Anderson.] 



Point Reyes Peninsula. 



139 



that of an acid rhyolite. This hypothesis, however, could hardly 

 account for the whole of the vast accumulations of sediments that 

 appear to be physically so similar throughout the Coast Ranges, for 

 while there are acid rhyolites found in these ranges, no source has 

 yet been discovered adequate to furnish so large a body of material. 



On the other hand, it has been urged by Fairbanks 1 and con- 

 clusively shown by W. S. T. Smith" that at least a large portion of 

 the series is of organic origin. The evidences of this derivation are 

 mainly three: (1) The shales are everywhere more or less bitumin- 

 ous. No other source of the bituminous matter is more probable 

 than that of organic remains. Yet fossil shells and bones have not 

 been found in great abundance in them, and therefore the bitumen 

 must have been derived from organisms, the hard parts of which 

 have largely disappeared. (2) Scattered tests and fragments of 

 Diatomace?e and other siliceous organisms have been found in 

 many of the beds, generally in an imperfect state of preservation. 

 These have been most satisfactorily found at Monterey and on the 

 island of Santa Catalina. (3) The colloid nature of the silica, which 

 often forms as much as go per cent of the rock, is such as is commonly 

 the result of organic secretion. Samples of the shale tested by W. S. 

 T. Smith, lost by a treatment with potassium hydrate over 70 per 

 cent of their weight. Fairbanks states that in a sample treated by 

 him with the same solvent the loss was as great as 89 per cent of 

 the original powder, which must therefore have been almost the 

 whole of the silica. A sample from the peninsula of Point Reyes 

 obtained from the beach south of White Gulch was subjected to a 

 similar treatment, with the result that all but 26.75 per cent of the 

 powder was dissolved. These tests clearly show the opaline nature 

 of the silica which in these shales usually is either amorphous or 

 has an organic structure. 



In support of the volcanic origin of a very large part of the 

 series, however, each of the above authors has contributed reliable 

 evidence. On the island of Santa Catalina volcanic tuff is asso- 

 ciated with the Miocene shale. 



1 Bull. Dept. Geol. Univ. Cal., Vol. 2, pp. 9-14. 



'■'Proc. Cal. Acad. Sci., 3d series, Vol. 1, No. 1, pp. 43-49. 



