RFA'IEWS 643 



and aspects assuiued bv the rock are functions of a process of chemical 

 alteration from peridotite or pyroxenite and a mechanical disintegra- 

 tion of the rock thus altered." 



The rocks of the Merced series rest unconformably upon the 

 Franciscan series, and occur in two areas, one on either side of 

 Montara Mountain. The series consists chiefly of soft sandstone. 

 There are, however, among the series, hard shell beds firmly cemented, 

 and soft shell beds, uncemented ; hard beds of gravel, lignitic beds, 

 and a bed of volcanic ash. The northern of the two areas dips to the 

 northeast, and is exposed along the beach for a distance of 20,000 feet. 

 It is 5S00 feet thick, and is the heaviest Pliocene deposit known in 

 North America. 



The most marked structural feature of the region consists of two 

 fault blocks, both tilted to the northeast, and designated as the San 

 Bruno block and the Montara block. It is this tilting of the Montara 

 block that gives the Merced series its constant dip to the northeast. 



• That the San Bruno block is older than the Montara block is- 

 proved by the fact that the Merced series has all been removed from 

 the former, as well as by the fact that its topography is more mature 

 than the latter. 



Two cycles of erosion are indicated on the San Bruno block — an 

 early one which is manifested only above an altitude of 300 or 400 

 feet, and a later one which has modified the old topography. The 

 modification of the old topography has been effected in two ways. 

 The first is that due to atmospheric and stream agencies, the second to 

 "destructive and constructive shore action at the various stages of the 

 uplift." The crest of San Bruno Mountain is thought to be due to 

 the same cause which determined the 1200-foot terrace on the higher 

 Montara block; and the 750-foot terrace on San Bruno mountain is 

 doubtless due to the same shore line that formed the 700-foot terrace 

 on the Montara block. 



On the Montara block there is only the later cycle of erosion 

 manifested. During this time there were formed the two terraces 

 above mentioned. These are well marked on the northeast slope, and 

 less so on the southwest slope. 



The consequent streams on the northeast slope of the Montara block 

 which came into existence on its emergence from the ocean, became 

 superimposed streams on reaching the hard terranes beneath the soft 

 Merced series. Later, as a result of the longitudinal faulting, these 



