ORIGIN OF LAKE BASINS. 29 



must have resulted in the fonnatiou of many Likes ; l)ut examples of water 

 bodies of tliis type are rare at the })resent time, principally for the reason 

 that the deformation of the earth's crust usually goes on slowly and the 

 depressions formed are di-ained or filled with sediments as rapidly as 

 they are formed. 



The best examples on this continent of basins formed by the upheaval 

 of mountains around them, occur in the great area of interior drainage 

 between the Rock}- mountains and the Sierra Nevada. The majority of 

 the minor basins in this region, however, are due to secondary causes, 

 but the vast seas, such as lakes Bonneville and Lahontan, \\hich for- 

 merly existed there, occupied basins of the character here referred to. 



The Laurentian lakes are held in basins produced in part by crustal 

 movements affecting large areas, and in part by conditions resulting from 

 other causes. Basins are also produced by less extensive elevations and 

 depressions of the earth's crust. The corrugation of a region, owing to 

 the formation of a series of approximately parallel folds, known as anti- 

 clinals and synclinals, as in the case of the Appalachian mountains, must 

 frequently produce basins in which water would be retained, were the 

 process allowed to go on without some counteracting agency ; but here 

 again, the movements are usually so slow that, especially in humid regions, 

 the depressions produced are destroyed as rapidly as they are formed. 

 While lakes in synclinal basins might be expected to be of common occur- 

 rence, they are in reality so rare that, so far as I am aware, none of the 

 tens of thousands of the lakes of America can be pointed to as examples. 



There is still another variety of earth movements in many instances 

 less gradual tlian those referred to above, to which many lakes owe their 

 origin. 



Fractures in the earth's crust occur in disturbed regions and may be 

 scores or even hundreds of miles in lengfth. The edg-es of the broken 

 strata on one side of a fracture are sometimes elevated, or those on the 

 opposite side depressed, thus forming what is known as a " fault." The 

 growth of faults sometimes goes on so slowly that no 2)ronounced changes 

 in topography result, for the reason that the rocks on the upheaved side 

 of the fracture are eroded away as fast as they are raised. At other 

 times, however, mountain ranges are produced, in which the strata are 

 inclined away from the steep, broken face overlooking the line of fracture. 

 In regions where such mountain ranges have l)een foi-med with comparative 

 rapidity and where denuding agencies are weak, great disturbamcs in the 

 drainage result, and "fault l)asins" are connnon. Numerous basins of 



