WATER AS A MECHANICAL AGENT. 199 



ward. They either belong to the continental river systems, and are river- 

 system lakes, or they are confined or imprisoned lakes. 



1. Imprisoned lakes fail of the third method of discharge and are rela- 

 tively few in number. They lie in basins or depressions that had been made 

 or left in the surface by orographic movements, or had become cut off in 

 some way from a river system, or possibly where the rocks, having little 

 firmness, had been excavated by former glacier action. Some of small size 

 occupy craters of extinct volcanoes. The Caspian Sea, Dead Sea, Great Salt 

 Lake of Utah, and lakes in the Great Basin, are examples. They are most 

 likely to exist under dry climates, where the supply of water is small and 

 evaporation large ; and they may vary from dry beds to lakes in the chang- 

 ing climates of the year. Some imprisoned lakes have had surficial dis- 

 charge in former eras. A confined river system usually supplies the waters, 

 and carries in what can be gathered from the rocks around by solution and 

 otherwise, as explained on page 118. 



2. Lakes connected with river systems occur in all climates and latitudes, 

 and at various heights. They are often situated in lines or clusters over 

 the nearly level summit region of a Continental Interior, where the great 

 rivers are gathering waters and deciding on their courses. They sometimes 

 occupy profound depressions in the earth's crust, like the Great Lakes 

 of North America, or follow the nearly level median line of continental 

 drainage, as the Winnipeg series of British America. 



The basins may be a result of geosynclinal movements, like that of Lake 

 Superior ; or otherwise of orographic origin, as the intermontane lake basins 

 of many mountain regions ; and even a consequence of the feeblest flexures 

 of the earth's crust. They have commonly been made within the area of a 

 river system by damming with transported material. Unusual floods may 

 make barriers by local depositions ; more easily, tributaries may throw across 

 a valley dams that have a degree of permanence ; still more effectively, ice 

 may carry along gravel and sand and block the deep and narrow channel ; or 

 better, in regions of glaciers, more formidable deposits of drift may make 

 obstructions in valleys and give outlines to many lakes over nearly level 

 regions. After a period of elevation when the valleys were excavated to 

 great depths, a period of lower level may have come, in which the transport- 

 ing waters were in great force and made obstructing deposits, especially when 

 water and gravel were afforded in vast quantities for the purpose by a melt- 

 ing glacier. Lake Geneva, in Switzerland, 45 miles long and 1095 feet deep, 

 the surface 1230 feet above tide-level, is supposed to have been made in the 

 way last mentioned ; and even also, Lago Maggiore, of northern Italy, which, 

 although only three miles wide, is 2613 feet deep, with 1920 of this below the 

 sea. Another view attributes the depth of Lake Geneva to a subsidence of 

 the lake bottom since the Glacial period. 



Further, a large river in its more aged or decrepit portion may so wall 

 itself in and raise its bed by depositions either side of and along its chan- 

 nel, that every flood makes temporary lakes ; and extraordinary floods may 



