I 918 ] Davis: The Radiolarian Cherts of the Franciscan Group 371 



of the silica of these cherts was derived from radiolarian skeletons, 

 and regarded them as the equivalents of radiolarian oozes. He says : 136 



I have found every gradation in the specimens from those in which the 

 radiolaria are distinctly marked, as Professor Lawson says, to those in which 

 they are only faintly distinguishable from the matrix, or apparently absent. 

 In my opinion this state of things gives good ground for the view that, owing 

 to possible transformations through the action of sea water, and the secondary 

 changes which are known to have taken place, there is no valid reason for denying 

 the organic origin even when no organic remains are distinguishable. 



Evidence opposed to the Idea that Matrix was derived from 

 Radiolaria. — One who holds that the cherts are radiolarian oozes, 

 altered on the sea floor, or changed after a partial consolidation, must 

 regard the radiolaria that are now present in these rocks as survivors 

 of a once very much larger number which made up the radiolarian 

 ooze. He must believe that the greater proportion of the silica has 

 been reworked by the actions above set out, until it is now found in 

 the matrix. 



There are cherts, which seen in thin section, might be taken as 

 supporting the above contentions, provided such cherts existed alone. 

 In them, one can see large numbers of radiolarian skeletons, among 

 which there appear to be all transitions between radiolaria in a fair 

 state of preservation, down to irregular areas of clear silica which 

 are apparently fusing with the matrix of the rock. 



There are, however, numerous other sections in which the radio- 

 laria are not so abundant, but in which they are in an excellent state 

 of preservation, and the apparent gradation to imperfect and much 

 altered forms is not observable (plate 30a, b, c, and e). If one 

 regards the matrix as due to reworking of the silica originally con- 

 tained in the skeletons of the radiolaria, he is at a loss to account for 

 the good preservation of the very few surviving forms, since they do 

 not appear to be of any more resistant types than some which, in other 

 rocks, are partially obliterated. 



Moreover, some of the beds in a given outcrop will show numerous 

 radiolaria, apparently being crowded with them. Other beds in the 

 same section show only a few radiolaria in a matrix of red silica. 

 Such differences can hardly be ascribed to anything other than to vari- 

 ations in original deposition. At one time there may have been more 

 radiolaria in the surface waters than at another, or else there may 

 have been a variation in the silica contributed from some inorganic 



isn.Jour. Geol., vol. 5, p. 66, 1897. 



