60 
GEOLOGY OF SW. IDAHO AND SE. OREGON. 
[BULL. 217. 
by water and the other the rocks which have cooled from a molten 
condition. The former includes the material laid down in lakes and 
by streams, aided in each case in a minor way, no doubt, by the 
transporting action of the wind; and the latter embraces the various 
products of volcanoes or volcanic rocks, and the similar magmas 
which have cooled in fissures, below the earth's surface, forming 
dikes. These more general groups of rocks are naturally subdivided 
in reference to mode of origin, composition, etc., as is indicated in the 
following table 
1. Sedimentary rocks. 
Conglomerates, sandstones, shales, unconsolidated gravel, 
sand, clay, etc. (together with beds of volcanic dust 
and volcanic gravel or lapilli). 
2. Igneous rocks. 
a. Volcanic rocks _ 
b. Dike rocks 
Lava flows 
rBasalt. 
JRhyolite. 
Fragmental Basaltic scoria, bombs, 
products of] lapilli, etc. 
volcanoes. ] Rhyolitic lapilli and 
dust. 
Magmas like the molten material dis- 
charged by volcanoes, but which 
cooled in fissures below the earth's 
surface, forming dikes. 
All the rocks referred to above are placed provisionally in the Ter- 
tiary division of geological history, excepting, as already noted, in 
the case of certain of the products of volcanoes which are of later 
date, and belong to the Pleistocene and recent times. The sedimen- 
tary rocks of Tertiary age also grade into more recent deposits of 
similar character and like mode of origin. At present it is impracti- 
cable to designate any well-defined boundary between either the vol- 
canic or sedimentary rocks of the Tertiary and the similar terranes of 
more recent origin. While terranes older than the Tertiary are cer- 
tainly present beneath the areas occupied by rocks of that age, they 
are believed not to be exposed at the surface along the route followed 
during the reconnaissance which furnished the data for this report. 
SEDIMENTARY ROCKS. 
The stratified rocks of the region examined — or those composed of 
fragments of older rocks, spread out in layers or strata by the waters 
of lakes and streams — comprise conglomerate or pudding stone, sand- 
stones, soft, highly calcareous, usually nearly white, shales, and loose 
gravel, sand, white volcanic dust, and dark usually yellowish volcanic 
gravel or lapilli. These beds were for the most part deposited in 
lakes, and belong to the Tertiary division of geological history. 
Examples of the conglomerate and sandstone referred to may be 
seen in the hills near where the Owyhee joins Snake River, in the 
isolated hills near Vale, and in the conspicuous bluffs to the southeast 
of Malheur and Harney lakes, as well as many other localities. These 
