Page:Popular Science Monthly Volume 46.djvu/241

Rh the Illinois is cutting a narrow trench, must be due to the great volume of the old lake outlet which once filled the channel from bluff to bluff, just as the present tributary rivers fill their valleys. The old outlet river fitted its broad valley as well as the existing rivers fit their narrow valleys. It must have been large compared with its tributaries, because the breadth of its channel increases so slowly in a distance of nearly a hundred miles. Like the Niagara below Buffalo and the St. Lawrence among the Thousand Isles, the old outlet near Joliet split up into a more or less complicated network of channels, and its discharge seems to have continued just about long enough for the selection of one of these as the survivor. But it is manifest that the old outlet, like all its fellows from other expanded lakes of the close of the Glacial period, did not exist long enough to broaden its channel by lateral meanderings. The special phases of climatic periods by which these constrained river courses were determined were too short-lived to allow the development of meandering rivers—far too brief to measure so long a part of a river's history. The old outlet of Michigan endured long enough to clear off the drift from its path, and to make a beginning of cutting its channel down into the underlying rock; but it does not seem to have cut the rock down as deep as it might have done if more time had been allowed, for even the smaller rivers of to-day have trenched the floor of the old channel since the outflow of the lake has been turned over another path. To be sure, something of the ability to do this may be ascribed to the change in the attitude of the land, a presumable elevation to the north since the ice went away; but we have no definite measures of the amount of this elevation in the district here considered.

The good fortune of having maps of this interesting district should bring it clearly before many students who may not see it on the ground. It appears to have so many features common to the other examples of its class that it may well be taken as their type. As other similar channels are mapped, it will be interesting to see how far their essential features are merely repetitions of those so clearly shown on the maps of the old outlet of Lake Michigan.

reference to Croll's and Ball's theories of ice ages and genial ages, Mr. Edward P. Culverwell has shown, on the basis of calculations of the daily distribution of solar heat on different latitudes at the present time and in the supposed glacial and genial ages, that the winter temperature of Great Britain in the glacial age, as dependent on sun heat, would be no lower than that from Yorkshire to the Shetlands, and similarly that, from 40º to 80º of north latitude, the shift of the winter isothermals would be only about 4º of latitude, a result wholly inadequate to produce an ice age. The shift of isothermals in the genial age was found to be still smaller.