THE ANCIENT OUTLET OF LAKE MICHIGAN. 229 



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 fel- 

 lows from other expanded lakes of the close of the Glacial period, 

 did not exist long enough to broaden its channel by lateral mean- 

 derings. 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 interest- 

 ing to see how far their essential features are merely repetitions 

 of those so clearly shown on the maps of the old outlet of Lake 

 Michisran. 



'^O' 



With reference to Oroll'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 tliat from 

 Yorkshire to the Shetlands, and siujilarly that, from 40 to 80 of north latitude, 

 the shift of the winter isothermals would be only about 4 of latitude, a result 

 wholly inadequMte to produce an ice age. The shift of isothermals in the genial 

 age was found to be still smaller. 



