12 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



the quartz, cutting through the rock regardless of previous fractures. 

 The extinction of the calcite is not wavy, due to the fact that it is 

 later than the period of movement. 



Shdes were made of sandstone from the specimens taken from 

 within four inches of the contact of the serpentine and sandstone 

 at the Presidio and the Potrero. No metamorphism w^as observed, 

 except for a sHght difference in the cementing material, showing 

 that no contact metamorphism has taken place here, even at so short 

 a distance from the serpentine. A slide made from sandstone taken 

 from the face of a small slip zone in one of the open quarries, shows 

 the results of movement in this rock. The cementing material had 

 been changed to a fibrous green mass* that was not affected by polar- 

 ized light. The quartz was all recrystallized into the fine aggre- 

 gate texture mentioned before, but in some cases the original form 

 or outline of the sand grains could still be seen. 



This is interesting to show the amount of metamorphism pro- 

 duced by the intrusion of a large mass of periodotite, and by move- 

 ment resulting in a small fault. 



Tuffs. — The much weathered tuffs appear as dark green or 

 much iron stained crumbly beds, and are well characterized by their 

 popular name of " rotten rock." They are much broken up beds 

 that can easily be dug into with a hammer, and possess no visible 

 bedding planes unless associated with sandstone or shale. This is 

 the form of the rock under ordinary weathering but when metamor- 

 phosing agencies have acted upon it, it becomes a hard green rock, 

 w^ith much quartz and has the appearance of a metamorphosed 

 jasper. 



Slides w^ere made from this tuff from Baker's beach, Point 

 Lobos beach. Silver Terrace hills, Potrero and several other locali- 

 ties, the best specimens being obtained from Silver Terrace. 



At the last named place the tufaceous nature of the rock can be 

 plainly seen in the weathered specimens. In a quarry near Silver 

 avenue and First street the relation of the fresh to the weathered 

 rock is seen. Here there appears to be a dike of green rock in- 

 truded into the tuff, but a microscopic examination shows the appar- 

 ent green dike and the brown tuff to be the same. In a street cut 

 on Railroad avenue near Nineteenth avenue the cause of the meta- 



