NEOCENE STRATIGRAPHY. 277 



seen on the narrow divides or is broken down so as not 

 to be recognizable. It will be noted too that the streams 

 do not always run direct to the Bay, but frequently spread 

 out in their headwaters at right angles to their general 

 course, as though they had found strata which were more 

 easily eroded than those further down. At two places, 

 west of San Mateo and west of Redwood Cit}^ large out- 

 crops of serpentine give to the top of the hill the flat as- 

 pect of a plateau, and from this rise abruptly high cone- 

 shaped knolls of phthanite. Near Belmont and San Carlos 

 this phthanite forms abrupt hills close to the salt marsh 

 bordering the bay. These foothills are for the most part 

 destitute of brush or trees, with smooth rounded surfaces 

 and showing but few rock exposures. 



The second topographic feature of importance is the 

 long valley running from Mussel Rock to Black Mountain. 

 This valley is occupied at the northern end by several 

 small creeks and lakes; in the middle by San Andreas 

 Lake and Creek, and Crystal Springs Lakes; and at 

 the southern end by Bear Creek, and San Francisquito 

 Creek. 



On account of the difference in the erosion of the 

 broad valley which San Francisquito Creek has cut in the 

 foothills, and the narrow precipitous valley of San Mateo 

 Creek, the present outlet, it has been suggested that the 

 drainage of the whole valley was originally through the San 

 Francisquito Creek. According to that view San Mateo 

 Creek has been simply one of the backward cutting 

 streams of the northeast slope, which has finally cut back 

 far enough to tap the valley and claim a large share of its 

 drainage. There are a number of other places where 

 the same process is approaching similar results. Near 

 Searsville the valley is quite broad and to the southeast 



