NO. 2226. 



DISTRIBUTION OF ENT0M0STRACA—D0DD8. 



69 



The lakes which I have designated as montane are of two main 

 types: (1) Rock-basin lakes on the upper courses of the streams, 

 just below the cirques; (2) morainal lakes inclosed by the morainal 

 ridges in the valleys and on the lower hillsides These lakes are 

 similar to each other and different from the alpine lakes in that they 

 are surrounded by forests (pine, fir, and spruce), ar;d that there is 

 an abundance of other vegetation growing about them and at the 

 water's edge, as a result of which there may be much plant debris 

 and considerable silt on the bottoms of the lakes. There may also be 

 a considerable growth of algae and other aquatic plants. These 

 features, together with the longer season and warmer temperatures. 



Fig. 8.— Curves showing the approximate distribution of .surface temperature throughout 



THE year in three SORTS OF LAKES. 



set them off distinctly from the lakes of the preceding group, and 

 we shall see later that the fauna is also quite distinct. 



The rock-basin lakes on the upper stream courses are about the 

 same size, on the average, as the alpine lakes in the cirques above 

 but probably have less depth. There may be one or more of these 

 lakes on a stream, which between the lakes usually descends over a 

 steep terrace often several hundred feet high. (See upper fig., pi. 14.) 

 The temperature conditions here are somewhat less rigorous than in 

 the alpine lakes. The cold water flowing out from the higher lakes 

 beccr es somewhat warmed, and temperatures from 55° to 60° F. 

 are jonmion, the latter figure about corresponding to 52° in the higher 

 lakes. The time of breaking up of the ice in the spring is about a 

 month earlier than in the higher lakes. Though most of the lakes 

 of this type are above 10,500 feet, and there are none to correspond 

 to them at lower elevations, it seems, probable that, if lakes were 

 present on the stream courses lower down, the difference in elevation 



