314 CALIFORNIA ACADEMY OF SCIENCES. 



Just below the sheet of basalt south of Stanford Uni- 

 versity the sandstone is coarse grained, nearly white and 

 very hard. Above the sheet of basalt it has been leached 

 out until the fragments on the surface are almost as por- 

 ous as a sponge. 



Along the Seven-Mile Beach in particular there are 

 many thin layers of hard conglomerate. The pebbles 

 are usually water-worn fragments of phthanite and the 

 other metamorphics, usually less than an inch in diame- 

 ter. These layers are, as a rule, very hard and seldom 

 more than a foot or two thick, though sometimes there 

 are many layers close together. At several places along 

 the Seven-Mile Beach section they resist weathering and 

 stand "out very prominently from the softer surrounding 

 strata, one layer at lowest tide being traceable several 

 hundred feet out into the ocean. Most of these fine con- 

 glomerates contain many commuted fragments of shells, 

 which may account for their hardness. In some the pro- 

 portion of shell fragments becomes so great that they 

 would more properly be called shell breccias or brecciated 

 limestone. Such a breccia outcrops quite prominently 

 in the foothills west of San Bruno; also along the con- 

 tact running southeast from Mussel Rock between the 

 Miocene and metamorphic or igneous rocks. Just above 

 the sheet of basalt south of Stanford University occurs a 

 similar brecciated limestone, largely composed of frag- 

 ments of Balanus, and rather soft and friable. In all the 

 ravines running into San Francisquito Creek above Sears- 

 ville a ledge of soft limestone is crossed. It is more 

 strictly a calcareous sandstone, and is full of fossils which 

 undoubtedly furnished the lime. 



A number of layers of lignite from an inch or two thick 

 up to a foot in thickness, occur in the section along Seven- 

 Mile Beach. In some cases the structure of the wood 



