66 GEOLOGY OF SW. IDAHO AND SE. OEEGON. [bull. 217. 
instances firmly cemented, and forming a rhyolite tuff, are far more 
numerous and more extensive than the associated rhyolitic lava sheets. 
The beds of white volcanic dust described above as forming a part of 
the sedimentary deposits are composed of the finest of the fragments 
blown into the air by the volcanoes from which came the material 
now forming the sheets of rhyolite and of rhyolitic tuff. 
The compact massive rhyolite is lighter colored than basalt, and in 
Idaho and Oregon is usually purplish on fresh surfaces, but weathers 
to a rich brown or red. It contains conspicuous crystals and grains 
of quartz, feldspar, and other mineral, and hence usually appears 
spotted. Frequently the crystals and grains referred to exhibit a 
linear arrangement, such as would be produced by a flowing motion 
in the glassy base in which they were embedded. At times the molten 
material cooled too quickly for crystals to form and produced a com- 
pact black glass, termed obsidian, fragments of which occur thickly 
over hundreds of square miles in eastern Oregon. 
The rhyolitic tuff is composed of angular fragments of the rock just 
described, which, in most instances, are firmly cemented, so as to form 
sheets that are nearly, if not fully, as resistant to atmospheric condi- 
tions as the rock which cooled from fusion without being shattered, 
and frequently forms "rim rocks" on the sides of canyons and 
valleys. 
Extensive exposures of rhyolite and of rhyolitic tuff occur in the 
mountains on the northeast side of Harney Valley, sometimes termed 
the Crow Creek Mountains. The conspicuous rim rocks on each side 
of Rattlesnake Creek, near Harney, are of rhyolitic tuff, while certain 
of the beds lower down in the walls of the same canyon, usually cav- 
ernous on the weathered outcrops, are of rhyolite, which has a pecul- 
iar concentric or spherulitic structure. Again, in the bluffs of Silvies 
River, to the west of Burns, the edges of a thick sheet of compact tuff 
are well exposed, and similar rock has a wide distribution in the 
forest-covered mountains in which Silvies River rises. Other out- 
crops of tuff and of compact rhyolite occur about Silver Lake at Iron 
Mountain and in the Mahogany Mountains. The conspicuous rim 
rocks in the vicinity of Diamond are composed of tuff, and similar 
rock has a wide distribution in the hills near Smith and Mule. 
An interesting fact in connection with the sheets of rhyolite and of 
rhyolitic tuff in southeastern Oregon is that they are younger than 
much of the basalt of the same region, and overlie it. There have 
been eruptions of basalt also since the youngest observed sheet of 
rhyolite or of rhyolitic tuff was spread out, as rim rocks of basalt 
above the thick deposits of rhyolitic tuff are of common occurrence. 
Dike rocks.— The molten magmas which rise in volcanoes and in 
part overflow, so as to form lava sheets, also in part cool below the 
surface in the conduits of volcanoes and in fissures. Many distinc- 
tive features in the rocks produced from the same molten magma arise 
in this way, which it is unnecessary to consider at this time. 
