22 



L^ELL'S ELEMENTS OP GEOLOGY. 



Metamorphic Rocks and their Origin. 



limestones, replete with shells and corals, are turned into white 

 statuary marble, and hard clays into slates called mica-schist and 

 hornblende-schist, all signs of organic bodies having been oblite- 

 rated. 



Although we are in a great degree ignorant of the precise 

 nature of the influence here exerted, yet it evidently bears some 

 analogy to that which volcanic heat and gases are capable of 

 producing ; 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 electricity or any 

 other causes have co-operated with heat to produce this influ- 

 ence, may be matter of speculation, but the plutonic influence 

 has sometimes pervaded entire mountain masses of strata. The 

 phenomena, therefore, being sometimes on so grand a scale, we 

 must not consider that the strata have always assumed their 

 crystalline or altered texture in consequence of the proximity of 

 granite, but rather that granite itself, as well as the altered strata, 

 have derived their crystalline texture from plutonic agency. 



In accordance with this hypothesis I have proposed (see 

 Principles of Geology,) the term " Metamorphic" for the altered 

 strata, a term derived from y-£fa, meta, trans, and ^opf^iq, morphe, 

 forma. 



Hence there are four great classes of rocks considered in 

 reference to their origin, — the aqueous, volcanic, plutonic, and 

 metamorphic, all of which may be conceived to have been 

 formed contemporaneously at every geological period, and to be 

 now in the progress of formation. By referring to the Frontis- 

 piece, the reader will perceive what relative positions the mem- 

 bers of these four great classes A, B, C, D, may occupy in the » 

 earth's crust, while in the course of simultaneous production. 

 Thus, while the aqueous deposits A, which are expressed by the 

 yellow colour, have been accumulating in successive strata at the 

 bottom of the sea, the volcanic cone B, has been piled up during 

 a long series of eruptions, and the other igneous rocks coloured 

 purple have also ascended from below in a fluid state. Some of 

 these last have been poured forth into the sea, and there mingled 

 with aqueous sediment. On pursuing downwards either the 

 small dikes or large masses of volcanic rock, we find them pass 

 gradually into plutonic formations, D, which are coloured red, 

 and which underlie all the rest. These last again are seen to 

 be in contact with a zone of contemporaneous metamorphic 

 strata, C, coloured blue, which they penetrate in numerous veins. 



In that part of the section which is uncoloured, a more ancient 

 series of mineral masses are seen, belonging also to the four 



