Lawson 1 



I'ALACHEj 



The Berkeley Hills. 



361 



sandstone. It is mapped with the Chico doubtfully since it differs 

 materially from the ordinary sandstone of that formation. 



Conglomerate, Aporhyolite, and Serpentine. — The continuous 

 section of Shasta-Chico sandstone and shales which is observed 

 by ascending Skyline Ridge from a point near the mouth of 

 Strawberry Canon, by Panoramic Way, is not paralleled on the 

 south side of Hamilton Gulch. If we ascend the ridge in the 

 line of Dvvight Way, the first rocks met with after leaving the 

 alluvium are an exposure of a pebble conglomerate of very limited 

 extent near the residence of Mrs. Gibbs. This conglomerate 

 resembles a conglomerate which is traceable for many miles along 

 the front of the range between the Knoxville and Chico. Imme- 

 diately above this we meet with a soil which is of a rusty red 

 color and is made up largely of small fragments of an oxidized 

 volcanic rock. The true character of the rock would not be 

 guessed at from an inspection of these chips, but on following it 

 around into Hamilton Gulch, an excavation which has been made 

 in the hillside affords an exposure, and outcrops of the fresh rock- 

 are to be found higher up on the slope. From those it is evident 

 that the rock in its less decomposed condition is a compact, bluish- 

 white, microcrystalline rock of a hard and silicious character. It 

 frequently contains numerous minute crystals of pyrite, and it is 

 the oxidation of these which gives the fragments and the soil 

 derived from them their ocherous appearance. A microscopic study 

 of this rock shows that it is made up essentially of quartz and feldspar, 

 with very little of the ferro-magnesian silicates, and it is regarded 

 as an altered form of an ancient flow of rhyolite lava. It is there- 

 fore designated an aporhyolite. 



This aporhyolite occupies the precipitous hill at the head of 

 Dvvight Way to an altitude of more than 300 feet above the point 

 where it is first met with. The steep slopes and red soil to which 

 it gives rise are in marked contrast to the gentler slopes and gray 

 soil yielded by the Shasta-Chico sandstones on the north of 

 Hamilton Gulch. Followed into Hamilton Gulch, the aporhyolite 

 extends only to the creek line, the lower limit of the formation 

 keeping approximately on the same level. On the south side it 

 extends up the creek for about 700 feet, abutting squarely upon the 



