chap. in. Volcanic Bombs. 43 



irregular, studded with projecting points, and even 

 concave. Their surfaces are rough, and fissured with 

 branching cracks ; their internal structure is either 

 irregularly scoriaceous and compact, or it presents a 

 symmetrical and very curious appearance. An irregular 

 segment of a bomb of this latter kind, of which I 

 found several, is accurately represented in the accom- 

 panying woodcut. Its size was about that of a man's 

 head. The whole interior is coarsely cellular ; the cells 

 averaging in diameter about the tenth of an inch ; but 

 nearer the outside they gradually decrease in size. This 

 part is succeeded by a well-defined shell of compact 

 lava, having a nearly uniform thickness of about the 

 third of an inch ; and the shell is overlaid by a some- 

 what thicker coating of finely cellular lava (the cells 

 varying from the fiftieth to the hundredth of an inch in 

 diameter), which forms the external surface : the line 

 separating the shell of compact lava from the outer 

 scoriaceous crust is distinctly defined. This structure 

 is very simply explained, if we suppose a mass of viscid, 

 scoriaceous matter, to be projected with a rapid, rota- 

 tory motion through the air ; for whilst 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 vapours 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 l 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 structure of a softened bomb, in 



1 NichcTs 'Architecture of the Heavens.' 



