78 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
the absence of evidence of rapid rotation on the surfaces of the vesicu- 
lar bombs in question and the correspondence of their internal struc- 
ture with what should be expected to occur if lava on cooling from 
fusion extrudes previously dissolved vapors or gases favor the hypoth- 
esis here suggested. 
The compact bombs usually, and, as it seems, invariably, have oval 
forms and twisted and projecting ends, and are composed of black 
stony lava throughout, but are never entirely free from steam cavities. 
The openings are irregular in size and shape and are frequently much 
distorted, but when seen in sections made at right angles to the longer 
axis of the projectile are many times arranged in rudely concentric 
bands. In these same sections, also, there are concentric lines or 
cracks produced by a rolling up of the material, or developed by 
weathering. The variations in form from globular to ovoid, and 
more or less irregular, and in internal structure from highly vesicu- 
lar to compact and stony, with but few irregular and frequently much 
elongated vesicles, appear to be due to variations in plasticity or 
liquidity of the material at the time it was projected into the air, and 
variations also in the length of its nights. The more plastic the lava 
and the longer its flight, the greater its tendency to assume a spher- 
ical form. So far as I am aware, the bombs exhibiting a gradation in 
the size of the vesicles occupying the interior are nearly spherical, 
while the oval or spindle-shaped examples with projecting ends are, 
as a rule at least, compact in the interior and contain relatively few 
and frequently unsymmeirical cavities. The absence of scoria? in the 
compact bombs is seemingly accounted for on the assumption that the 
lava was without a high percentage of contained steam or gases, or 
that the liquidit}^ of the material was such that they escaped before 
consolidation was far advanced. There are large quantities of thin, 
nearly flat cakes of compact or but slightly vesicular lava strewn 
about the Cinder Buttes, which will be described below, and which 
were, I suppose, formed by the cooling of liquid lava blown out of the 
volcanoes, after coming to rest. The compact bombs, with more or 
less spiral bands or vesicles in their interior and spiral and curved 
ridges on their surfaces, seem to have originated from masses of vis- 
cous lava, similar to those which formed the flat cake, but which 
cooled in the air and, after losing their previously contained steam, 
were rolled together on account of their rotation so as to assume 
more or less football-like shapes. 
In this connection, however, it should be noted that no forms inter- 
mediate between the flat cakes produced apparently by the ejecting 
highly liquid lava and the well-defined globular or ovoid bombs were 
noted. There is, so far as I am aware, an absence of bombs which 
were sufficiently plastic to become conspicuously flattened on striking, 
