GEOGRAPHICAL EVOLUTION. 555 



low, "while still at a great depth, and have been laid bare to the light 

 after the removal of the pile of rock under which they originally lay. 



By noting these and other characters, geologists have learned that, 

 besides the regions of still active volcanoes, there are few large areas 

 of the earth's surface where proofs of former volcanic action or of the 

 protrusion of igneous rocks may not be found. The crust of the earth, 

 crumpled and fissured, has been, so to speak, perforated and cemented 

 together by molten matter driven up from below. 



2. Metamorphic. The sedimentary rocks of the land have under- 

 gone many changes since their formation, some of which are still far 

 from being satisfactorily accounted for. One of these changes is ex- 

 pressed by the term Metamorphism, and the rocks which have under- 

 gone this process are called Metamorphic. It seems to have taken 

 place under widely different conditions, being sometimes confined to 

 small local tracts, at other times extending across a large portion of a 

 continent. It consists in the rearrangement of the component mate- 

 rials of rocks, and notably in their recrystallization along particular 

 lines or laminae. It is usually associated with evidence of great pres- 

 sure ; the rocks in which it occurs having been corrugated and crum- 

 pled, not only in vast folds, which extend across whole mountains, but 

 even in such minute puckerings as can only be observed with the mi- 

 croscope. It shows itself more particularly among the older geologi- 

 cal formations, or those which have been once deeply buried under 

 more recent masses of rock, and have been exposed as the result of the 

 removal of these overlying accumulations. The original characters 

 of the sandstones, shales, grits, conglomerates, and limestones, of 

 which no doubt these metamorphic masses once consisted, have been 

 almost entirely effaced and have given place to that peculiar crystal- 

 line laminated or foliated structure so distinctively a result of meta- 

 morphism. 



An attentive examination of a metamorphic region shows that here 

 and there the alteration and recrystallization have proceeded so far that 

 the rocks graduate into granites and other so-called igneous rocks. A 

 series of specimens may be collected showing unaltered or at least 

 quite recognizable sedimentry rocks at the one end, and thoroughly 

 crystalline igneous rocks at the other. Thus the remarkable fact is 

 brought home to the mind that ordinary sandstones, shales, and other 

 sedimentary materials may in the course of ages be converted by un- 

 derground changes into crystalline granite. The framework of the 

 land, besides being knit together by masses of igneous rock intruded 

 from below, has been strengthened by the welding and crystallization 

 of its lowest rocks. It is these rocks which rise along the central crests 

 of mountain -chains, where, after the lapse of ages, they have been un- 

 covered and laid bare, to be bleached and shattered by frost and storm. 

 Proceedings of the Royal Geographical Society. 



