418 THE ICE AGE IN NORTH AMERICA. 



as the Pacific coast, it is readily answered : " The terrestrial 

 globe in some of its behavior may be compared to an India- 

 rubber ball filled with water. If indented by pressure in one 

 place, there must be a protuberance equal in volume in 

 another place. In a ball of uniform composition, the pro- 

 tuberance would be spread over the entire surface beyond 

 the region indented, and the effect in one particular spot 

 might be insignificant. Should a small area of the caout- 

 chouc be thinner than the rest, that part would be protruded 

 to a greater extent than other parts of the surface. Should 

 there be small holes or fissures through it, the water would 

 escape and flow over the surface — that is, the protuberance 

 resulting from local pressure would be chiefly on the outside 

 of the shell. As we ordinarily conceive it, the water would 

 be squeezed out like the juice of a squeezed orange."* 



Another supplementary theory which has been invoked 

 to account for the loess, is, that it has, in certain sections 

 at least, been brought to the surface by the agency of earth- 

 worms and other animals which burrow in the ground. The 

 remarkable facts adduced by Mr. Darwin concerning the ac- 

 tivity of these humble agents give respectability to the 

 theory ; and, indeed, the power of these agencies can be seen 

 by any observer who will take the pains to notice the count- 

 less marks of angle-worms which frequently appear upon the 

 surface of the soil after a rain. Mr. Darwin estimated that 

 the amount of soil brought to the surface by worms was in 

 one case at the rate of nearly two tenths of an inch a year. 

 It is estimated, also, by competent authority, that the number 

 of worms to an acre is as great as fifty-four thousand and 

 that they would weigh three hundred and fifty-six pounds. 

 Trustworthy estimates also show that these creatures some- 

 times raise annually to the surface fourteen tons, and again 

 eighteen tons, to the acre.f From this it is easily seen that 

 the predominance of fine earth upon the surface is due to the 



* "Some Effect of Pressure of a Continental Glacier," p. 139. 



f See Darwin, on " Vegetable Mould and Earth-Worms," chaps, iii and iv. 



