ANT STRUCTURES 465 



certain physical conditions, such as the amount and distribution of the 

 rains, the character of the soil, the area over which the necessary plants 

 or food can be obtained, etcetera. Of course, the mounds are of different 

 sizes according to their ages; but considering only the largest and oldest 

 ones made by a single species, and foimd in various different localities, it 

 is noteworthy that there is a great difference in the sizes of the largest of 

 them. Just what determines this variation I can not say positively, but 

 the influences referred to above — that is, rainfall, character of soil, and 

 vegetation — naturally suggest themselves as possible influences. 



Bates has the following in regard to the mounds made by the saubas 

 in the vicinity of Para :^^ 



"In our first walks we were puzzled to account for large mounds of earth, 

 of a different colour from the surrounding soil, which were thrown up in the 

 plantations and woods. Some of them were very extensive, being forty yards 

 in circumference, but not more than two feet in height. We soon ascertained 

 that these were the work of the saubas, being the outworks, or domes, which 

 overlie and protect the entrances to their vest subterranean galleries. On 

 close examination, I found the earth of which they are composed to consist of 

 very minute granules, agglomerated without cement, and forming many rows 

 of little ridges and turrets. The difference in colour from the superficial soil 

 of the vicinity is owing to their being formed of the undersoil, brought up 

 from a considerable depth. It is very rarely that the ants are seen at work 

 on these mounds ; the entrances seem to be generally closed ; only now and 

 then, when some particular work is going on, are the galleries opened." 



Nowhere do I remember to have seen more or larger ant-hills than 

 along Eio Utinga, in the diamond regions of the interior of the State of 

 Bahia. From the town Eiachao, down the river to the village of Pegas, 

 the examples are big and abundant. In a few places they are so close 

 together that, big and little, they appear to cover half of the ground. My 

 notes, written on the spot, say "more than half of the ground." Such 

 places, however, are exceptional. The distribution is always more or less 

 irregular — bunched apparently on account of characteristics of soil or 

 drainage, or for some other reason that does not appear. In some areas 

 of from 10 to 20 acres the ant-hills occupy from a fifth to a third of the 

 ground, while over larger tracts they take up from one-eighth to a seventh 

 of the ground. In height the mounds are often as much as 5 meters 

 high, with bases 15 or 16 meters in diameter. In the forests these mounds 

 are generally overgrown with young trees. On many of the big mounds 

 I have seen trees more than 30 centimeters in diameter. At the village 

 of Antonio Jose the people have planted pineapples upon the mounds. 



27 Naturalist on the River Amazons, 4th ed., p. 10. London, 1875. 

 XXXIII— Bull. Geol. Soc. Am., Vol. 21, 1909 



