russell.] TERTIARY FORMATIONS. 65 
through the action of the wind, as sheets of volcanic dnst, volcanic 
gravel, or lapilli, and angular fragments frequently of considerable 
size. 
The basalt was spread over the surface of the land mostly in a 
molten and even highly fluid condition, and the beds of fragmental 
material produced are relatively small; while the rhyolite to a great 
extent was shattered at the time it was erupted, and the resulting 
beds of fragments were probably of greater extent and thickness than 
the sheets of the same material which were spread out as lava flows. 
This difference in the behavior of the volcanoes from which the com- 
paratively fusible basalt was erupted, and of the volcanoes from which 
the more refractory rhyolite was discharged, is of much significance 
in reference to the nature of volcanoes, but can not be discussed at 
this time. 
The basalt is a black, compact rock, but is frequently cellular on 
account of the presence of steam cavities, and in many localities is 
columnar. In the region under review it occurs in widely extended 
sheets which in general vary in thickness from about 20 to 80 feet. 
Fine examples may be seen all along Snake. River, but more espe- 
cially on the northern side of its canyon, in the canyon of Bruneau 
River, and in the hills and mountains of Malheur and Harney counties, 
Oreg. The finest exposure of basalt in the region visited by me in 
1002, if not the most remarkable in the world, is to be seen in Stein 
Mountain. The eastern slope of that splendid mountain is composed 
of the broken and eroded edges of sheets of basalt, which dip west- 
ward at an angle of about 3° and present an aggregate thickness of 
not less than 5,000 feet. Between the sheets of basalt, as already 
noted, there are in at least 18 instances beds of coarse sandstone, 
varying in thickness from a few inches to 6 feet. Where the beds of 
sandstone occur the sheets of basalt are in general about 60 feet thick. 
From this and other similar evidence the total number of lava flows 
which occur in Stein Mountain is estimated at between 80 and 100. 
The widely extended sheets of basalt just referred to, like the sedi- 
mentary beds with which they are intimately associated — the sheets 
of basalt and the beds of sandstone, shale, clays, etc. — belong prin- 
cipally to the Tertiary period of geological history and, as seems evi- 
dent, form the lowest member of the widety distributed formation 
termed the Columbia River lava. Volcanoes have been active, how- 
ever, in Idaho and Oregon at intervals from the time the oldest sheets 
of basalt were poured out in a molten condition down to almost the 
present day, as is shown by the Cinder Buttes, Jordan Craters, and 
other volcanic features already briefly described. 
The rhyolite, like the basalt, occurs both as massive sheets, formed 
by the cooling of molten rock, and as fragmental deposits, which also 
form well-defined beds. Of the layers of rock originating in these two 
ways, the sheets composed of angular fragments, now in many 
Bull. 217—03 5 
