520 



CHARLES DARWIN 



cells decreasing in size towards the exterior; where there 

 is a shell-like case about the third of an inch in thickness, 

 of compact stone, which again is overlaid by the outside 

 crust of finely cellular lava. I think there can be little 

 doubt, first that the external crust cooled rapidly in the state 

 in which we now see it; secondly, that the still fluid lava 

 within, was packed by the centrifugal force, generated by 



the revolving of the bomb, against the external cooled 

 crust, and so produced the solid shell of stone; and lastly, 

 that the centrifugal force, by relieving the pressure in the 

 more central parts of the bomb, allowed the heated vapours 

 to expand their cells, thus forming the coarse cellular mass 

 of the centre. 



A hill, formed of the older series of volcanic rocks, and 

 which has been incorrectly considered as the crater of a vol- 

 cano, is remarkable from its broad, slightly hollowed, and 

 circular summit having been filled up with many successive 

 layers of ashes and fine scoria?. These saucer-shaped layers 

 crop out on the margin, forming perfect rings of many dif- 

 ferent colours, giving to the summit a most fantastic appear- 

 ance ; one of these rings is white and broad, and resembles 



