44 GEOLOGY OF SW. IDAHO AND SE. OEEGON. [bull. 217. 
extrusion from the cooling lava of steam or gases previously held in 
solution in the molten rock. The regeneration of steam or gases in 
this manner would tend to expand a bomb, thus leading to a cracking 
of the first formed crust and a widening of the cracks, as is illustrated 
on PL X, A. The change referred to would also favor the formation 
of larger and larger vesicles within a bomb as the cooling and stif- 
fening of the material composing it progressed. In addition to the 
facts referred to tending to support the hypothesis here restated, it 
may be noted that the bombs seen about the Cinder Buttes which 
show the internal structure, neither in their forms nor in their sur- 
face markings exhibit evidences of rapid rotation, and moreover occur 
on the crests and inner slopes of craters or in situations that do not 
indicate a long aerial flight. The impression that one gains from see- 
ing large numbers of these bombs in the position in which they fell 
is that they are formed of masses of plastic lava which were tossed 
out of craters with only sufficient rotary motion to give a spherical 
form, but were not fired high in the air and did not acquire the spin- 
dle shape with twisted projections at either end, so characteristic of 
bombs that rotate rapidly while cooling. 
Bombs of the various external shapes and with the wide range in 
interior structure described above occur in great numbers about the 
Cinder Buttes, and in fact furnish a very considerable portion of the 
fragmental material of which they are composed. Mingled with the 
bombs in the walls of many of the craters, as previously described/ 
are great quantities of thin, nearly flat, cake-like masses of lava, 
which were formed by the cooling and hardening of small bodies or 
splashes of lava that had been projected into the air and were still 
liquid when they fell. These flat cakes furnish illustrations of one 
extreme of the many variations presented by the material thrown out 
by volcanoes during explosive eruptions; a series which includes 
scoria, clots, several varieties of volcanic bombs, and angular frag- 
ments, such as lava blocks, lapilli, volcanic dust, etc. This wide 
range in the products of volcanic exj)losions in realitj^ presents an 
orderly sequence, dependent on the degree of fluidity, plasticity, or 
rigidity of the material at the time it was blown into the air. 
If the material forming the summit portion of the column of lava 
within the conduit of a volcano becomes rigid before steam explosions 
beneath cause it to be fractured and the fragments blown into the air, 
angular blocks of lava, lapilli, dust, etc., are produced. If the mate- 
rial is plastic or viscous, clots may be blown out, and cool as scoria- 
ceous masses, or so-called cinders, which have rough, irregular, but 
not fractured surfaces. If such clots are projected high in the air 
and cool while rotating, bombs result. Should the lava be still more 
thoroughly fused at the time it is projected into the air, it may still 
be liquid on striking, and form lava cakes, or even spread over the 
a Bull. U. S. Geol. Survey No. 199. 
