RUM. GEOLOGY. 495 



ture the indication rather than the solid substance of the 

 crystal is visible ; when broken again, perhaps the whole 

 disappears. This peculiarity seems to arise from a general 

 polar tendency in the irregular and minute crystals that 

 form the mass, causing them to affect that disposition 

 which, acting on the molecules at liberty, would have 

 constituted a perfect crystal. Thus the lamina of the 

 crystal may be distinctly seen in the laminse of the 

 smaller parts, while its perfect continuity is at the same 

 time interrupted by the unevenness arising from the 

 partial independence of these parts. In other instances 

 it would seem as if numberless minute crystals or grains 

 of the rock had been enveloped in a crystallizing base ; 

 the case being here analogous to that of the well known 

 arenaceous spar of Fontainebleau. Although I have enu- 

 merated this rock among the common traps, as if it was 

 ascertained to consist of hornblende and felspar, there 

 seems much reason to suspect that, like the fine grained 

 rock at Harris already described, it is also a mixture of 

 augit with the latter substance. This opinion is founded 

 on the imbedded crystals of augit which it contains, on its 

 high specific gravity, its very sonorous property, and the 

 great resistance it offers to decomposition, the last being 

 a character in which most varieties of augit rock agree. 

 But it is so difficult to distinguish augit and hornblende 

 when in this state of minute aggregation, that I shall 

 be excused for leaving this matter undetermined. 



All the rocks, the sandstone, the syenite, the augit, 

 the trap, and even the basaltic veins thus described,- are 

 traversed by a second set of basaltic veins. These are 

 often minute, not exceeding an inch in breadth, but very 

 persistent. They are of a very fine grain and nearly 

 black ; and are easily distinguished even in their courses 

 through the first set of veins, by their resistance to de- 

 composition. 



While on the subject of basaltic veins I must not omit 

 to mention a very remarkable one to be seen on the shores 



