Vol. 4] Osmont. — Geological Section of Coast Manges. 



ir, 



Also, we can safely correlate the Wilson Ranch beds with the 

 typical Merced, with a strong probability, from the abundance 

 of area trilincata, that it belongs to about the horizon repre- 

 sented at Capitola. Becker's Cache Lake beds may be their 

 fresh-water equivalent. 



Now, it was thought in the beginning of this work, that the 

 very characteristic pumiceous tuff called in this paper the So- 

 noma Tuff would serve as a datum-plane to connect the forma- 

 tions encountered with those of Contra Costa County, for pumi- 

 ceous tuff is extensively developed throughout the Mt. Diablo 

 region. Unfortunately, a difficulty has arisen, which will now 

 be discussed. In the Contra Costa district a pumiceous tuff 

 identical in appearance to that found in the writer's territory 

 exists at the base, or interbedded near the base, of an extensive 

 fresh-water deposit, called by Lawson the Orindan, and described 

 as being a "Thick formation of pebbly conglomerate, feebly co- 

 herent sandstones, thin beds of volcanic tuff generally decom- 

 posed and of a dark brownish color, beds of blue and slate col- 

 ored clay containing fresh-water ostracods, sandstones contain- 

 ing ostracods and molluscs, and occasional patches of lava of 

 quite limited extent intercalated ( ?) with sedimentary beds. 

 No marine organisms have been found in any part of the beds." 



This Orindan formation mantles indifferently over the blue 

 San Pablo, the Monterey and the older rocks, but, save for the 

 pumiceous tuff near its base, it is nonvolcanic in its nature, and 

 precedes all the lava flows of the Bei'keley Hills. In its gravels 

 certain vertebrate remains have been found by Lawson, Merriam 

 and Sinclair, the most important of which are horse-teeth. These 

 were referred to Dr. Gidley, who reports upon them as follows : 



"Number 1324 is a very interesting tooth as it is indistin- 

 guishable from Hipparion ricMhofeni, a species from Eastern 

 China. It is possible it may represent a species of true Hippa- 

 rion. Number 1323 is an upper molar of a smaller species, prob- 

 ably NcoMpparion." . Hipparion ricMhofeni is reported from 

 the lower Pliocene. 



The above evidence points to these gravels being of late Mio- 

 cene or early Pliocene age. Certainly they are later than the 

 marine blue San Pablo, and older than the Merced. At Rodeo, 



