NO. 2226. DISTRIBUTION OF ENTOMOSTRACA—DODDS. 67 



rises to 90° F. in small bodies of water and probably at times to 100° F. 

 Upper figure, plate 13, illustrates a typical lake of the plains. 



A description of the lakes in the mountain region is not so simple 

 a matter, and, in view of the fact that the greatest interest of the 

 present studies centers in the alpine fauna, must be given in greater 

 detail. 



At their western border the plains pass, for the most part, abruptly 

 into the mountains, so that the first rank of foothills often rises 

 within a distance of a mile or two from one to three thousand feet 

 above the plains. In sharp contrast to the topography of the plains, 

 the relief in the mountains is great and the streams run in narrow 

 valleys a thousand or more feet in depth. The highest part of the 

 mountains, the Continental Divide, crosses the State from north to 

 south, in much of its course being from 11,000 to 13,000 feet in 

 elevation, with peaks rising to 14,000 feet. 



In the mountain region west of Boulder there are very many small 

 lakes, from 106 of which I have made collections. Inasmuch as nearly 

 all of these lakes are of glacial origin, some account of glaciation and 

 glacial topography is necessary. Though no part of Colorado was 

 covered by the continental glacier there were in the higher moim- 

 tains at the same time very many glaciers, only a few remnants of 

 which remain. In the Tolland region these extended downward 

 from the Continental Divide to an elevation of 8,000 to 9,000 feet, 

 reaching eastward in the valleys as tongues of ice a distance of 5 to 

 10 miles. On the western slope glaciation in this region was less 

 extensive. 



The cirques in which these glaciers had their origins, just below 

 the divide, are now one of the conspicuous topographic features of 

 the higher parts of the mountains. Each cirque, separated from 

 those adjacent to it by high, narrow ridges extending outward from 

 the divide, is shut in on three sides by steep rock walls a thousand 

 feet or more in height. In nearly every cirque is a lake fed by water 

 from the huge snow banks, some of them perennial, which accumu- 

 late on its walls in the winter. 



These lakes in the cirques at the heads of streams are the highest 

 bodies of water to be considered and present the most extreme alpine 

 coi ditioiiS. They lie just at or above the upper limits of timber, 

 nearly aU of them at elevations above 11,000 feet, the highest one 

 studied being Ice Lake at 12,188 feet. I have designated these as 

 alpine lakes, and those at lower elevations in the mountains will 

 be spoken of as montane, the division, as will be explained later, 

 beirg made on the basis of faunal as weU as physical peculiarities. 

 I have made collections from 24 alpine lakes. 



None of these is more than one-fourth mile in length, and, while 

 they are considered locally to be very deep, I suspect that few, if any 

 of them, are over 50 feet, though as boats are not available it can 



