chap. in. Basaltic Rocks, 41 



some parts extremely vesicular, in others little so ; it is 

 of a black colour, but sometimes contains crystals of 

 glassy feldspar, and seldom much olivine. These 

 streams appear to have possessed singularly little 

 fluidity ; their side walls and lower ends being very 

 steep, and even as much as between twenty or thirty 

 feet in height. Their surface is extraordinarily rugged, 

 and from a short distance appears as if studded with 

 small craters. These projections consist of broad, 

 irregularly conical, hillocks, traversed by fissures, and 

 composed of the same unequally scoriaceous basalt with 

 the surrounding streams, but having an obscure ten- 

 dency to a columnar structure ; they rise to a height 

 between ten and thirty feet above the general surface, 

 and have been formed, as I presume, by the heaping up 

 of the viscid lava at points of greater resistance. At 

 the base of several of these hillocks, and occasionally 

 likewise on more level parts, solid ribs, composed of 

 angulo-globular masses of basalt, resembling in size 

 and outline arched sewers or gutters of brickwork, but 

 not being hollow, project between two or three feet 

 above the surface of the streams ; what their origin 

 may have been, I do not know. Many of the super- 

 ficial fragments from these basaltic streams present 

 singularly convoluted forms ; and some specimens could 

 hardly be distinguished from logs of dark-coloured wood 

 without their bark. 



Many of the basaltic streams can be traced, either 

 to points of eruption at the base of the great central 

 mass of trachyte, or to separate, conical, red-coloured 

 hills, which are scattered over the northern and western 

 borders of the island. Standing on the central eminence, 

 I counted between twenty and thirty of these cones of 

 eruption. The greater number of them had their trun- 

 cated summits cut off obliquely, and they all sloped 



