Anderson. 1 



Point Reyes Peninsula. 



141 



A similar explanation has been offered for the well-known fact 

 of the silicification of fossils, that is, for their replacement by 

 silica. Professor LeConte has suggested a similar explanation for 

 the silicification of wood. 1 



The probable agency of an organic solvent for the silica of 

 the bituminous shales was suspected from an interesting oc- 

 currence on the peninsula. The remains of a cetacean were 

 discovered buried in the bituminous shales of Drake's Bay. They 

 were encased in an oblong oval crust of compact siliceous matter, 

 which had the general appearance of a siliceous nodule of large 

 dimensions. From one end of this concretion (?) the bones pro- 

 truded, and even these were crusted over with a bead-like layer of 

 opaline silica. There lacks, therefore, no link in the chain of 

 evidence that points to the organic derivation of a large part, if not 

 even the larger part, of the white shales of the Coast Range 

 Miocene. Yet the volcanic source of some of the strata is no 

 less satisfactorily shown. 



Terrace Formations . — The Pleistocene deposits, only a part of 

 which are represented upon the map, are usually coarse arkose 

 detritus with an indistinct horizontal stratification. They are found, 

 generally, in the larger depressions of the peninsula, and range in 

 elevation from 500 feet downward. They form a series of low, 

 broad hills, extending along the middle of the valley near Olema, 

 and occur at intervals upon both shores of Tomales Bay, forming 

 there a system of low bench-like terraces below 200 feet in height. 

 West of the main ridge they are found in occasional patches around 

 the flanks of the hills at the head of Drake's Estero, and north of 

 Abbott's Lagoon. Along the county road, crossing the hills, they 

 have an elevation of about 500 feet, and consist of granitic gravels 

 and pebbles of white shales. No fossils have yet been found in 

 them. A beautiful section of these deposits is to be seen in the 

 cliffs east of Drake's Bay, where 50 or 60 feet of gravels are 

 found capping the Miocene shales. 



Recent Sands. — Along the western margin of the peninsula there 

 are deposits of irregularly stratified or unstratified sand which has 



1 Am. Jour. Sci., Vol. 19, p. 181. — Elem. Geol., p. 193. 



