SUMMARY. 459 



The massive rocks. — Tlie m.issive Tocks met witli in tills investigation are 

 granite, diabase, cliorite, andesites, rhyolite, and basalt. The granites seem 

 to underlie tlie entire Coast Ranges and to form the lower and central por- 

 tion of the Sierra Nevada. They are on the whole pretty uniform and pi-e- 

 sent no known peculiarit}^ Diabase occurs in the Mesozoic conglomerates 

 of .Steamboat Springs and seems to be identical with the diabase which 

 forms the hanging wall of the Comstock lode. Dioi-ite is represented 

 chiefly by pebbles in the Neocomian conglomerates of the Coast Ranges. 



The andesites are divisible into two groups, an older and a youngei'. 

 The younger group is found at Steamboat Springs and elsewhere in and 

 near the Sierra Nevada, at Mt. Shasta, and from Clear Lake to ^It. Diablo. 

 It presents several varieties : one containing pyroxene, a mere trace of 

 hornblende, and no mica; a second containing pyroxene and mica, but no 

 hornblende; a third containing hornblende, with very small quantities of 

 pyroxene, together with mica in quantities ranging from nil to a very large 

 percentage. All of these pass over into one another, sometimes within a few 

 feet, and in masses evidently not due to separate eruptions. Nearly or quite 

 all of them are rough, soft rocks, such as were formerly supposed to be 

 trachyte. They form a natural group, which should be recognized. I have 

 proposed the name asiicrite to suggest their resemblance to trachyte. As- 

 perites, then, are a group of andesites with external characteristics similar 

 to those of trachyte. 



Both the asperites and the basalts near Clear Lake pass by transitions 

 into enormous masses of obsidian. The transitions have been traced in tlie 

 field, in the chemical laboratory, and under the microscope. Tlie glasses are 

 more acid than the crystalline rocks into which they pass, l)ut they contain 

 much more alkali and much less lime and magnesia; their specific gravity 

 is also much smaller. They have cooled as glasses, instead of as crystalline 

 aggregates, Ijecause of their peculiar composition, and not because thev have 

 been sul)jected to different physical conditions from the associated, sensibly 

 holoerj'stalline lavas. 



The origin of the massive rocks of California is discussed in Cliapter 

 IV. It is shown to be probable that portions of the granitic rocks repre- 

 sent pai'ts of the original crust of the earth, or that they are primeval rocks. 



