BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 34. PP. 703-720 December so, 1923 



GEOTHEEMS OF LAKE SUPEEIOR COPPEE COIJ^ TEY ^ 



BY ALFRED C. LANE 



{Presented before the Society December 30, 1922) 



CONTENTS 



Page 



Summary 703 



Historical > 704 



Summary of data 705 



Lake Superior not efficient 710 



Effect of last Ice Age 711 



Mathematical discussion 715 



Other causes of low gradient .• 719 



Other regions 720 



Summary 



The rate of increase of temperature in the Lake Superior mines aver- 

 ages about one degree per 105 feet, but is greater in depth, about one 

 degree Fahrenheit per 90 feet. While the mean air temperature in 

 Calumet is about 39 degrees Fahrenheit, the mean ground temperature 

 is 43 degrees (42.6 degrees Fahrenheit), owing to the blanketing effect 

 of the snow in winter. 



This increase of gradient is due to an increase of surface temperature 

 about 10,000 years ago; for, by taking the gradient below 4,000 feet and 

 producing it to the surface, we find a surface temperature of 32 degrees 

 Fahrenheit, that which the ice bottom may have had. The time since 

 the rise in temperature is found by seeing how much the scale of time 

 may be enlarged in the probability curve connecting the temperature and 

 time for a flow of heat due to a sudden rise of temperature, and yet have 

 the curve lie under one constructed to show the excess of observed tem- 

 perature over that due to the bottom gradient. 



There are indications of a milder climate between the Ice Age and the 

 present. 



^ Manuscript received by tlie Secretary of the Society February 20, 1923. 



(703) 



