300 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



These termites live partly in their houses and partly in the ground. 

 Their nests all open into the soil beneath, so that the acids and gases 

 produced by these vast numbers of insects are poured into the ground 

 and eventually carried down to the rocks beneath. 



The galleries of the ants proper spread out on all sides of the main 

 entrances and anastomose so as to form a vast network of underground 

 channels, into which the ants drag their food and the leaves of plants, 

 where their dead bodies decaj'', and where their breath so long as they 

 live contributes carbonic acid* to the other organic acids formed by the 

 decay of this animal matter, all of which attack the rocks and soil and 

 are washed by the rains to lower levels.f 



One fact which suggests the great numbers of these ants is that there 

 are in Brazil several species of armadillos and ant-eaters which subsist 

 in part or entirel}^ on ants and termites ; and the tamanduas or ant- 

 eaters are no small animals. Gardner tells of having killed one that 

 measured 6 feet in length, not counting the tail, and 10 feet with the tail. J 



Chemical work of 'plants. — The effect of vegetation upon rock disinte- 

 gration comes from, first, the mechanical effects of penetrating roots, 

 which break up the material ; second, making courses or roads of the 

 roots along which the surface waters readily penetrate ; third, the ab- 

 straction from the earth of such materials as are available in plant 

 growth; fourth, in the formation of acids upon the decay of the plants. 



I. The mechanical work of penetrating roots scarcely need be dwelt 

 upon here. Every one is acquainted with the ]:)r3'ing off of boards from 

 houses by ivy and Virginia creeper and the lifting of sidewalks b}^ the 

 roots of trees passing beneath them. On a smaller scale this kind of work 

 loosens the earthy particles and opens incipient cracks and even sepa- 

 rates large blocks. 



II. The second point is well understood b}^ hydraulic engineers and 

 by any one, indeed, who has had occasion to build earth dams. Roots 

 left in the earth beneath the dam are almost certain to guide the water 

 along incipient breaks. It seems to be next to impossible to prevent 

 leaks in ground containing such roots, and for this reason careful engi- 

 neers always have such grounds cleaned of stumps and roots before fills 

 are made. 



III. Besides abstracting a large part of the plant's food directly from 

 the soil, the roots of plants are able to attack and dissolve the rocks them- 



* I know of no determination of the carbonic acid exhaled by ants. Moleschott's researches on 

 the production of carbonic acid by animals relate only to frogs. His results may or may not be 

 applicable to ants. Comptes Rendus, xli, 1855, pp. 3G3. 456, 961. 



f James E. Mills is the only person who, so tar as I am aware, has called attention to the geo- 

 logical work of ants (.\mer. Geologist, June, 1889, p. 351). It is spoken of by Henry Drummond 

 (Good Words, May, 1885, p. 303), but only the mechanical importance of their work is suggested. 



t Travels in Brazil. George Gardner. London, 1845, pp. 398, 399. 



