464 THE ICE AGE IN NORTH AMERICA. 



Zembla. We may, therefore, drop this second theory out of 

 consideration. 



The third theory, so ably advocated by Professor Whitney, 

 that the Ice age was the direct result of the excessive moisture 

 of earlier periods, and that the disappearance of glaciers is to 

 be accounted for by a general drying up of the earth, is ruled 

 out by the fact that there is evidence, among other things, 

 from the vast deposits of salt existing in numerous parts of 

 the world, that the work of desiccation has been going on in 

 some portions of the earth from the earliest geological ages. 

 For example, central New York is, at the present time, one 

 of the best-watered portions of the world; but it is underlaid 

 by deposits of salt sixty or seventy feet in thickness, and these 

 extend under much of the area of Upper Canada and Michi- 

 gan. To produce this amount of salt there must have oc- 

 curred, during the Upper Silurian age, the drying up of an 

 inland sea over that region a mile in depth. We are com- 

 pelled, therefore, to regard the era of the saline group of 

 rocks, rather than the present, as the great age of desiccation. 

 Besides, moisture in the atmosphere is efficient as a glacial 

 cause only when it is precipitated as snow, and this must be 

 determined by general meteorological conditions. There is 

 probably moisture enough always in the air to produce an 

 ice age if the conditions can be combined to precipitate it in 

 the right form and at the right place to encourage the growth 

 of glaciers. 



The fourth theory is that argued at great length by Profes- 

 sors Chamberlin andSalisbury ("Geology," vol. ii, 665, vol. iii, 

 432). According to this theory the presence of an excessive 

 amount of carbon dioxide in the atmosphere increases the 

 warmth of the earth's surface by reducing the radiation. 

 Therefore, whatever reduces the carbon dioxide in the envel- 

 oping atmosphere of the earth will reduce its superficial 

 temperature. It is well known that preceding the glacial 

 epochs of the permian and pliocene periods there was a great 

 extension of land areas on both continents accompanied by 

 the formation of great mountain systems. This greatly in- 



