220 METAMORPHIC ROCKS. 



the surface, over wide regions. Nearly the whole of Canada and Labra- 

 dor, a large strip on the eastern slope of the Appalachians, and a large 

 portion of the mountainous regions of the western border of this con- 

 tinent, are composed of them. Beneath the surface they probably un- 

 derlie all other stratified rocks, and are therefore the most widely dif- 

 fused of all rocks. Their thickness is also often immense. The Lau- 

 rentian series of Canada is probably 50,000 feet thick, and metamorphic 

 throughout. 



Principal Kinds. — The principal kinds of metamorphic rocks are : 

 Gneiss, mica-schist, chlorite-schist, talcose-schist, hornblende-schist, 

 clay-slate, quart zite, marble, and serpentine. 



Gneiss, the most universal and characteristic of these rocks, has the 

 general appearance and mineral composition of granite, except that it is 

 more or less distinctly stratified. Often, however, the stratification can 

 only be observed in large masses. Gneiss runs by insensible gradations, 



on the one hand, into 

 granite, and on the other, 

 through the more per- 

 fectly stratified, schists, 

 into sandy clays or clayey 

 sands. 



The schists are usu- 

 ally grayish fissile rocks, 

 made up largely of scales 

 of mica, or chlorite, or talc. Hornblende-schist is similarly made up of 

 scales of hornblende, and is therefore a very dark rock. The fissile 

 structure of schists is due to the presence of these scales, and is there- 

 fore wholly different from that of slates. It is called foliation-struct- 

 ure. 



Serpentine is a compact, greenish magnesian rock. The other va- 

 rieties need no description. Hornblende-schists run by insensible gra- 

 dations into clay-slates on the one hand, and into diorites and syenites 

 on the other. 



All these kinds may be regarded as changed sands, limestones, and 

 clays, the infinite varieties being the result of the difference in the original 

 sediments and the degrees of metamorphism. Sands and limestones are 

 often found very pure ; such when metamorphosed produce quartzite 

 and marble. Clays, on the contrary, are almost always impure, con- 

 taining sand, lime, iron, magnesia, etc. Such impure clays, if sand is 

 in excess, produce by metamorphosis gneiss, mica-schist, and the like; 

 but if lime and iron are in considerable quantities they produce horn- 

 blende-schist or clay-slate ; if magnesia, talcose-schist. The origin of 

 serpentine is not well understood ; but it is evidently in many cases a 

 changed magnesian clay. All gradations between such clays and ser- 



Fig. 191.— Gneiss. 



