. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 metamorp
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 metamo
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
