1906.] Wheeler, The Ants of the Grand Canon. 331 



down through several different belts of vegetation, the comparatively 

 limited number of woody desert plants rendered the journey some- 

 what disappointing from the standpoint of the main object of our 

 trip. For the first 2,600 feet of the descent the trees continue, but 

 from that point to the river the slopes are treeless and the vegetation 

 of a desert character. One of the most striking features is extensive 

 fields of a rosaceous shrub, Coleogyne ramosissima, which extends in 

 an almost pure growth over the canyon terraces at an elevation of 

 about 3,600 feet in a soil seemingly well supplied with lime. There 

 is a notable absence of many shrubs which would be present in the open 

 desert at the elevations afforded by the lower parts of the canyon 

 and which have a seemingly good route for extension up the canyon 

 from the Mojave desert. The absence of these plants is presumably 

 connected with the narrowness of the canyon, which besides producing 

 abnormal air currents and temperature conditions is responsible for 

 a rainfall greater than would occur at the same elevations in the 

 open desert. A cloud sheet precipitating rain on the 7,000-foot 

 plateau through which the canyon passes would presumably continue 

 to precipitate as it drifted across the canyon, whereas if it should 

 drift off the plateau over a desert of low elevation its precipitation 

 would be greatly lessened or would cease altogether." 



In walking down the Bright Angel Trail from the beautiful Koho- 

 nino Forest to the river, one can hardly fail to be impressed by the 

 dryness of the walls of the vast chasm, the steadily increasing temper- 

 ature, and the wonderful, omnipresent evidences of erosion caused 

 by the torrential floods which at certain seasons must rush down the 

 precipitous inclines. As steepness and dryness of slope are always 

 unfavorable to ant-life, we are not surprised to find their colonies 

 few and widely scattered, of small size and showing other evidences 

 of adverse conditions, especially in unusual forms of nest architecture. 

 At an altitude of 3670 feet on the trail, at a place called Indian 

 Garden, there is a beautiful stream overgrown by low willows and hence 

 known as Willow Creek, where the much less precipitous and more 

 humid soil favors the development of colonies. On the adjoining 

 Angel Plateau (3700 feet) which is covered with the Coleogyne ramo- 

 sissima mentioned and figured by Coville and McDougal, the level is 

 also more favorable, but here the soil is very dry and stony and 

 actually much poorer in species than the deserts of the Lower 

 Colorado. Finally, in the granite strata which extend to an elevation 

 of 1000 or 1200 feet above the river and constitute the hard, sombre 

 walls of the "inner canon," there are no ants except where the sand 



