THE CAUSE OF THE GLACIAL PERIOD. 327 



geological times as they have done in any previous geo- 

 logical periods. When one reflects, also, upon the size 

 of the earth, a continental elevation of 3,000 or 4,000 feet 

 upon a globe whose diameter is more than 40,000,000 feet 

 is an insignificant trifle. On a globe one foot in diameter 

 it would be represented by a protuberance of barely one 

 thousandth of an inch. A corresponding wrinkle upon a 

 large apple would require a magnifying-glass for its de- 

 tection. Moreover, the activity of existing volcanoes, the 

 immense outflows of lava which have taken place in the 

 later geological periods, together with the uniform increase 

 of heat as we penetrate to deeper strata in the crust of 

 the earth — all point to a condition of the earth's interior 

 that would make the elevations of land which we have 

 invoked for the production of the Glacial period easily 

 credible. Physicists do not, indeed, now hold to the entire 

 fluidity of the earth's interior, but rather to a solid centre, 

 where gravity overcomes the expansive power of heat, and 

 maintains solidity even when the heat is intense. But 

 between the cooling crust of the earth's exterior and a 

 central solid core there is now believed to be a film where 

 the influences of heat and of the pressure of gravity are 

 approximately balanced, and the space is occupied by a 

 half-melted or viscous magma, capable of yielding to a 

 slow pressure, and of moving in response to it from one 

 portion of the enclosed space to another where the press- 

 ure is for any cause relieved. 



As a result of prolonged enquiries respecting the na- 

 ture of the forces at work both in the interior and upon 

 the exterior of the earth, and of a careful study of the 

 successive changes marking the geological period, we are 

 led to believe that the continental elevations necessary to 

 produce the phenomena of the Glacial period are not 

 only entirely possible but easily credible, and in analogy 

 with the natural progress of geological history. In the 

 first place, it is easy to see that two causes are in operation 



