2 8 Terceira. 



PART I. 



as is probable, a similar origin, denudation has been 

 here effected on an enormous scale. Near the base of 

 this hill, I observed beds of white tuff, intersected 

 by numerous dikes, some of amygdaloidal basalt and 

 others of trachyte ; and beds of slaty phonolite with 

 the planes of cleavage directed NW. and SE. Parts 

 of this rock, where the crystals were scanty, closely re- 

 sembled common clay-slate, altered by the contact of a 

 trap-dike. The lamination of rocks, which undoubtedly 

 have once been fluid, appears to me a subject well deserv- 

 ing attention. On the beach there were numerous frag- 

 ments of compact basalt, of which rock a distant facade 

 of columns seemed to be formed. 



Terceira in the Azores. — The central parts of this 

 island consist of irregularly rounded mountains of no 

 great elevation, composed of trachyte, which closely 

 resembles in general character the trachyte of Ascension, 

 presently to be described. This formation is in many 

 parts overlaid, in the usual order of superposition, by 

 streams of basaltic lava, which near the coast compose 

 nearly the whole surface. The course which these 

 streams have followed from their craters, can often be 

 followed by the eye. The town of Angra is overlooked 

 by a crateriform hill (Mount Brazil), entirely built of 

 thin strata of fine-grained, harsh, brown-coloured tuff. 

 The upper beds are seen to overlap the basaltic streams 

 on which the town stands. This hill is almost iden- 

 tical in structure and composition with numerous crater- 

 formed hills in the Galapagos Archipelago. 



Effects of steam on the trachytic rocks. — In the 

 central part of the island there is a spot, where steam 

 is constantly issuing in jets from the bottom of a small 

 ravine-like hollow, which has no exit, and which abuts 

 against a range of trachytic mountains. The steam is 

 emitted from several irregular fissures : it is scentless, 



