350 University of California Publications in Geology [Vol. ll 



that the cherts were metamorphosed Knoxville shales, Fairbanks, at 

 this time, believed that the cherts were metamorphic rocks. 



In 1894, F. L. Ransome 114 described the geology of Angel Island 

 and during his study of this area the true nature of the organic re- 

 mains was discovered. While Becker had recognized the presence of 

 the remains of organisms in the cherts, he had failed to distinguish 

 their true nature. Ransome 's ideas regarding the origin of the cherts 

 may be seen from the following quotation : 



. . . Having found the remnants of siliceous organisms in the less siliceous 

 portions of the cherts, they were subsequently recognized in the more silicified 

 faeies as little spherical bodies of clear cryptocrystalline silica, possessing more 

 or less definite boundaries. From the wavy extinctions observed upon turning 

 the stage between crossed nicols, it is supposed that the silica is in the form of 

 chalcedony. It is possible, if not probable, that, although the radiolaria are best 

 preserved in the more earthy and less siliceous portions of the rock, they were 

 originally much more abundant in the jaspery varieties, but have there become 

 obscured by the solution and recrystallization of the abundant silica furnished 

 by their remains. 



Whatever may be the source of the silica forming the cherts and jaspers in 

 other portions of the Coast Ranges, on Angel Island it appears unnecessary to 

 call upon any hypothesis of a general secondary silicification to explain their 

 origin. It is a more simple, and, in the light of the foregoing facts, a more 

 probable supposition that they were originally siliceous deposits, the silica being 

 derived largely from organic remains and varying in amount in different portions 

 of the series, both vertically and horizontally, with the former abundance or 

 scarcity of the siliceous organisms. When these were few, the non-siliceous com- 

 ponents of the deposit would predominate and furnish a dark matrix for the 

 preservation of the radiolaria. When, on the other hand, the latter were very 

 abundant, the deposit would be largely opaline silica, in a condition susceptible to 

 solution, and the organic structure would become obliterated. 



Ransome suggested that these rocks be thereafter spoken of as 

 ' ' radiolarian cherts ' ' : 



Although, in many localities the name "bedded jaspers" may be applied to 

 these rocks with perfect propriety, yet the investigation of their representatives 

 upon Angel Island, appears to indicate that ' ' radiolarian chert ' ' is, on the whole, 

 a better designation for them, inasmuch as it easts some light upon their origin, 

 and suggests their relationship with similar • cherts in Europe. 



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. 



In an appendix to Ransome 's paper, J. G. Hinde described the 

 radiolaria in specimens of the chert from Angel Island and in speci- 

 mens collected by Professor Lawson from Buri-Buri Ridge on the 



11* Geology of Angel Island, op. tit, vol. 1, pp. 199-200, 1894. 



