210 ON THE ORIGIN, MATERIALS, COMPOSITION, 



confirm, both this view, and what was just said 

 respecting the primary calcareous strata them- 

 selves. 



As to diallage rock, and the more antient red sand- 

 stone, the same processes of reasoning apply to them 

 as to those rocks to which they are analogous ; but 

 hornblende schist requires a particular consideration. 

 This is an extremely fusible compound, and its pe- 

 culiar crystalline texture proves that it could not have 

 been deposited from water; in which indeed its earths 

 are insoluble, and from which they could not have 

 been precipitated in such a form. It is, besides, pre- 

 cisely analogous to many greenstones of the trap 

 family; from which indeed it is often so little distin- 

 guishable, that it has been confounded with them 

 under the name of primitive greenstone. That it is, 

 further, actually produced by heat, is evinced by find- 

 ing that the argillaceous schists, when in contact with 

 granite, are actually converted into it. Whether 

 simple, or compounded of hornblende and felspar, 

 the same reasoning applies to it. It is nevertheless 

 admitted, that its original materials have been deposited 

 from water, and thus its laminar and stratified dis- 

 position is explained. That it has, further, consisted 

 of clay or schist, is rendered probable, not only by 

 the numerous facts occurring in the trap rocks, but 

 by that very striking analogy in which beds of shale 

 beneath trap are actually converted into Lydian stone; 

 a substance differing from it, almost solely in the 

 compactness and uniformity of its texture. 



But indeed, as far as a single fact can prove such a 

 case, the origin of hornblende schist from clay slate 

 is completely established by the occurrence, in Shet- 

 land, of a mass of the latter substance alternating 

 with gneiss and approximating to granite. Here, 



