5 68 THE POPULAR SCIENCE MONTHLY. 



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 seons 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 lime- 

 stone, 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 sea- weed. 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 sand- 

 stone 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 toaPteropod 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 



