120 Galapagos Archipelago. part i. 



subaqueous deposit ; whilst the upper beds, round the 

 entire circumference, consist of a harsh, friable tuff, 

 of little specific gravity, but often containing fragments 

 of rock in layers. This upper tuff contains numerous 

 pisolitic balls, about the size of small bullets, which 

 differ from the surrounding matter, only in being 

 slightly harder and finer grained. The beds dip away 

 very regularly on all sides, at angles varying, as I 

 found by measurement, from 25 to 30 degrees. The 

 external surface of the crater slopes at a nearly similar 

 inclination ; and is formed by slightly convex ribs, 

 like those on the shell of a pecten or scallop, which 

 become broader as they extend from the mouth of the 

 crater to its base. These ribs are generally from eight 

 to twenty feet in breadth, but sometimes they are as 

 much as forty feet broad ; and they resemble old, 

 plastered, much flattened vaults, with the plaster 

 scaling off in plates : they are separated from each 

 other by gullies, deepened by alluvial action. At their 

 upper and narrow ends, near the mouth of the crater, 

 these ribs often consist of real hollow passages, like, 

 but rather smaller than, those often formed by the 

 cooling of the crust of a lava-stream, whilst the inner 

 parts have flowed onward ; — of which structure I saw 

 many examples at Chatham Island. There can be no 

 doubt but that these hollow ribs or vaults have been 

 formed in a similar manner, namely, by the setting 

 or hardening of a superficial crust on streams of mud, 

 which have flowed down from the upper part of the 

 crater. In another part of this same crater, I saw 

 open concave gutters between one and two feet wide, 

 which appear to have been formed by the hardening 

 of the lower surface of a mud-stream, instead of, 

 as in the former case, of the upper surface. From 

 these facts I think it is certain that the tuff must have 



