200 DYNAIVnCAL GEOLOGY. 



carry off waters that excavate a course through the alhivium to neighboring- 

 depressions and thus make a more permanent lake. 



Salton Lake, in the southeastern corner of California, 130 miles long by 40 in greatest, 

 breadth, resulted, in July, 1891, from the overflow of the Colorado River on the west side 

 below Yuma. The alluvial region either side of the river between Yuma and the head of 

 the California Gulf, 50 miles distant, had been gradually built up by river depositions, 

 until a large depression, Coahuila valley, now 300 feet below the sea where deepest, had 

 been separated from the head of the gulf and left as a nearly dry desert basin. The 

 flooded waters, pressing westward along the westward course of New River, succeeded in 

 passing the low summit level, and then quickly excavated a way to the depression and filled 

 it. Owing to the hot and extremely dry climate, evaporation will sooner or later make it 

 an empty lake-basin, as it was essentially before. The river at Yuma is about 150 feet 

 above the gulf. Nearly 100 miles north of the Salton Lake is Death Valley, 225 feet 

 below the sea, also situated in the line of the California Gulf. 



W. P. Blake traveled over the desert in 1853 (Geol. Beconn. Cal., 4to, 1858), and 

 describes it as having, in general, a barren, clayey surface, with some saline springs along, 

 the margin and elsewhere. On the rocks of the shore, there was a thick horizontal belt 

 of whitish calcareous tufa about 15 feet (where examined) above the level of the desert, 

 indicating a former water level, and proving that the desert was the dry basin of a former 

 lake. He found that the Indians had a tradition of the existence of a great lake filled 

 with fish ; of its slowly drying up, and of a sudden return of the waters, when many 

 were drowned. The recent event is evidently not the only one of the kind in the region. 



Other lake-basins have been made by glacier-damming (page 238), and 

 possibly, as above stated, by glacier-excavation. Still others of small size 

 are a result of underminings, especially through removals of clay-beds by 

 pressure ; others have come from a damming against the sea by beach-made 

 deposits (page 224), converting inlets into sea-border basins. 



The large lakes of the world, after the Caspian, are the Great Lakes of North America,. 

 Lake Baikal in Asia, and Lake Victoria in east Central Africa. The map. Fig. 185, gives 

 the positions of the American Great Lakes, and the line of greatest depth, the deepest 

 point in each, and also the limits of the several drainage areas. Lake Superior has an. 

 area of 31,200 square miles ; Huron, of 23,800 square miles ; Michigan, of 22,450 ; Erie, 

 of 9960 ; Ontario, of 7240. The heights of the water above mean sea level are : Lake 

 Superior, 601-8'; Huron and Michigan, 581-3' ; Erie, 572-9' ; Ontario, 246-6'. The section. 

 Fig. 186, shows their depths, and the extension below the sea level. (Schermerhorn,. 

 Amer. Jour. Set, 1887.) Lake Champlain is 402' deep, 300' of it below the sea level. 



The heights of some other American lakes are as follows : Winnipeg, 630' ; Lake of 

 the Woods, 1640' ; Great Salt Lake, 4218' ; Yellowstone Lake, 7788' ; Shoshone Lake, 

 7870'; Great Bear Lake, 5931'. 



The Caspian has an area of 170,000 square miles, a depth of 500', and descends 90' 

 below the sea level. Lake Baikal in Siberia (really among the high Altai Mountains 

 and near Central Asia) is 397 miles long, 54 miles in maximum width, and has a 

 depth in some parts of over 300 fathoms, nearly 500' of which is below the sea level. The 

 great African Lake, Victoria, has an area of about 27,000 square miles, and is 3300 feet 

 above the sea level. The Assat Lake lies in a depression east of Abyssinia, 600' below the 

 level of the Red Sea, and is salt. 



Rivers tend to obliterate the lakes along them in two ways : by the depo- 

 sition of detritus in their still waters and along their borders, and by erosion 



