1918] Davis: The Badiolarian Cherts of the Franciscan Group 349 



Idria — into ordinary shales. Under the microscope, also, the most highly indu- 

 rated specimens are found to contain fossils. , 



... In the mass of the phthanites included between the quartz veins remains 

 of clastic structure are often visible, especially in reflected light. The most inter- 

 esting constituents of foreign origin are round spots, which often retain evidences 

 of organic character. Professor Joseph Leidy, at my request, has examined some 

 of the thin sections containing such spots, which he regards as probably foramini- 

 ferous shells. 



Becker seems also to have believed the Monterey cherts to be meta- 

 morphic also, since he speaks of them in the following words : ' ' light 

 cream-colored schists of Miocene age which occnpy a narrow strip 

 along the coast of California from the neighborhood of Santa Cruz 

 southward. ' ' 



Up to this time none of the geologists working in this region had 

 noticed the presence of the radiolaria in the cherts. This is not re- 

 markable, however, since no thin sections were made until Becker's 

 work. 



H. W. Turner, 112 writing in 1891, accepts Whitney's evidence of 

 transition and metamorphism, as shown at Bagiey Canon, Mount 

 Diablo. In an appendix to Turner's paper, Melville gives analyses of 

 various rocks from Mount Diablo. He states that the "common red 

 shale ' ' passes into ' ' silicified shale or phthanite. ' ' He writes : 



The great mass of these deposits in Mount Diablo is exceedingly folded and 

 distorted, yet shows the alternating layers of the two components, a phenomenon 

 quite usual in this class of rocks in the Coast Eanges. This rock is very friable, 

 and the phthanite can be readily separated from the shale. 



In 1892 and 1893, H. W. Fairbanks 113 in two successive papers, 

 showed the probability of the presence of an unconformity between 

 the Franciscan rocks and the Knoxville. He pointed out that Aucellae 

 were quite common in the Knoxville and that, although some of the 

 Franciscan sandstones and shales were little metamorphosed, Aucellae 

 were never found even in these slightly altered rocks. He showed 

 that there were sharp contacts above which unaltered Knoxville 

 shales rested on considerably disturbed and altered rocks of the older 

 series. He pointed out, also, that at Mount Diablo an " amygdaloidal 

 intrusive" cut the older series but was never found in the Knox- 

 ville. While these considerations tended to overthrow the old idea 



112 The Geology of Mount Diablo, California, Bull. Geol. Sbc. Am., vol. 2, 

 p. 383, 1891. 



113 The Pre-Cretaceous Age of the Metamorphic Rocks of the California Coast 

 Ranges, Am. Geol., vol. 9. p. 153, 1892. 



Notes on Farther study of the Pre-Cretaceous Rocks of the California Coast 

 Ranges, ibid., vol. 11, p. 69, 1893. 



