chap. v. Ejected Fragments. 127 



a body cf cooled and crystallised lava, which has after- 

 wards been broken up and re-liquefied ; the crust being 

 less acted on by the renewed heat and movement. 



Concluding remarks on the tuff-craters. — These 

 craters, from the peculiarity of the resin-like substance 

 which enters largely into their composition, from their 

 structure, their size and number, present the most strik- 

 ing feature in the geology of this Archipelago. The 

 majority of them form either separate islets, or promon- 

 tories attached to the larger islands ; and those which 

 now stand at some little distance from the coast are 

 worn and breached, as if by the action of the sea. 

 From this general circumstance of their position, and 

 from the small quantity of ejected ashes in any part 

 of the Archipelago, I am led to conclude, that the tuff 

 has been chiefly produced, by the grinding together of 

 fragments of lava within active craters, communicating 

 with the sea. In the origin and composition of the 

 tuff, and in the frequent presence of a central lake of 

 brine and of layers of salt, these craters resemble, though 

 on a gigantic scale, the c salses,' or hillocks of mud, which 

 are common in some parts of Italy and in other countries. 1 

 Their closer connection, however, in this Archipelago, 

 with ordinary volcanic action, is shown by the pools of 

 solidified basalt, with which they are sometimes filled up. 



It at first appears very singular, that all the craters 

 formed of tuff have their southern sides, either quite 

 broken down and wholly removed, or much lower than 

 the other sides. I saw and received accounts of twenty- 

 eight of these craters ; of these, twelve form separate 



1 D'Aubuisson's £ Traile de Geognosie,' torn. i. p. 189. I may 

 remark, that I saw at Terceira, in the Azores, a crater of tuff or 

 peperino, very similar to these of the Galapagos Archipelago. From 

 the description given in Freycinet's ' Voyage,' similar ones occur at 

 the Sandwich Islands ; and probably they are present in many other 

 places. 



