On Mineral and Metallic Veins. 



489 



common origin, we need not be surprised at finding any of the 

 primary rocks in the character of intrusive rocks, among those 

 which he above them in the geological series. Granite, fre- 

 quently occurs intrusively. Masses of granite are often found 

 intersected by veins of granite differing from them somewhat in 

 their constituent parts. Gneiss and the slates lying above it are 

 in like manner intersected by veins of granite. Figures 5 and 6, 

 plate xiii. represent the granite traversing gneiss in the Vallee 

 de Vallorsine in Switzerland. In various parts of the world 

 streams of lava have been poured out from beneath the granite ; 

 the ancient volcanoes of Central France rest upon a granite 

 plain sixteen hundred feet above the level of the river Allier, 

 and their lavas, which have flowed since the valleys were formed 

 into which they have run, pass into the state of compact basalt. 

 Among other proofs of a common origin for all these intrusive 

 rocks, whether lavas or traps, is that shown by Mr. McCuiloch 

 of a trap vein traversing granite, in the Isle of Arran, as de- 

 scribed in fig. 7, plate xiii. 



Where an evident displacement of rocks is observable, it 

 generally occurs that some evidence of the cause is not very far 

 off. In fig. 3, we see the strata displaced and raised, and infer 

 that the displacement has been occasioned by an expansive 

 subterranean force, of which the dividing trap vein is the evi- 

 dence, the fissure in which it is contained being a sort of safety 

 valve. At fig. 8, plate xiii. we have another instance of this 

 subterranean force. The striped laminag of the gneiss rock, and 

 their continuation interrupted, that part lying to the left being 

 raised above the part to the right. The fissure occasioned by 

 this displacement, or shifting, is, in this instance, filled with a vein 

 of granite. This occurs in Coll, one of the western islands. Fig 

 9, from Fudia, is still more instructive : the lamina? of the gneiss 

 are here shifted as in fig. 8. A vein of granite which had in- 

 tersected the gneiss, appears to have been in its turn shifted, by 

 the subsequent intersection of a vein of quartz. On the other 

 side of this mass of gneiss, another vein of granite intersects it, 

 and is in its turn intersected by a vein of trap. From all these 

 appearances, we may suppose that the veins of granite, and 

 perhaps the trap vein, had penetrated the gneiss, perhaps before 

 it had taken its indurated state, and that, at some epoch after 

 every thing had become hard, the shifting took place, and the 



Vol. I.— 62 



