374 University of California Publications in Geology [Vol. ll 



Another objection to the idea is based on the nature of the Bar- 

 bados earths. Here diatoms are found in considerable amounts in 

 the radiolarian earths, and they appear not to have been greatly 

 changed. This, however, may be explained as due to the Tertiary age 

 of these beds and the fact that they have not been subjected to as many 

 changes as have the older rocks. 



Another objection to all hypotheses of this type is found in the 

 cherts of the Monterey. These show, as pointed out in detail pre- 

 viously, no evidence of having been derived from diatomaceous earths, 

 and they appear to have been formed by the precipitation of gela- 

 tinous silica. The Monterey cherts are independent in origin from 

 the diatomaceous earths which are associated with them. 



Other Sources of Silica of Matrix. — Still other possibilities have 

 been suggested for the source of the excess silica. It may be possible 

 that the silica was derived from sponge spicules or volcanic glass 

 present with the radiolaria. No evidence can be presented in favor 

 of the idea that any of these were the source of the excess silica. The 

 sponge spicules would not be very easily altered and would be as 

 likely to be preserved as the radiolaria. It does not seem probable 

 that volcanic glass could be so completely altered that it would not 

 somewhere leave evidence of its presence in the cherts. Small amounts 

 of it, which have been little altered, are found in some of the red 

 shales. 



Possible Differences in Radiolaria. — So far in the discussion, it 

 has been assumed that radiolaria were all alike in their power of re- 

 sistance to alteration. Some radiolaria possess skeletons consisting of 

 pure silica ; others possess skeletons composed of chitinous material ; 

 and still others possess skeletons partly of chitinous material and 

 partly of silica. It may be that the skeletons which are in part chiti- 

 nous are easily decomposed by bacterial action, leaving the silica 

 which they contain in a very finely divided condition. This finely 

 divided silica, organic in its source, might form a paste in which 

 siliceous skeletons were embedded. On consolidation this would give 

 a matrix showing no organic structure. This explanation of the 

 matrix would clear up many of the difficulties and still permit us to 

 regard the cherts as entirely of organic origin. It is opposed, how- 

 ever, by certain facts regarding the bedding, which will be pointed 

 out later. 



Hypothesis of Later Silicification. — Another possibility is that the 

 cherts are really radiolarian oozes which have been changed into 



