704 A. C. LAXE GEOTHERMS OF LAKE SUPERIOR COPPER COUNTRY 



The deep gradient of one degree Fahrenheit in 90 feet, equal to one 

 degree centigrade in 49.5 meters, is not so far from that which is normal 

 to the earth, but it is easily explained by the absence of pyrite or signs of 

 any recent exothermal reactions in the rock, by exhaustion of heat from 

 below in early times, by the abnormal thickness of the crust indicated by 

 the observations for isostasy, and the less radioactivity of basic rocks. 

 Eeactions leaving CaClg in solution, and hence endothermal, do not need 

 to be inferred. 



Lake Superior has no appreciable effect on it, first, because the deeper 

 temperature of Lake Superior (39 degrees Fahrenheit) is practically the 

 same as that at Calumet; secondly, because the gradient near the lake is 

 nearer the normal ; and, thirdly, because the lake is too far from Calumet, 

 compared with the difference in elevation, to have an appreciable effect. 

 Similar postglacial heat waves should be and may be noted in the tem- 

 perature of other deep wells. 



Historical 



The first observations on underground temperatures on Keweenaw 

 Point were made back in 1847 and were reported by C. T. Jackson,^ and 

 are tabulated in my report on page 759. He rightly assumed 43 degrees 

 as the surface soil temperature — that is, it is probably within one degree 

 of this for the yearly average, which varies a little from year to year, as 

 the table shows. The greatest depth then reached (with no account of 

 the topography) was 236 feet, where the temperature was 45 degTees 

 Fahrenheit. No account has been taken of mean depth from the surface, 

 nor allowance for the topography in any of the measurements. 



The next important observations were made by H. A. Wheeler.^ Sub- 

 tracting one degree in 105 from his upper measurements, we get 41.7 

 degrees Fahrenheit for the initial soil temperature; and, doing the same 

 thing for his lower measurements, we find a temperature of 42.5 degrees 

 Fahrenheit. 



In 1894 J. F. Roberts tested the bottom of the Central Mine. 



In 1901 I found in the Champion copper mine a temperature of 44 

 (5 degrees) in the first level, 130 feet below surface, and 45 degrees in 

 the second level, 250 feet down. Making the same deductions of one 

 degree per 105 feet, we have 43.3 degrees Fahrenheit respectively, 42.6 

 degrees for the surface temperature. In August a strong flow from (as 

 was told me) 80 feet down in Calumet and Hecla hole 10, on section 16, 



- Annual message to Congress and accompanying documents, 1849-50, Part III, 

 3 Amei'ican Journal of Science, vol. xxxii, 1886, pp. 125 to 138 ; also in Transactions 

 St. Louis Academy of Science, my report, p. 760. 



