

 of these rocks. The following quotation will indicate the conclusions 

 arrived at : 



. . . As regards the organic origin of the silica of which the chert is com- 

 posed, it seems to the writer that there are features both in the slides and in the 

 field occurrence of the formation which do not harmonize with this supposition. 

 In the slides having the radiolarian remains, the latter generally occur as easts 

 of forms embedded in a matrix of silica which shows no evidence whatever of 

 organic origin. The cavities of the Radiolaria have been filled with chalcedonic 

 silica, and are in definite contrast with the non-chalcedonic matrix. The discrete 

 character of the fossils is significant of their mode of accumulation. The silica 

 seems to have been an amorphous chemical precipitate forming in the bottom of 

 the ocean in which the Radiolaria thrived. The dead Radiolaria dropped into 

 this precipitate, became embedded in it, and were so preserved. The state of 

 preservation is variable, but this is ascribable to the molecular changes that have 

 been in progress in the rock since its solidification, or to the solvent action of the 

 same agencies which held in solution and precipitated the silica. 



It thus seems to the writer that the bulk of the silica cannot be proved to be 

 the extremely altered debris of Radiolaria. The direct petrographieal suggestion 

 is that they are chemical precipitates. If now we accept this hypothesis, it be- 

 comes apparent that there are three possible sources for the silica so precipitated, 

 viz.: (1) Siliceous springs in the bottom of the ocean, similar to those well known 

 in volcanic regions; (2) radiolarian and other siliceous remains, which may have 

 become entirely dissolved in sea water; and (3) volcanic ejectmenta, which may 

 have become similarly dissolved. The last is the least probable, because we are 

 not actually familiar with such a reaction as the solution of volcanic glass by 

 sea water. Our ignorance is, however, no proof that such solution may not take 

 place under special conditions. Setting this third possibility aside, let us consider 

 to which of the other two the field evidence points. 



If the silica were derived from the solution of organic remains by sea water, 

 or indeed, directly from organic debris, we should expect to find the cherts having 

 a vast extent of fairly uniform thickness. On the other hand, if the silica were 

 derived from siliceous springs we might have the formation developed in lens- 

 like masses of varying thickness at different centers. The field evidence agrees in 

 an unmistakable way with the second of our supposed conditions. The radio- 

 larian cherts occur throughout the field, and indeed, throughout the Coast Ranges 

 in a sporadic way. Although some of the occurrences are many hundred feet 

 thick, they appear to thin out rapidly and do not form sheets comparable in 

 extent to the San Francisco sandstone. Most of the individual occurrences, 

 moreover, are of very limited extent, occupying only a few acres, or only a frac- 

 tion of an acre, and it seems impossible to conceive that they had any other than 

 a very local origin. Great numbers of these small patches of chert occur in the 

 sandstone which are so small that they can not be represented in the mapping 

 without gross exaggeration. The hypothesis of the derivation of the silica from 

 siliceous springs and its precipitation in the bed of the ocean in local accumula- 

 tions, in which radiolarian remains became embedded as they dropped to the 

 bottom, seems, therefore, the most adequate to explain the facts, and there is 

 nothing averse to it so far as the witer is aware. The abundance of the Radio- 

 laria may be due to the favorable conditions involved in the excessive amount 



