eitssell.] VOLCANIC BOMBS. 77 
by it, but more frequently this outer cellular layer is absent. Within 
the thin crust the hollow vesicles or steam cavities are small and 
increase in size toward the center, where they frequently measure half 
an inch in diameter. A photograph of the broken surface of such a 
bomb, showing about one-fourth of its circular cross section, is repro- 
duced on PI. XI, B. The appearance of some of the vesicular bombs 
when seen in section is almost precisely the same as the interior of 
a similar body from Ascension Island, a drawing of which has been 
published by Charles Darwin. a It has been suggested by Darwin 
that the gradation in the size of the vesicles in the bombs found on 
Ascension Island, from large at the center to small near the inner 
surface of the inclosing rind, is due to the rapid rotation of the mate- 
rial while in the air, the ''centrifugal force" thus generated tending 
to relieve pressure at the center and to permit the steam-filled vesicles 
inclosed in that part of the body to expand more than those near the 
surface. The bombs from the Cinder Buttes having the internal 
structure described and figured by Darwin were found near the 
praters from which they came, in several instances lying on their 
inner slopes, and do not seem to have made long aerial flights. 
Besides they are nearly spherical and without the spindle-like form 
with projecting ears, such as characterize associated bombs which 
cooled while rotating. For these reasons it does not seem that Dar- 
win's explanation is applicable to the examples observed, and as an 
alternative I venture to suggest the following hypothesis: 
As has been observed, metals on cooling from fusion give out gases 
which were previously dissolved or occluded in the molten mass, and 
it may reasonably be supposed that lava, especially if basic and rich 
in iron, would behave in a similar way. If, then, a mass of highly 
heated, viscous lava should be thrown out from a volcanic crater 
and its surface cooled quickly so as to form a crust, the vapors and 
gases given off by the mass within as it cooled would be retained 
and tend to form vesicles. As cooling progressed the vapors and 
gases would be forced toward the center and there form the largest 
vesicles, room being furnished in part by the shrinkage of the magma. 
This supposed process, by which the vesicles are produced and 
increased in size from the rind toward the center of a bomb, is analo- 
gous to the formation of a crust on a loaf of bread with a highly 
vesicular interior. In this connection attention may be again directed 
to the second figure on PI. XII, which shows the appearance of the 
surface of a bomb which became cracked, and the cracks widened 
owing to expansion from within. This is analogous to the widening 
of a cut on the surface of a loaf of bread as it bakes. In brief, both 
aGeological observations on the volcanic islands visited during the voyage of H. M. S. Beagle. 
London, 1844, p. 36. 
