350 CALIFORNIA ACADEMY OF SCIENCES. 



form a continuous slope from the foothills to the center 

 of the bay itself. In some areas, as between San Mateo 

 and Coyote Point, the layer forming the surface is a black 

 earth full of commuted fragments of shells. Small areas 

 of apparently similar deposits occur at the surface of the 

 Quaternar}^ near the mouths of nearly all the little valleys 

 which open out upon the main valley of the Bay of San 

 Francisco. Smaller patches of a few acres occur at many 

 points on the flanks of the foothills, sometimes several 

 hundred feet above the bay. 



The most abundant shells among these fragments are : 



Cardinm corbis Martyn. 

 Cerithidea californica Haldemann. 

 Macoma nasuta Conrad. 

 Mytilus californianus Conrad. 

 Ostrea lurida Carpenter. 



The study of the Quaternary on the side of the moun- 

 tains toward the bay is not easy, on account of the difficulty 

 of distinguishing between subaerial and marine deposits. 

 Much of the flat land of the valley appears to be of sub- 

 aerial origin, as shown in cuts made by streams, but this 

 seems to be overlain by bay deposits. 



Deposits in Hills near Seven-Mile Beach. — At a number 

 of places recent erosion has exposed fresh water and 

 wind deposits. These have already been noticed under 

 the Merced series in the paragraph on structure. In some 

 of the drainless basins these deposits are at present form- 

 ing, in some cases, according to those living in the vicinity, 

 gaining an inch or two a year. During the rainy season 

 the deposit is a water deposit, and during the long dry 

 summer the wind carries off the lighter sand or carries in 

 sand from the surrounding higher ground. 



The short distance that these secondary deposits have 

 been transported has produced a marked similarity in ap- 

 pearance between these beds and some of the upturned 



