1918] Davis: The Radiolarian Cherts of the Franciscan Group 351 



San Francisco Peninsula. He pointed out the fact that no other 

 microscopic organisms could be recognized in the cherts, but stated 

 that it was quite possible that diatoms had been originally inter- 

 mingled with radiolaria. He believed that the more delicate diatoms, 

 if originally present, might have been obliterated, while the coarser 

 structures of radiolaria were only partly obliterated. 



Later, a paper by H. W. Fairbanks 115 appeared, in which he de- 

 scribed the cherts of the Franciscan formation. With regard to the 

 origin of these rocks, he followed Ransome in believing that they were 

 the equivalents of radiolarian ooze, and that practically all their 

 silica was originally in the tests of radiolaria. He believed that a 

 considerable part of the radiolarian tests had been obliterated in the 

 changes to which the rocks had been subjected. After describing the 

 microscopic character of the cherts, he concludes : 



The lessons to be drawn from these facts are that the jasper in its essential 

 character is not a metamorphic rock, and that it was formed of siliceous sedi- 

 ments resulting in great measure from organic life, as has been demonstrated to 

 be the case with similar rocks in other parts of the world. 



His description of the general geological relations of the cherts, is 

 in part as follows : 



The entire freedom of the jaspers from any fragmental material deposited in 

 the ordinary way near a shore would indicate their formation in deep or at least 

 quiet waters. The very rare occurrence, however, of limestone in this series and 

 the abundance of sandstone would seem to indicate the absence of deep-sea con- 

 ditions during the deposition of the greater portion. 



No one has yet worked out the stratigraphic position of the jasper beds in the 

 series, and ascertained if they are distributed through it or confined to a single 

 horizon. The wide occurrence of the jasper beds may not, perhaps, result so 

 much from any great extent vertically as from the extremely crushed and broken 

 condition of the series as a whole. As a result of this condition, strata of the 

 same or nearly the same horizon might be exposed in many places. So far as the 

 writer is aware, jaspery beds are absent from the recognized Cretaceous, but in 

 the Miocene there again appear flinty beds of probably the same origin, but 

 wholly free from the secondary silicification so characteristic of the earlier ones. 



In 1895, Professor A. C. Lawson's 116 report on The Geology of the 

 San Francisco Peninsula Was published. At this time there was 

 presented the first complete and satisfactory description of these 

 cherts and their associated shales. 



Professor Lawson here recognized the fact that the silica of these 

 cherts was not entirely due to the radiolarian skeletons present in 



us A Eeview of our Knowledge of the Geology of the California Coast Range, 

 Bull. Geol. Soc. Am., vol. 6, p. 82, 1895. 



n« 15th Ann. Rep. U. S. Geol. Surv. 1893-94, pp. 401-76, 1895. 



