J. D. Dana on the characters distinguishing Kinds of Rocks. 431 
the earth’s crust is a recurrent phenomenon, it may help us to 
trace out its cause. 
Approximate practical conclusions.—The only practical con- 
clusion which can be drawn from the preceding discussion seems 
to be that the origin of the excavation of the valley is compara- 
tively modern, and that it was from the operation of forces 
producing probably uniform results, and in a way that we have 
some approximate comprehension of it in general, from our 
knowledge of special localities. 
Note.—Descriptions of the best known sections of the valley 
follow in the report. 
Art. LI.—On some points in Lithology; by James D. Dana. 
[Continued from page 343.] 
his term “ hornblende-granite” is at variance also with the 
fundamental idea and nature of granite. Granite is eminently 
Presence of a little iron in the original material having appar- 
ently determined the formation of the latter where it occurs. 
the contrary the hornblende of such rocks contains usually 
