720 A. C. LANE GEOTHERMS OF LAKE SUPERIOR COPPER COUNTRY 



(6) The lieat of crushing and orogenic actions may very likely tend 

 to raise the average gradient in other places, but be absent here. 



On the whole, I must maintain still that "the low gradient is not incon- 

 sisteiit with endothermic reactions and with downward-working waters. 

 It will probably be a long time before we shall be able to say at all defi- 

 nitely which part of the low gradient must have been due to each factor." 

 This will probably come through comparative studies of the gradients 

 in different districts. 



Other Eegions 



If, however, the temperatures near the surface are higher than would 

 be inferred from the temperatures at the bottom, and the rate of change 

 there, and the consequent less average gradient for the whole depth, than 

 that at the bottom are due to postglacial warming, they must not be con- 

 fined to the Copper Country. The phenomena must be widespread, al- 

 though the amount of warming at the surface and the depth to which 

 the warming has penetrated should naturally vary from place to place. 



This is indeed the case. It would take us too far to follow it up in a 

 paper which was started as a study of one phase of the question of copper 

 deposition, especially as we may look for a monograph on the subject by 

 C. E. Van Or strand, who is doing much more accurate work than any I 

 have done; yet, just for illustration, we have plotted in figure 3 his ob- 

 servations of the temperatures in the Lake well.^^ It should be noted 

 that as the rocks are quite different, and the diffusivity also different, 

 and the well near the very edge of the farthest extension of the ice-sheet, 

 one should not expect the size nor the depth of penetration of the post- 

 glacial heat wave to be the same. In each case, however, there is a fairly 

 uniform gradient near the bottom of the well. Iji each case this gradient 

 points to a surface temperature near freezing. The depth of penetration 

 of the heat wave seems somewhat greater in West Virginia, though not 

 as much as might be expected. 



Since reading this paper my attention has been called to similar 

 facts and a similar explanation given by Prof. H. Arctowski, of Lwow 

 (Leopol)." 



The records of the deepest mine in the world, that of Saint Juan del 

 Eey, Brazil (6,726 to 6,426 feet), also show a much more rapid rise 

 toward the bottom. ^° 



18 Barton's U. S. Geol. Siu-vej^ BuU. No. 701, p. 94, and the reports of the West Vir- 

 ginia Survey ; also a separate pamphlet by I. C. White. 

 i^Kosmos, May 2, 1923. 

 20 Mining and Metallurgy, June, 1923, p. 283. 



