1907. 



CRANDALL— SAN FRANXISCO PENINSULA. 



9 



which in an open quarry obscure the original bedding planes. Pro- 

 fessor Lawson has attributed these joints to weathering but their 

 occurrence in fresh rock after the same fashion leads to the belief 

 that thev are of dynamic origin, though they are much more evident 

 in the wethered beds. In the open quarries, planes along which 



Fig. 4. 



Slickensided faces in jasper, shown at a quarry on Lobos Avenue near 



the ocean. 



slipping has occurred and small faults from a few feet to a hundred 

 feet, may be observed crossing each other at varied angles and in 

 many directions. In the small faults the sandstone may be seen 

 altered to a thickness from three to ten inches on either side of the 

 fault plane. Secondary silicification is widespread in these sand- 

 stones, giving rise to barren quartz veins in places, some of which 

 were the cause of the gold mining excitement on San Bruno moun- 

 tain some years ago. 



The tuffs also show marked effects of regional disturbances, 

 resulting in the formation of dark green, hard, compact rocks, which 

 were termed greenstones in the field until their true character was 

 ascertained. Examples of this rock may be seen in the small islands 

 along the cliff's of the Golden Gate, at Silver Terrace hill and at 

 ]\Iussel Rock. 



