276 CALIFORNIA ACADEMY OF SCIENCES. 



by an irregular series of ridges and valleys, often with a 

 difference of level of 1000 feet or more. These ridges 

 are sometimes parallel to the main axis and structural in 

 their nature, but more frequently have a trend away from 

 the main axis at various angles and are the result of ero- 

 sion. 



The main topographic features of the northern half of 

 the range are as follows: From the Seven-Mile Beach, 

 between Mussel Rock and Lake Merced, a line of hills 

 extends southeast to San Francisquito Creek, where there 

 is a broad wind-gap. Beyond this they continue in the 

 same direction until they run out in the Santa Clara Val- 

 ley. This line of hills is also cut in several other places : 

 by San Mateo Creek; by the creek which rising on the 

 " Jersey Farm " flows down to the bay close to San Bruno 

 station ; and by a branch of Twelve-Mile Creek and 

 several creeks to the south. Its general aspect is that of 

 a block tilted up, having been faulted along its south- 

 western edge. It will be seen from the map that the 

 drainage is all one way in this line of hills with the ex- 

 ceptions noted. Aside from the three streams mentioned, 

 the streams which run from these hills to the bay are 

 small and dry much of the year. Twelve-Mile Creek, 

 Seventeen-Mile Creek, Belmont Creek, Cordillas Creek 

 and Matadro Creek are among the largest north of San 

 Francisquito Creek. 



The topography of these hills is, in the main, like that 

 of a tilted block, or more correctly, a line of tilted blocks. 

 All the highest points are close to the southwest edge. 

 Near the northwest end two points reach 720 feet in 

 height, and to the southeast some points probably exceed 

 that height. Its northeast slope is not an even slope for 

 most of the streams have cut narrow V-shaped channels 

 several hundred feet deep ; so that the old slope is only 



