EFFECT OF LAST ICE AGE 711 



The water rose in an artesian flow from a depth somewhere between 709 

 and 950 feet at a temperature of about 55 degrees, and the gradient is 

 not far from one degree in 70 feet. It certainly can not be any lower 

 than one degree in 84 feet, as given by Darton. 



Effect of last Ice Age 



If Lake Superior has no cooling influence, what is the cause of the low 

 gradient ? It may be that this is the normal gradient, due to the flow of 

 internal heat from far below. Such a gradient would give a temperature 

 of 1,500 degrees centigrade in 73.5 kilometer, or 2,000 degrees centigrade 

 in 98 kilometer depths, from which lavas might be supposed to come. 

 Then others generally found would be abnormal and affected by chem- 

 ical reactions, and this is certainly often the case. But one thing that 

 clearly appears from the data should be considered, and that is that the 

 rate of increase is more rapid at the bottom. This, to which we shall 

 return, might be laid to approaching a source of heat below, for in that 

 case the curve of heat would be steeper as we go down, for not only is 

 the heat increasing, but the rate of increase of heat increases as we ap- 

 proach the source of heat. This would imply a continued acceleration 

 of the increase of heat with deeper mining. But in that case we should 

 expect an abnormally high gradient at the bottom, which is not true. 

 The Montana mines reach a greater temperature in half the depth — for 

 example, the North Butte Mine has at 2,800 feet a temperature of 107 

 degrees Fahrenheit, and the igneous activity there has been much more 

 recent. 



The same effect, however, would be produced if the surface tempera- 

 ture had grown milder in later years. It would be the same kind of 

 effect as the progress of a summer wave of heat downward, which has 

 been studied^ by Callendar and McLeod and many authorities of weather 

 bureaus and agricultural stations. 



That there has been such an effect is certain, for we know that not so 

 many thousands of years ago there was a time when the ice covered this 

 region. There are, indeed, kettle-holes and other signs of melting ice 

 blocks right over the Tamarack and Calumet and Hecla mines. Then 

 we know that this was succeeded by a lake — Lake Duluth — which 

 Leverett has studied,^ ° and that probably relatively soon thereafter the 

 land emerged. What the changes were thereafter in ground temperature 



^Transactions Royal Society of Canada, vol. i, 1895, sec. 3, flg. 6; see also vol. ii, 

 p. 109, and vol. iii, p. 31. 



^<* Michigan Academy of Sciences, 1917. 



