50 THE GEYSERS OF PLUTON RIVER, 
height, projecting quite out into the plain. The whole 
seemed full of obsidian, covered with a layer of earth, 
on which is a thick growth of trees and shrubbery, 
save on the summit, where there is less soil. Here in 
many places the surface was covered, from six to 
twelve inches in depth, with broken pieces and small 
boulders of this volcanic substance, resembling a 
newly made macadamized road. 
Taking the spade, I scraped away the fragments 
and loose pieces to reach themass below. This we 
found existing in a conglomerate state. The mass in 
which the obsidian is imbedded is quite soft and friable 
towards the surface; so that it was difficult to detach 
it with the obsidian adhering to it, except in very 
small pieces. The largest of the specimens obtained 
was about the size and shape of an ostrich’s egg, from 
which they diminished to that of a pea. Many pre- 
sented sharp angles, where they had come in contact 
and been broken. The substance in which the obsi- 
dian is imbedded resembles a coarse mortar of lime, 
sand, and gravel. I took a sketch of Napa Valley 
from these hills, showing Mount Diabolo in the dis- 
tance, which is plainly seen from San Francisco and 
Sacramento. 
Obsidian is used by the Indians for their arrow- 
heads in all parts of North America west of the 
Rocky Mountains. It is found too among many tribes 
to the east of this range. The ancient Mexicans made 
of it the knives which they used in their sacrifices. 
We found small fragments of it along the Gila, wher- 
ever there had been any Indian villages; and also 
among the ruins of the Casas Grandes, in Chihuahua, 
