ii2 Galapagos Archipelago. paet i. 



there occur two craters composed of two kinds of tuff; 

 one kind being friable, like slightly consolidated ashes ; 

 and the other compact, and of a different nature from 

 anything which I have met with described. This 

 latter substance, where it is best characterised, is of a 

 yellowish-brown colour, translucent, and with a lustre 

 somewhat resembling resin ; it is brittle, with an 

 angular, rough, and very irregular fracture, sometimes, 

 however, being slightly granular, and even obscurely 

 crystalline : it can readily be scratched with a knife, 

 yet some points are hard enough just to mark common 

 glass ; it fuses with ease into a blackish-green glass. 

 The mass contains numerous broken crystals of olivine 

 and augite, and small particles of black and brown 

 scorias : it is often traversed by thin seams of calcareous 

 matter. It generally affects a nodular or concretionary 

 structure. In a hand specimen this substance would 

 certainly be mistaken for a pale and peculiar variety of 

 pitchstone ; but when seen in mass its stratification, 

 and the numerous layers of fragments of basalt, both 

 angular and rounded, at once render its subaqueous 

 origin evident. An examination of a series of speci- 

 mens shows that this resin-like substance results from a 

 chemical change on small particles of pale and dark- 

 coloured scoriaceous rocks ; and this change could be 

 distinctly traced in different stages round the edges of 

 even the same particle. The position near the coast 

 of all the craters composed of this kind of tuff or 

 peperino, and their breached condition, renders it 

 probable that they were all formed when standing im- 

 mersed in the sea ; considering this circumstance, 

 together with the remarkable absence of large beds of 

 ashes in the whole archipelago, I think it highly 

 probable that much the greater part of the tuff has 

 originated from the trituration of fragments of the 



