GEOGRAPHICAL EVOLUTION 349 



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

 together by molten matter driven up from below. 



(6) Metamorphic. The sedimentary rocks of the land 

 have undergone many changes since their formation, some 

 of which are still far from being satisfactorily accounted 

 for. One of these changes is expressed by the term 

 Metamorphism, and the rocks which have undergone this 

 process are called Metamorphic. It seems to have taken 

 place under widely varied conditions, being sometimes con- 

 fined to small local tracts, at other times extending across 

 a large portion of the continent. It consists in the rear- 

 rangement of the component materials of rocks, and nota- 

 bly in their recrystallisation along particular lines or laminae. 

 It is usually associated with evidence of great pressure; 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 microscope. It shows itself more par- 

 ticularly among the older geological 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 more or less effaced, and 

 have given place to that peculiar crystalline 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 recrystallisation 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 recognisable 

 sedimentary 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 underground changes into crystalline 

 granite. The framework of the land, besides being knit 

 together by masses of igneous rock intruded from below, 



