446 THE JOURNAL OF GEOLOGY. 
This continued until the Miocene. Then the great thickness of 
sediment yielded to mountain-forming forces. Geéologically 
speaking this movement seems to have been quite rapid, if we 
may judge from the thorough way that the rocks have been 
folded, broken and crushed. For a time subaérial erosion was 
the principal dynamic factor at work in the area of the mountains. 
Then a slow subsidence set in, and the white shale of the 
Monterey series was deposited on the coast side of the mountains. | 
In some places: what is now the crest of the mountains was 
covered by the white shale. Then the sediment became sandy, 
the old mountains which had meanwhile been much eroded 
settled still more, and the Merced series was laid down.. Near 
San Francisco the conditions were favorable for rapid sedimenta- 
tion, and nearly a mile of thickness was laid down. Finally at 
the end of the Phocene, or beginning of the Pleistocene, move- 
ment began along the old axis. Great faults were formed and 
the center of the range rose, not to its present height, but to a 
position probably about 1000-1200 feet lower. This left the 
present flanks of the mountains under water, a fact which 
doubtless accounts for the preservation there of the soft 
Merced series. 
Il. THE PLEISTOCENE HISTORY OF SAN FRANCISCO PENINSULA. 
Topography.— The main portion of San Francisco peninsula is 
occupied by the northern end of the Santa Cruz Mountains and 
their foothills. The strike of these mountains to the northwest 
carries them to the seaboard a few miles south of the Golden 
Gate. In the southern part of the peninsula the mountains are 
separated from the bay by a broad plain. At the northern end 
this plain, narrowed down to a valley or windgap of some breadth, 
separates the mountains from the San Bruno Mountains, a parallel 
range only a few miles long, and from the irregular groups of 
hills in the: city of San Francisco...On the ocean sidejof the 
mountains are remnants of a similar plain. These plains on 
both sides of the mountains are one of the most marked features 
of the topography. They usually start quite abruptly from the 
