PART I. CHAPTER I. 



21 



Metamorphic Rocks. 



to distinguish them from the volcanic. The beginner will easily 

 conceive that the influence of subterranean heat may extend 

 downwards from the crater of every active volcano to a great 

 depth below, perhaps several miles or leagues (see Frontispiece,) 

 . and the effects which are produced deep in the bowels of the 

 earth may, or rather must be distinct ; so that volcanic and plu- 

 tonic rocks, each different in texture, and sometimes even in 

 composition, may originate simultaneously, the one at the sur- 

 face, the other far beneath it. 



Although granite has often pierced through other strata, it 

 has rarely, if ever, been observed, to rest upon them as if it had 

 overflowed. But as this is continually the case with the volca- 

 nic rocks, they have been styled from this peculiarity, " overly- 

 ing" by Dr. MacCulloch ; and Mr. Necker has proposed the 

 term "underlying" for the granites, to designate the opposite 

 mode in which they almost invariably- present themselves. 



Metamorphic rocks. — The fourth and last great division of 

 rocks are the crystaUine strata or schists, called gneiss, mica- 

 schist, clay-slate, chlorite-schist, marble, and the like, the origin 

 of which is more doubtful than that of the other three classes. 

 They contain no pebbles or sand or scorise, or angular pieces of 

 imbedded stone, and no traces of organic bodies, and they are 

 often as crystalline as granite, yet are divided into beds, corre- 

 sponding in form to those of sedimentary formations, and are 

 therefore said to be stratified. The beds sometimes consist of an 

 alternation of substances varying in colour, composition, and 

 thickness, precisely as we see in stratified fossiliferous deposits. 

 According to the theory which I adopt as most probable, and 

 which will be afterwarc^ more fully explained, the materials of 

 • these strata were originally deposited from water in the usual 

 form of sediment, but they were subsequently altered by subter- 

 ranean heat, so as to assume a new texture. It is demonstrable, 

 ^ in some cases at least, that such a complete conversion has 

 actually taken place. I have already remarked that alterations, 

 such as might be produced by intense heat, are observed in strata 

 near their contact with veins and dikes of volcanic rocks. 

 These, however, are on a small scale ; but a similar influence 

 has been exerted much more powerfully in the neighbourhood 

 of plutonic rocks under different circumstances, and perhaps in 

 combination with other causes. The effects thereby superin- 

 duced on fossiliferous strata have sometimes extended to a dis- 

 tance of a quarter of a mile from the point of contact. Through- 

 out the greater part of this space the fossiliferous beds have 

 exchanged an earthy for a highly crystalline texture, and have 

 lost all traces of organic remains. Thus, for example, dark 



