gether by reason of the westward attenuation, and finally the entire disappearance of the salina formation in which it is largely excavated. The ice was then thrust up on to harder rocks that form the basis of Northwestern Ohio and Northeastern Indiana. Lake Michigan was terminated southwardly by the eastward trend of the rocky outcrops at an angle that the ice could not follow. The Red-River flats of Minnesota correspond to the Winnipeg basin in the same way that the Black-Swamp district of Ohio does to the Lake-Erie basin, or the prairie district surrounding the southern end of Lake Michigan does to the basin of that lake.
It must be remembered, however, that throughout the continuance of the ice-period the motion of the ice brought it finally into a climate where it could not exist as ice. It gave rise to countless streams of water. The broad, level country of the Northwest was not sufficiently irregular to gather the ice, and consequently not the water, into valleys having a north-south direction. The water from the ice acted all along the ice-foot with a comparatively uniform energy. If it was ultimately gathered into large streams, it must have been at considerable distances from the glacial field. It must be remembered also that the accumulated precipitation of the entire year over broad, continental areas was preserved from thaw till it arrived at the latitude of the limitation of the glacier, and there its full volume was discharged. It was as if the entire precipitation of the continent—say from the latitude of Chicago to the north-pole—were concentrated on a belt of territory, say of fifty miles in width, running east and west across the continent, and having the direction of the marginal line of the ice-foot. Thus a constant sheet of turbid, running water would act on all objects over which the ice-foot retreated.
We must not forget, in recalling to our imagination the scenes and events of the ice-period, to inquire what were the position and the condition of the drift to which it gave origin.
In regions far to the north, the eye probably would not be able to discern any object except that of the universal ice. The surface of the ground would be thousands of feet below the traveller, if we may be permitted to presume so hardy a human being. Like Dr. Kane exploring the great Humboldt Glacier of Greenland, he would meet with countless obstacles and dangers. But those obstacles would consist of hummocky ice, or crevassed ice, or perpendicular ice-walls. He would see no soil, no rocks, no vegetation, no animal life. The winds would whistle, storms would rage, snow would be drifted about, and the ineffectual sun would rarely venture to smile on the dreary waste. Farther to the south, the explorer would find isolated spots of bare ground. He would see about them the accumulated débris of bowlders, gravel, and dust, from constant winds, spread more or less over the ice-field, staining its painful whiteness, and showing the more grateful aspect of earth and stones. Another hundred miles farther south,
