Ransome.] 



Geology of Angel Island. 



199 



to indicate that "radiolarian chert" is, on the whole, a better desig- 

 nation for them, inasmuch as it casts some light upon their origin, 

 and suggests their relationship with similar cherts in Europe.* 



The most instructive exposure of these cherts occurs on the 

 eastern side of Hospital Cove, as a bedded formation interstratified 

 with the sandstone, and running from the water's edge up into a 

 little canon, where it appears to terminate rather abruptly. The 

 change of sedimentation from sandstone to chert appears at this 

 point to have been somewhat gradual, as the two are not sharply 

 defined at the contact. Several facies can be distinguished in the 

 chert beds. There is the common red jasper with the usual quartz 

 veining, also a light-colored variety, lull of tiny cavities averaging 

 about half a millimeter in diameter and lined with iron oxide.' A 

 careful search was made for radiolaria in this rock. They were 

 found in abundance in the beds just described, being best preserved 

 in portions having a dark red color and dull earthy fracture, in 

 which they are sometimes barely discernible with a good lens. 

 Some microscopic slides, and additional chips, were forwarded by 

 Professor Lawson, with some specimens of his own from another 

 locality, to Dr. Hinde, and the results of his examination have been 

 appended in a note which he very kindly placed at Professor Lawson's 

 disposal. Having found the remnants of siliceous organisms in the 

 less siliceous portions of the cherts, they were subsequently recog- 

 nized in the more silicified facies 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 chal- 

 cedony. 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. t 

 *See appendix by Dr. Geo. J. Hinde, p. 235. 



t Note. — Becker probably refers to these radiolarian remains in certain of 

 his "phthanites" when he says: " The most interesting constituents of foreign 

 origin are round spots, which often retain evidences of organic character. 

 Professor Leidy, at my request, has examined some of the thin sections con- 

 taining such spots, which he regards as probably foraminiferous shells." 

 Quicksilver Deposits of the Pacific Slope. Monog. XIII, U. S. Geol. Surv., p. 108. 



