Page:The Journal of geology (IA journalofgeology11893univers).pdf/107

gneiss was found in two localities, and was indicated in several others; there is no evidence of irruptive contacts between the gneiss and limestone; the gneiss shows no evidence of sedimentary origin; therefore, the simplest hypothesis, but requiring more proof, is that the gneiss is an eroded metamorphosed plutonic rock, upon which the limestone was deposited. The marble is coarsely crystalline, and in age is next to the gneiss. Near the base of the limestone, and interbedded with it, are peculiar schistose rocks, which, while completely crystalline and resembling igneous rocks in composition, are indicated by their field relations to be of sedimentary origin. Near Gouverneur an outcrop of limestone contains abundant fragments of black schist, scattered through the limestone in a most irregular manner, and making up, perhaps, one-third of the rock. This and other outcrops show that the schist fragments are remains of once continuous schist layers, which have been completely shattered in the course of metamorphism, since between the continuous belts of schist and the Gouverneur locality there is every possible gradation. While the schists show the effects of foldings, contortions, stretchings and shattering, the limestone shows no traces of it, it appearing to have been a plastic mass in which the schists moved with considerable freedom. The conspicuous result of metamorphism in the limestone is crystallization. In the limestones are also pegmatitic veins, which have been much shattered by the dynamic action, reducing them to small lumps of quartz and feldspar, scattered through the limestone. So far as observed the pegmatite yields to strain only by fracturing, not showing preliminary contortions, so general in the schistose layers.

In the southern part of the area examined is a granite, not grading into gneiss, and which breaks through the limestone, causing great disturbance in strike and dip, enclosing masses of the rock many feet in diameter, and metamorphosing this rock to some extent. The sandstone at Gouverneur was found in direct contact with the limestone. Here it appears that the limestone surface has been subjected to erosion before the sandstone was deposited upon it. In confirmation of this are seen narrow irregular cracks extending several feet into the limestone, which have been filled with sandstone. The limestone was evidently completely lithified when the sandstone was deposited and sifted into it, and this implies discordance. This unconformity proves that the limestone is older than the upper Cambrian, the data being wanting for any more definite determination of its age. The metamorphism of the rocks of the limestone-bearing series occurred before upper Cambrian time, but the sandstone is metamorphosed, and this metamorphism must therefore belong to post-Potsdam time.

Comments.—The inquiry rises whether the second metamorphism spoken of, that of the sandstone, is produced merely by interstitial cementation, or is dynamic metamorphism. If the first is found to be the explanation, so far as