Russell.] RECENT VOLCANOES. 43 
The third common type of bombs includes those which are cellular 
within — the vesicles diminishing* gradually in size from center to cir- 
cumference — and inclosed in a thin rind or crust. In some instances 
there is a thin layer of cellular lava on the outside of the hard rind, 
but more frequently the surface is smooth, glossy black, and in many 
examples broken by shrinkage cracks which have been widened by 
the expansion of the material within after a thin surface crust had 
formed. (PI. X, A.) 
The interior structure of these bombs closely resembles that of 
certain bombs found on Ascension Island and described by Charles 
Darwin. In order to show this resemblance more definitely, a figure 
of a partial section of one of the bombs referred to, published by 
Darwin, a is reproduced on PI. IX by the side of a similar specimen 
from the Cinder Buttes. In each case there is a conspicuous and 
gradual increase in the size of the vesicles in the central scoriaceous 
mass, from the inner side of the inclosing rind to the center. The 
explanation of this peculiar internal structure advanced by Darwin 
is as follows: 
This structure is very simply explained, if we suppose a mass of viscid, scoria- 
ceous matter to be projected with a rapid, rotatory motion through the air, for 
while the external crust, from cooling, became solidified (in the state we now see 
it), the centrifugal force, by relieving the pressure in the interior parts of the 
bomb, would allow the heated vapors to expand their cells; but these being 
driven by the same force against the already hardened crust would become, the 
nearer they were to this part, smaller and smaller or less expanded until they 
became packed into a solid, concentric shell. As we know that chips from a 
grindstone can be flirted off when made to revolve with sufficient velocity, we 
need not doubt that the centrifugal force would have power to modify the struc- 
ture of a softened bomb in the manner here supposed. Geologists have remarked 
that the external form of a bomb at once bespeaks the history of its aerial course, 
and we now see that the internal structure can speak with almost equal plainness 
of its rotatory movement. 
As stated in my report on a reconnaissance in Idaho in 1901, b the 
observations then made concerning the cellular bombs strewn about 
the Cinder Buttes are not in harmony with the explanation quoted 
above. Later studies serve to strengthen this conclusion and to lend 
additional support to the hypothesis advanced in the bulletin referred 
to. In brief the explanation of the cellular condition of the bombs in 
question, and the increase in the size of the vesicles they contain 
from beneath the crust to the center, which apparently best satisfies 
the observed facts, is that masses of steam-charged lava, tossed into 
the air from pools of liquid rock in craters, cooled quickly at the sur- 
face and formed a dense crust, which prevented further escape of 
steam from within, and as the lava continued to cool, the change 
gradually progressing from the circumference inward, there was an 
a Darwin, Charles, Geological observations on volcanic islands visited during the voyage of 
H. M. S. Beagle, London, 1844, p. 36. 
& Bull. U. S. Geol. Survey No. 199. 
