Page:Popular Science Monthly Volume 68.djvu/200

196 is the Utica slate, near the top of the Ordovician, while between the Potsdam and the Utica appear in order from below, upward, the Beekmantown, Chazy and Trenton limestones. Except perhaps the Utica slate and the Trenton limestone, which is somewhat shaly, all these are firm, resistant rocks. The visible contacts of the Paleozoic strata with the old crystallines, especially on the northwest and west, are often those of sedimentary overlap, due to an advancing shore line, but on the east and northeast they are much more frequently due to block-faulting of a most interesting character and exceedingly significant as throwing light on the physiography of the interior mountains. Aside from this, however, the Paleozoic strata enter only in a very minor way



into the structure of the mountains. They occur around the edges, except for a few isolated outliers from five to forty miles within the Precambrian area.

After the deposition of the Utica, so far as the actual evidence is concerned, there were no more rocks laid down until the advent of the Labradorean ice sheet of the Glacial epoch. Whether later Paleozoics once existed and have been removed by erosion, or whether the area has been continuously land from the close of the Ordovician to the present, may be esteemed to a certain extent open to debate. From observations near Little Falls on the southern side, Professor H. P. Gushing has concluded that the Niagara limestone probably extended a long distance into the area of the crystallines if not entirely across. But no trace of it has been discovered in place, and the great gap in time from