Doc. No. 9. j 33 



tance from the intrusive material. The slates and sandstones when thus 

 acted upon assume either a sub-crystaline or completely metamorphosed 

 structure, and comport themselves in this particular much in the same 

 manner as similar formations on the west sloue of the Sierra Nevada. 



The area covered by the metamorphic rocks in the Coast Mountains is 

 not as extensive as those of the mining sections, but the changes, when 

 they do occur, are equally as perfect and complete. The most extensive 

 change of this character noticed among these mountains was that on the 

 Alameda Creek in Sunol Valley, eight miles north-east of the Mission 

 San Jose, and again after crossing this valley on the road to Livermore's. 

 The slates on the creek were changed into a hard, compact rock, for 

 the distance of one mile, and three-fourths of a mile beyond they had 

 assumed a porphyritic character. 



Following this series to the right of the road, among the hills which 

 divide Sunol's from Livermore's Valley, the greenstone-trap became 

 largely developed, with basaltic fragments among the drift of the arro- 

 yas. On the east side of Livermore's valley the fossiliferous rocks again 

 made their appearance, and continued south for eight miles, beyond 

 which to the south the mountains were not examined. 



The classification of the rocks in these mountains according to their 

 relative ages will follow as next in order. 



CLASSIFICATION OF THE EOCKS OF THE COAST MOUN 

 TAINS AND MONTE DIABLO KANGE. 



Our most northern point in this case will be Bodega Head and a line 

 forty-three miles north of Napa City, and in this case shall avail myself 

 of that part of Mr. Tyson's report on this part of the country, and which 

 is contained in Senate Doc. No 47, 1st Sess. 31st Cong., this being the 

 most northern geological section yet made. 



By reference to his section, we find that the primitive rocks occur as 

 far north in the Coast Mountains as the above locality named, and that 

 the rocks of sedimentary origin are found to rest directly on the primi 

 tive rocks for a considerable distance east from the coast line. 



After reaching eight or nine miles from the coast, the recent volcanic 

 group succeeds the primitive, when the latter is again replaced in the 

 hills bordering the western edge of the basin ; from this view of the 

 case it appears that all the rocks of this section are similar in their posi 

 tions with those detailed farther south. 



Commencing with the lowest in order we find : 



FIRST. The granite series and serpentines ; on these rest the older 

 sedimentary rocks. 



SECOND. The more recent volcanic rocks. These are more largely 

 distributed through the Monte Diablo Range. They consist of trachytes 

 and lavas, and have protruded through all other formations that preced 

 ed them. These rocks form at least three-fourths of the mountain range 

 extending from Point San Pablo to the head of the Canada San Juan, 

 5 



