-irbnatAtmli*. 





/'// yw/ 



ErrECT OF PLANT AND ANIMAL LIFE 379 



times to depths of many feet, bring about not merely a rear- 

 rangement of soil particles through a transfer of materials from 

 lower to higher levels, but also a condition of porosity whereby 

 air and water gain access to the deeper lying portions, there 

 to promote further chemical and physical changes. 



Naturally these insects limit their work to dry and light 

 soils, where their operations may be compared with that of earth- 

 worms whose operations are confined to moist ones. Shaler has 

 calculated^ that over a certain field in Cambridge (Massachu- 

 setts) the ants have made an average transfer of soil matter 

 from the depths to the surface sufficient to form a layer each year 

 of at least a fifth of an inch over the entire four acres under 

 observation. He further 

 mentions a curious effect 

 arising from the interfer- 

 ence of the ants with the 

 original conditions, in the 

 separation of the finer from pig. 40.— Effects of ant-hills on soils, aa, 

 the coarser particles. In sand accumulated in hill; l)h, material 



certain parts of New Eng- ^^^^^^ ^^^^ ^^^ «^^P^^' ^^^^^^^^ ^^^^ 

 T T , T ., _ - vegetable mould, 



land where sandy sous had 



laid for a long time uncultivated, fields were covered to a depth of 

 some inches with a layer of fine sand without pebbles larger than 

 the head of a pin, while below the level of perhaps a foot the 

 soil was mainly pebbles, with very little finer material. This 

 condition, it is argued, was brought about by the tens of thou- 

 sands of ants which each year, over every acre, in the process 

 of building their dwelling brought up the finer material and 

 deposited it in the form of a mound about the surface openings, 

 leaving behind the coarser particles, too heavy for them to 

 move. The common black and brown ants of the United States 

 (Formica exsectoides) build upon the surface mounds in many 

 cases from 1 to 2 feet in height, and 3 to 5 feet in diameter, 

 which are composed of materials brought up from below the 

 surface intermingled with twigs and shreds of bark and leaves. 

 The mass of some of these mounds has been calculated as equal 

 to 2 cubic yards. Being of unconsolidated, loosely coherent ma- 

 terial, such are constantly being degraded by wind and rain and 

 their particles distributed over the surrounding surface. ''"Where 



1 12tli Ann. Bep. TJ. S. Geol. Survey, 1890-91, p. 278. 



