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586 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
accompanied by a marked crystallization. The limestones have become 
marbles, the sandy beds quartzites, and the other strata have assumed the 
character of slate, mica-schist, chlorite-schist, and gneiss, among which 
hornblendic, augitic, hypersthenic, and chrysolitic zones occur. The 
geological horizon of these rocks is shown by the discovery in them at 
various localities of fossils belonging to the Trenton and Hudson River 
subdivision of the Lower Silurian system of eastern North America. 
The rocks have been ridged up and altered along a belt of country lying 
to the east of the Hudson and extending north into Canada. 
Other examples might be cited. <A long belt of regional metamor- 
phism extends through the Ardennes, and instructive areas occur in 
the Harz and in Greece. Some parts of the Triassic formations of the 
Sierra Nevada of Western North America have been found by Whitney 
in the condition of serpentine and mica-schist ; while on the Coast Range 
of California he has met with similar metamorphism of the Cretaceous ~ 
series. Itis probable that such alterations have repeatedly occurred in - 
successive geological periods over the surface of the globe. 
From the evidence of such examples, the conclusion may be drawn 
that there are extensive regions where ordinary sedimentary strata 
have been plicated, crumpled, and foliated, so as to assume the 
character of true crystalline schists. This change is precisely similar 
in its stages to that which may be traced in local metamorphism 
round bosses of granite. It is connected with, and proportional to, 
mechanical disturbance of the strata. It is unequal in extent, even 
over limited areas, being apt to attain sporadically a maximum 
development, particularly in the areas of greatest plication. Even 
in the midst of the metamorphosed tracts, bands of comparatively 
unchanged rock may be traced, the true clastic origin of which 
cannot be disputed. The process was not everywhere uniform, partly, 
no doubt, because of the varying composition of the rocks subjected 
to its operation, and partly because it really was more actively 
induced in areas of greater disturbance. 
From the evidence furnished by local metamorphism, there can 
be little hesitation in regarding the bedding of the crystalline rocks 
in a tract of regional metamorphism as generally representing 
original layers of deposit. In some cases, however, the foliation may 
represent cleavage, as pointed out by Sedgwick and Darwin. So far, 
indeed, as a rock continued homogeneous in chemical composition 
and general texture, foliation might be induced along any dominant 
divisional planes. If these planes were those of cleavage, the resultant 
foliation might not appreciably differ from cleavage along original 
bedding planes. But it may be doubted whether a cleavage foliation 
could run without sensible and even very serious interruptions over 
wide areas. Jor, in the first place, in most large masses of sedimen- 
1 See Dana, Amer. Jowrn. Sci. xiii, xiv. xvii. The identification of the so-called 
Taconic schists of New England with altered Lower Silurian rocks has been called in 
question by Sterry Hunt, but the stratigraphical evidence collected by A. Wing, Dana, - 
and others, and the testimony of the fossils collected by Dana, Dwight, &c., have 
sustained it. In the Punjab a series of gneisses and schists overlies infra-Triassic rocks, 
Wynne, Geog. Mag. 1880, p, 314. 
