Page:Popular Science Monthly Volume 1.djvu/584

568 sheet of ice. If we can trust astronomical data (Stone's Tables of the Eccentricity of the Earth's Orbit), the glacial epoch lasted about 50,000 years. Add this to the age of the present channel, and 25,000 years for the preglacial channel, and we have 275,000 years as an approximation to the age of Niagara River.

Of course these figures are given merely as an approximation to the truth. To the general reader the time seems immense. But to the geologist it seems short, and his concern is to account for the æons in which the lakes and their water-shed must have stood above the ocean, but which the Niagara has not registered. Let us attend for a little while to the earlier history of this Niagara region.

From the Old Suspension Bridge three geologic systems can be seen on the river-banks. The lowest is a red, mottled, shaly sandstone, the Medina sandstone. It is marked 3 on the section (Fig. 1). Above this, and having the same dip, is a thin group of green shale and gray limestone, the Clinton group, No. 4 of the section. Overlying the Clinton is dark shale, and over the shale a thick band of gray limestone, the two forming the Niagara group, designated on the section by Nos. 5 and 6.

Below the escarpment at Lewiston, as the diagram will show, the lowest member of the Medina sandstone (No. 1, Fig. 1) appears as the surface rock. We find it ripple-marked and carrying the Lingida cuneata and Fucoides Harlani, its characteristic shell and seaweed. It underlies a good part of Western New York and Canada, and extends southward into Pennsylvania and Virginia, with everywhere the same characters, indicating a quiet, shallow sea, fed by rivers which for ages brought down the same sediments. It is eighth in the series of palaeozoic rocks which form the first volume of the world's history after the beginnings of life, and is the oldest rock which shows itself about the Falls.

Up the river, about two miles from Lewiston, the railroad, which descends the river-bank, takes us to the junction of the Medina sandstone with the Clinton group. The green shale is barren here, but at Lockport we have found it full of Agnostics lotus, a little ill-defined crustacean. The overlying limestone is exceedingly rich in fossils, Atrypa neglecta being the characteristic shell. The sea had changed both its life and the rock material on its bottom.

Another change, and to the Clinton succeeded the Niagara period. The change was not abrupt, for many species, common in the Clinton sea, lived in the Niagara as well.

In the Niagara shale we have found Conularia Niagarensis, a shell which must be referred to a Pteropod mollusk. Pteropods of the living world are seen only on mid-ocean. They flap themselves over the water by wing-like appendages from the side to the head. Their shells do not drift ashore, but the dredge has brought them up from the ooze of the deep-sea bottom. Now, this Niagara shale is only the hardened