296 CALIFORNIA ACADEMY OF SCIENCES. 



In many places over considerable areas hand specimens 

 show a schistose structure full of slickensides and evi- 

 dently bearing no relation to its original structure and 

 character, sometimes appearing as though it had been re- 

 duced to a pasty mass and afterward consolidated. 



In many places this old sandstone is further distinguished 

 by secondary silicification, the rock being filled with a net 

 work of fine quartz veins. 



In some regions the metamorphic sandstone can be 

 recognized at once. In others the resemblance to the 

 sandstone of the Pescadero series is so great that it is im- 

 possible to draw the line between them. For example, 

 in the San Bruno Mountains the northeastern side is clearly 

 made up of the Pescadero sandstones showing the bed- 

 ding and characteristic features. As the southern end of 

 the mountain is rounded the bedding becomes more ob- 

 scure and the dip less regular; this continues, the beds 

 being found at all angles until finally the bedding only 

 shows here and there, phthanite occurs to some extent, 

 secondary silicification is quite marked in some places, 

 and the rock as a whole on the west side of the mountain 

 would be pronounced the metamorphic sandstone. Yet 

 the occasional presence of characteristic Pescadero sand- 

 stone, the gradual transition from the one sandstone to 

 the other, together with the structure of the mountain, 

 would seem to suggest that the metamorphic sandstone 

 and accompanying phthanites simply made up the lower 

 part of the Pescadero series. 



This relation is suggested by the exposures on many of 

 the hills in and around San Francisco. Indeed, it is not 

 to be wondered at that all the early geologists and down 

 to within a few years have placed the metamorphic sand- 

 stones, the phthanites, and the younger sandstones in one 

 series, for such a relationship appears to be the true one 

 on the San Francisco peninsula. 



