212 SPARKS FROM A GEOLOGIST'S HAMMER. 



of the regions contiguous to the lakes are somewhat 

 raised by the lake influence. The cooling effect in winter 

 is not equal to the warming effect in summer. In other 

 words, the mean temperature of the lake is a few degrees 

 higher than that of the land. As this fact cannot be 

 attributed to an influx of river water from more south- 

 ern latitudes, and would seem to be only partially ex- 

 plained by the probably higher temperature of river waters 

 in the same latitudes, it remains to seek an explanation 

 of the higher mean temperature of the lake. Now, let it 

 be remembered that the waters of the lake penetrate 900 

 feet toward the heated interior of the earth; and that it 

 has been ascertained that on the land every fifty-five feet 

 of descent beneath the plane of constant temperature 

 brings us one additional degree of heat. It will thus 

 appear that if the depth of constant temperature in the 

 mean latitude of Lake Michigan is 60 feet, the water of 

 the lake reaches a depth where the terrestrial temperature 

 should be 15 higher than the constant temperature be- 

 neath the land, which would probably be about the mean 

 annual temperature of the locality. The writer has ven- 

 tured heretofore to suggest that, though the cooling influ- 

 ence of the local annual mean must have been felt by 

 the earth in the bottom of the lake, it must be still true 

 that the bottom of the lake has felt somewhat the warm- 

 ing influence of the normal terrestrial temperature at 

 that depth. It seems, therefore, entirely reasonable to 

 maintain that the heat of the earth's internal fires con- 

 tributes something to the excess of the lake's mean warmth 

 over the mean warmth of the land. The great lake 

 may, therefore, be conceived as held in a vast natural 

 dish, which is warmed over the imperishable fire which 



