1918 J Davis: The Radiolarian Cherts of the Franciscan Group 263 



bands. The banding runs parallel to the bedding and apparently is 

 original, yet there is no evidence that the vein cuts the dark portion of 

 the chert. It seems to come from it and be a part of it. It is sug- 

 gested that the black and white bands were laid down together, but that 

 the white portions consolidated first, and during some slight move- 

 ment became fractured so that the unconsolidated clearer silica was 

 forced into the fissure. 



MASSIVE WHITE CHEETS 



Another variety, referred to before as massive white chert, occurs 

 in large bodies in Marin County and in the San Miguel Hills in San 

 Francisco. It also occurs elsewhere in lenses in the midst of other 

 varieties of chert, This variety is characterized by the fact that only 

 small amounts of shale appear in the partings. 



Cherts of this variety differ in appearance from the black and 

 white cherts before mentioned in that they are not laminated. They 

 are usually pale, creamy white in color. The radiolaria are not readily 

 apparent in hand specimens. 



In thin sections the matrix is seen to be composed of nearly clear 

 silica, clouded by extremely minute mineral grains. In this matrix, 

 there are rounded or elliptical areas of clear silica which represent 

 radiolaria with rather poorly preserved outlines. Between crossed 

 nicols the matrix is seen to consist largely of cryptocrystalline silica 

 in which there may be occasional areas of amorphous silica. 



THE MATEIX OE THE CHEETS 



This subject will be discussed more fully later, when the question 

 of the origin of the cherts is considered, but it should be here empha- 

 sized that all these radiolarian cherts contain more silica than may be 

 accounted for by the radiolaria which they now contain. The radio- 

 larian skeletons are always embedded in a matrix of silica, which gives 

 no evidence of organic derivation (plates 30a-e). As previously 

 noted, the number of radiolaria present varies between wide limits. 



CONCEETIONS IN CHEET 



The occurrence of concretions in certain cherts in the Santa Ynez 

 Mountains and in North Berkeley has been referred to (page 252). 



In thin sections, these concretions are seen to be much like the 

 cherts which enclose them, save that they contain less iron oxide 



