40 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
fragments of the breached crater from which the lava came, and that 
they were floated on the surface of the plastic lava. The floated frag- 
ments are composed of rock which, on account of its porous and usually 
scoriaceous condition, is lighter than the usually compact lava on 
which they occur, but they depressed the viscous surface on which 
they rested in the same manner that a brick might depress the sur- 
face of pitch. The floated fragments were not carried to the end of 
the lava flow, for the reason that a crust was formed on the stream 
of molten rock from beneath which the still liquid material escaped. 
Owing to the outflow beneath, the crust remained stationary and 
with its freight of tuff was gradually lowered to the position it now 
occupies. That the crags to which attention has been directed are 
not islands in the lava stream is clearly shown b}^ comparing them 
with well-characterized occurrences of that nature, one of which is 
shown on PI. IV, A. The stranded blocks of lava about an island in a 
lava stream which has subsided are inclined away from it, as may be 
seen in an example of this nature shown in the photograph just 
referred to. In such instances the result is much the same as may be 
observed on the borders of a river which has subsided after a sheet of 
ice had formed on its surface; in instances of this nature, as is well 
known, cakes of ice left stranded on the banks of the stream are 
inclined downward toward its center. About the islands in a lava 
stream which has been lowered by the outflow of the liquid rock from 
beneath a solid crust, the portions of the crust left stranded slope 
away from the central part of the island and nothing resembling the 
moat-like depression observed about the floated crags described above 
is even suggested. 
In some instances observed about the Cinder Buttes, the subsidence 
of the central part of a lava appear to have been so gentle that the 
surface sheet of stiffened but still somewhat plastic material was not 
seriously broken, and still remains in the condition of pahoehoe. At 
other times a more rapid lowering of the surface, or the presence 
of a rigid and brittle crust, caused a large amount of fracturing and 
the heaping up of the pieces produced so as to make a characteristic 
aa surface. About the bases of the floated crags on the Northwest 
lava flow, described above, typical examples of each of these classes 
of lava surfaces are to be found. In some instances the crags rise 
from smooth, swelling, and frequently corrugated pahoehoe surfaces, 
and at other places are surrounded by the chaos that is characteristic 
of aa surfaces. 
Volcanic bombs. — The volcanic bombs strewn about the Cinder 
Buttes are so abundant that in several instances they form a large 
part of the material of which the craters are composed and present 
several well-characterized varieties. 
The term "volcanic bomb" is usually employed to designate the 
class of projectiles blown out of volcanoes, which, on account of their 
