chap. in. Tr achy tic Rocks. 49 



grains of specular iron, and black microscopical points, 

 which latter, from being easily fused, and then becom- 

 ing magnetic, I presume are hornblende. The greater 

 number of the hills, however, are composed of a quite 

 white, friable stone, appearing like a trachytic tuff. 

 Obsidian, hornstone, and several kinds of laminated 

 feldspathic rocks, are associated with the trachyte. 

 There is no distinct stratification ; nor could I distin- 

 guish a crateriform structure in any of the hills of this 

 series. Considerable dislocations have taken place; 

 and many fissures in these rocks are yet left open, or 

 are only partially filled with loose fragments. Within 

 the space, 1 mainly formed of trachyte, some basaltic 

 streams have burst forth ; and not far from the summit 

 of Green Mountain, there is one stream of quite black, 

 vesicular basalt, containing minute crystals of glassy 

 feldspar, which have a rounded appearance. 



The soft white stone above mentioned is remarkable 

 from its singular resemblance, when viewed in mass, to 

 a sedimentary tuff: it was long before I could persuade 

 myself that such was not its origin ; and other geologists 

 have been perplexed by closely similar formations in 

 trachytic regions. In two cases, this white earthy stone 

 formed isolated hills ; in a third, it was associated with 

 columnar and laminated trachyte ; but I was unable to 

 trace an actual junction. It contains numerous crystals 

 of glassy feldspar and black microscopical specks, and 

 is marked with small darker patches, exactly as in the 

 surrounding trachyte. Its basis, however, when viewed 

 under the microscope, is generally quite earthy; but 

 sometimes it exhibits a decidedly crystalline structure. 

 On the hill marked ' Crater of an old volcano,' it passes 



1 This space is nearly included by a line sweeping round Green 

 Mountain, and joining the hills, called the Weather Port Signal, 

 Holyhead, and that denominated (improperly in a geological sense) 

 ' the Crater of an old volcano.' 



