6o Types of Aquatic Environment 



innumerable, but in the great belts of com and cotton, 

 and on the plains to the westward, they are few and 

 far between. They are abundant in the regions of more 

 recent volcanic disturbance in our western mountains, 

 but are practically absent from the geologically older 

 Appalachian hills. They lie in the depressions between 

 the recently uplifted lava blocks of southern Oregon. 

 They occur also in the craters of extinct volcanoes. 

 They are apt to be most picturesque when their setting 

 is in the midst of mountains. There are probably no 

 more beautiful lakes in the world than some of those in 

 the West, such as Lake Tahoe (altitude 6200 ft.) on the 

 California-Nevada boundary, and Lake Chelan in the 

 state of Washington*, to say nothing of the Coeur 

 d'Alene in Idaho and Lake Louise in British Columbia. 

 Eastward the famous lake regions that attract most 

 visitors are those of the mountains of New York and 

 New England, those of the woodlands of Michigan and 

 Wisconsin and those of the vast areas of rocks and water 

 in Canada. 



Lakes are temporary phenomena from the geologists 

 point of view. No sooner are their basins formed than 

 the work of their destruction begins. Water is the 

 agent of it, gravity the force employed, and erosion 

 the chief method. Consequently, other things being 

 equal, the processes of destruction go on most rapidly 

 in regions of abimdant rainfall. Inwash of silt from 

 surrounding slopes tends to fill up their basins. The 

 most extensive filling is about the mouths of inflowing 

 streams, where mud flats form, and extend in Deltas 

 out into the lake. These deltas are the exposed sum- 

 mits of great mounds of silt that spread out broadly 

 tindemeath the water on the lake floor. At the shore- 

 lines these deposits are loosened by the frosts of winter, 



*Descriptions of these two lakes will be found in Russell' s Lakes of North 

 America. 



