For some reason or other the temperature gradually fell, and great glaciers forming in the northern regions, the highlands of Canada and the Arctic Circles, submerged Northern Europe and reduced Canada and half of the United States to the present condition of Greenland. Those who see glaciers to-day can form little idea of the enormous possibilities of semi-fluid ice. Only in Alaska, where the Muir Glacier empties itself into the Muir inlet at the rate of seventy feet a day, can we form any idea of the glacier as a destructive agency. This glacier empties two hundred million cubic feet of ice into the sea every day; that is to say, 45,000 tons of ice fall into the water every minute in avalanches with detonations which sound like the booming of a cannonnade. The very earth seems to tremble, and the sea boils and foams with the continual discharge of fresh icebergs. “From observations upon living glaciers,” says Dr. Wright, “and from the known nature of ice, we may learn to recognize the track of a glacier as readily and unmistakably as we would the familiar foot-prints of an animal.” By the effects of ice-grinding, rocks are smoothed and polished, rounded and mammillated. They are, moreover, striated. “These may be called glacial hieroglyphics; glacial deposits are equally distinctive. They are of three different kinds—ground moraine, terminal moraine, and erratic bowlders. The heights to which the ice-flood rose are frequently self- registered on the mountains which once breasted its flow. They serve, in Dr. Wright’s phrase, as ‘glaciometers.’ Thus it has been learned that the ice was a mile thick in New England and a couple of thousand feet thick in Pennsylvania. The date of the close of the Glacial Epoch in the United States can scarcely, then, be placed earlier than 6,000 B. C. For it was, we repeat, the withdrawal of the ice that set the chronometer of the Falls going. The Falls of Niagara, indeed, constitute in themselves, in Dr. Wright’s apt phrase, ‘a glacial chronometer.’” [Illustration: TAKU GLACIER, ALASKA.] It was this tremendous agency of glacial action that gave us Northwest America as we have it at present. “The inexhaustible fertility of the Far West is an endowment from vanished glaciers.”