132 Trachyte and Basalt. parti. 



CHAPTER VI. 



TRACHYTE AND BASALT. DISTRIBUTION OF VOLCANIC 



ISLES. 



The sinking of crystals in fluid lava — Specific gravity of the consti- 

 tuent parts of trachyte and of basalt, and their consequent separa- 

 tion — Obsidian — Apparent non-separation of the element of plutonic 

 rocks — Origin of trap-dikes in the plutonic series — Distribution of 

 volcanic islands ; their prevalence in the great oceans — They are 

 generally arranged in lines — The central volcanos of Von Buck 

 doubtful — Volcanic islands bordering continents — Antiquity of 

 volcanic islands, and their elevation in mass — Eruptions on 

 parallel lines of fissure within the same geological period. 



On the separation of the constituent minerals of lava, 

 according to their specific gravities. — One side of 

 Fresh- water Bay, in James Island, is formed by the 

 wreck of a large crater, mentioned in the last chapter, 

 of which the interior has been filled up by a pool of 

 basalt, about 200 feet in thickness. This basalt is of a 

 gray colour, and contains many crystals of glassy albite, 

 which become much more numerous in the lower, 

 scoriaceous part. This is contrary to what might have 

 been expected, for if the crystals had been originally 

 disseminated in equal numbers, the greater intumescence 

 of this lower scoriaceous part would have made them 

 appear fewer in number. Von Buch l has described a 

 stream of obsidian on the peak of Teneriffe, in which 



1 ' Description des Isles Canaries,' pp. 190 and 191. 



