AMERICAN UYMENOPTERA. 207 



facts fail accurately to satisfy the inquiry as to the age of the hills, 

 they are interesting as showing to some extent this capacity for 

 work. I have good reason to believe that some of the hills are at 

 least thirty years old. They probably do not grow after having 

 reached a certain bulk. 



Itiiildiiig by CoinpaNN.— One other point in the architecture of 

 the hills engaged luy attention. The mounds were observed very 

 generally to have the longest face of the cone toward the west. Is 

 this merely the result of gravity and the wash of the rains, since the 

 mountain slopes toward the west? Or, is it a characteristic habit of 

 the hill, fixed by the purpose of the ant? I was led into this inquiry 

 by Ruber's statement concerning tlie Yellow ants (Formica flava), of 

 the Alps. Their habitations there take an oblong and almost regular 

 shape. They lie in a direction east and west. Their summit and the 

 greatest slope always faces the east; but they incline also on the oppo- 

 site side. This peculiarity, which was verified upon thousands of ant 

 hills is not preserved in the plains, probably because of exposure to 

 derangement by men and the lower animals. 



Iluber's description of the position of the yellow ant lulls corre- 

 sponds with the fallow ant hills at ('amp Kiddle, except that the 

 position of the greatest slope is reversed; i. e., it is toward the west 

 instead of the east. Jiy the greatest slope I mean the longest slope, 

 as I suppose Mr. Iluber also n)eans. Of a large number of hills 

 examined and recorded by myself, 94 per cent, had the long slope 

 westward, the steepest slope eastward. Of the remainder, more than 

 half had the long slope toward the southwest. To determine, if possi- 

 ble, whether a change in their general form would follow a change in 

 the slope of the land, I noted particularly the position of a few hills 

 upon a ravine in the face of the mountain whose sides sloped nearly 

 north and soutti ; also of those upon level ground, and of hills built 

 upon the sides of a deep cutting made by ore miners for purposes of 

 drainage. The result left my mind in doubt. Several cases were 

 noted in whicli the general tendency had plainly prevailed over such 

 influence as a different slope and gravity might have exerted. On the 

 other hand some hills were found the longest slopes of which appeared 

 to be carried from the general direction by a corresponding slope of 

 the land* The impression left upon my mind is that the habit of the 



* Extract from note book ; " 1 hill clearly determined by the slope of the gnlly 

 to face (longest slope), N. W. 2 hills long slope S. W. with tendency to W.; 

 apparent struggle to face W., in one case nearly successful. 1 hill B. E. appa- 



