10 



CRANDALL— SAN FRANCISCO PENINSULA. 



[January 4, 



The jaspers more than any other rocks, show the result of crush- 

 ing in the contortions and wrinkHngs of their layers. They seem 

 to be beds of great elasticity, that bend and twist without breaking. 



Fig. 5. 



♦ 



Local folding and faulting of jaspers at Stow Lake, Golden Gate Park. 



In nearly every place where they are exposed they show either paral- 

 lel successions of close folds, or wavy structure, which is particularly 

 well emphasized by their thin bedding. 



Petrography of the Franciscan Rocks. 



Sandstones. — The sandstone, in the hand specimen, when fresh, 

 is a bluish gray rock; when weathered it is a dull yellowish brown. - 

 In the field this sandstone is characterized by two of its constituents, 

 the small flakes of shale and the presence in certain localities of 

 considerable amounts of brown mica. Slides of this rock from var- 

 ious places are very similar except for the presence of mica, and in 

 nearly all cases show that it is sandstone derived almost entirely 

 from the weathering of an igneous rock — in other words it is an 

 arkose. 



The grains are small and angular rather than rounded, set widely 



