292 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



about the present site of Sonoma Mountain. Between these 

 successive eruptions there were time intervals sufficiently long 

 for soils to form from the lavas and for the growth of great 

 forests of pine and redwood. In other portions of the shore 

 during these time intervals, lakes were formed as attested by 

 freshwater cherts, in the Neohipparion beds. In this lake 

 vegetation accumulated in sufficient amount to form a three 

 foot vein of coal. Whether this lake was directly connected 

 with Lake Orinda of the Berkeley Hills region was not de- 

 termined, but they were, beyond much doubt, synchronous. 

 This is the period during which Neohipparion gidleyi and 

 other Pliocene horses flourished in this region. 



Great changes took place at the close of the Pliocene. 

 The two outer blocks were lowered and at least the major 

 portions of these blocks were swept at times by the Pleisto- 

 cene sea. It is very possible that the Point Reyes Triangle 

 was never completely submerged as a Pleistocene Tomales 

 Bay is evidenced by the deposits of the Millerton formation. 

 The immediate environs of Tamalpais were under active wave 

 erosion or sub-aerial erosion during a portion of Pleistocene 

 time as no Merced is now found within this area. Further 

 north, however, the planation effects of the Pleistocene sea 

 across the soft sandstones and shales of the Merced forma- 

 tion are preserved to this day in the vicinity of Freestone. 

 After a marine plain was developed across the San Francisco- 

 Marin Block, the mainland was slightly uplifted. Short, 

 rapid East- West consequent streams developed across this 

 plain. Later, the mainland blocks were tilted and the ve- 

 locities of the streams in lower courses were sufficiently ac- 

 celerated, so that practically all cut gorges of greater or lesser 

 lengths at their mouths. Some of the larger consequent 

 streams, such as the Mark West Russian River and, for a 

 time, Pleistocene Walker Creek, maintained courses across the 

 two mainland blocks. Some of the smaller streams, however, 

 were quickly ponded in their middle courses as a result of 

 this tilting and were early spilled out to the east by way of 

 their head- water tributaries. 



That marked climatic changes took place in the San Fran- 

 cisco Bay Region as well as in the Sierra Nevada is shown 

 bv two distinct faunas and floras of Pleistocene asre. Both 



