5 METAMOEPHIC EOCKS. [Ch. L 



and Mr. Necker has proposed the term " underlying" for the granites, 

 to designate the opposite mode in which they almost invariably present 

 themselves. 



Metamorphic, or stratified crystalline rocks. — The fourth and last 

 great division of rocks are the crystalline strata and slates, 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 scoriae, 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, corresponding in 

 form and arrangement to those of sedimentary formations, and are 

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

 nation of substances varying in color, composition, and thickness, pre- 

 cisely as we see in stratified fossiliferous deposits. According to the 

 Huttonian theory, which I adopt as the most probable, and which will be 

 afterwards more fully explained, the materials of these strata were origi- 

 nally deposited from water in the usual form of sediment, but they were 

 subsequently so altered by subterranean heat, as to assume a new texture. 

 It is demonstrable, in some cases at least, that such a complete conversion 

 has actually taken place, fossiliferous strata leaving exchanged an earthy for 

 a highly crystalline texture for a distance of a quarter of a mile from their 

 contact with granite. In some cases, dark limestones replete with shells and 

 corals, have been turned into white statuary marble, and hard clays, contain- 

 ing vegetable or other remains, into slates called mica-schist or hornblende- 

 schist, every vestige of the organic bodies having been obliterated. 



Although we are in a great degree ignorant of the precise nature of 

 the influence exerted in these cases, yet it evidently bears some analogy 

 to that which volcanic heat and gases are known to produce ; and the 

 action may be conveniently called plutonic, because it appears to have 

 been developed in those regions where plutonic rocks are generated, and 

 under similar circumstances of pressure and depth in the earth. "Whether 

 hot water or steam permeating stratified masses, or electricity, or any 

 other causes have cooperated to produce the crystalline texture, may be 

 matter of speculation, but it is clear that the plutonic influence has some- 

 times pervaded entire mountain masses of strata. 



In accordance with the hypothesis above alluded to, I proposed in the 

 first edition of the Principles of Geology (1833), the term " Metamorphic*' 

 for the altered strata, a term derived from fxsrot, meta, trans, and popcpYj, 

 rnorphe, forma. 



Hence there are four great classes of rocks considered in reference to their 

 origin, — the aqueous, the volcanic, the plutonic, and the metamorphic. In 

 the course of this work it will be shown, that portions of each of these four 

 distinct classes have originated at many successive periods. They have all 

 been produced contemporaneously, and may even now be in the progress 

 of formation on a large scale. It is hot true, as was formerly supposed, 

 that all granites, together with the crystalline or metamorphic strata, 

 were first formed, and therefore entitled to be called " primitive," and 



