CHAP. III. 



Veins. 5 1 



from slight differences in the composition of the same 

 mass of molten stone, and from the abrading action of 

 one such part still fluid on another part already solidi- 

 fied. The curiously formed veins have, I believe, been 

 formed by siliceous matter being subsequently segre- 

 gated. But my chief reason for believing that these 

 soft earthy stones, with their extraneous fragments, are 

 not of sedimentary origin, is the extreme improbability 

 of crystals of feldspar, black microscopical specks, and 

 small stains of a darker colour occurring in the same 

 proportional numbers in an aqueous deposit, and in 

 masses of solid trachyte. Moreover, as I have remarked , 

 the microscope occasionally reveals a crystalline struc- 

 ture in the apparently earthy basis. On the other 

 hand, the partial decomposition of such great masses of 

 trachyte, forming whole mountains, is undoubtedly a 

 circumstance of not easy explanation. 



Veins in the earthy trachytic masses. — These veins 

 are extraordinarily numerous, intersecting in the most 

 complicated manner both coloured varieties of the 

 earthy trachyte : they are best seen on the flanks of 

 the ' Crater of the old volcano.' They contain crystals 

 of glassy feldspar, black microscopical specks and little- 

 dark stains, precisely as in the surrounding rock ; but 

 the basis is very different, being exceedingly hard, 

 compact, somewhat brittle, and of rather less easy 

 fusibility. The veins vary much, and suddenly, from 

 the tenth of an inch to one inch in thickness ; they 

 often thin out, not only on their edges, but in their 

 central parts, thus leaving round, irregular apertures ; 

 their surfaces are rugged. They are inclined at every 

 possible angle with the horizon, or are horizontal ; they 

 are generally curvilinear, and often interbranch one 

 with another. From their hardness they withstand 

 weathering, and projecting two or three feet above the 



