632 



GEOLOGY. 



its ejection from beneath. As a penetrated body must have an existence prior to its 

 penetration, nothing can be plainer than that the limestone was formed prior to the 

 invasion of the granite ; while it is equally clear, that, by thus overlying and interstratifying 

 the limestone, and forming veins as at c c, the granite must have been erupted in a melted 

 condition. A remarkable change is often observed in the character of the sedimentary 

 strata in immediate contact with the granite or any other igneous rock, arising from 

 exposure to the action of the heated mass. Thus the celebrated Carrara marble, used in 

 sculpture, once supposed, from the absence of fossils, and its crystalline texture, to be a 

 primitive rock, is only a limestone of a comparatively modern period, whose characteristic 

 fossils have been obscured or destroyed, and the whole fused into a uniform rock, by 

 plu tonic action. Clay and sandstone also have been converted into substances as hard as 

 flint, and coal into coke, by the invasion and interjection of the igneous rocks changes 

 analogous to what may now be produced in those materials by artificial means of a similar 

 nature. 



Thus the unstratified rocks, of 'which granite only has been mentioned, as the most 

 important example, occur in irregular masses beneath the stratified ; in disrupting masses, 

 which have broken asunder the strata, and inclined and contorted them in an endless variety 

 of ways, forming some of the highest mountains of the globe ; in incumbent masses, having 

 flowed over the surface of strata, while in a state of fusion ; as interposing masses, filling 

 cracks or fissures in strata produced by the disturbing force which has acted upon them ; 

 and as veins, which are similar insinuations of the melted mineral matter, but with many 

 slender ramifications or branches, often containing angular fragments, broken from the 

 invaded rock. 



When Hutton was groping his way to the doctrine which he was the first to propound, 

 that granite was the result of igneous fusion, or, to employ his own language, a body trans- 

 fused from the subterraneous region, and made to break and invade the strata, he rightly 

 regarded this as a question only to be determined upon the spot where it is found in 

 immediate contact with those bodies which are evidently stratified. " I wanted to see," 

 he remarks, "whether the granite mass, in poit of time, had been prior or posterior 

 to those water -formed bodies, the Alpine strata, or primary schists (transition slates) ; 

 and, as to the manner of operation, I particularly desired to know if the granite had been 

 made to flow, in that state of fusion, among the broken and dislocated strata. Having 

 suspended my opinion till I should find some decisive appearance by which this important 

 question might be determined, I considered where I might be most likely to find the 

 junction of the granite country with the Alpine strata ; and having an engagement with 

 Mr. Clerk, of Eldin, to visit the Duke of Athol at Blair, I concluded that from that 

 place it could not be far before the great mass of granite, which runs south-west of 

 Aberdeen, would be met with in the river Tilt, or some of its branches. But," he adds, 



" Mr. Clerk and I were resolved to find it out, to whatever 

 distance the pursuit might lead us among the mountains of 

 this elevated tract.** The result was the discovery in Glen 

 Tilt, in the bed of the stream, of the expected junction, 

 which so delighted Hutton and his friends, that the guides 

 thought they had detected some mine of gold. " Here," he 

 states, " I had every satisfaction I could possibly desire, 

 having found the most perfect evidence that the granite had 

 been made to break the Alpine strata, and invade that 

 country in a fluid state." The actual appearance of the 

 junction in Glen Tilt is represented in fig. 34., which shows 

 the undulating outline of the granite forming veins, intruding itself into the beds of clay 



