ii8 PRINCIPLES OF STRATIGRAPHY 



2. Volcanic barriers 



a. Volcanoes. 



b. Lava flows. 



3. Chemical : Tufa barrier basin. 



4. Ice barrier basin. 



5. Organically built barrier. 



a. Coral reef barrier. 



b. Vegetal growths. 



6. Detrital barrier basin. 



a. Marine and lacustrine (barrier beach). 



b. Fluviatile, or river-built : Fan-delta barriers ; mar- 



ginal barriers (drift-wood barriers, etc.). 



c. Glacial, or ice-built : Drift and morainal barriers. 



d. Atmospherically built. 



(i) Land-slip barrier. 

 (2) Dune barrier. 



e. Artificial (built by organisms from foreign sub- 



stances), 

 (i) Beaver dams. 

 (2) Man-made dams. 



While pure types of these basins probably exist, the majority 

 of lake basins are of a more complex order, falling under more 

 than one class. 



A. Defonnational or tectonic basins. 



These are due to faulting, folding, or warping, so as to produce 

 a closed depression, i. Fault basins are not uncommon, the best 

 examples being Albert and Summer Lakes of Oregon (Fig. 19), 

 and perhaps Lakes Tanganyika and Nyassa of Africa (Fig. 20). 

 2. Lakes due only to folding or the formation of mountain troughs 

 are rare, but those in which folding takes a part are not uncommon 

 in young mountain regions. Here belong, in part at least, Lake 

 Baikal of Asia. Lake Nicaragua, and the western end of Lake Supe- 

 rior in Central and North America, respectively. 3. Basins due 

 wholly to warping are unknown, but Lake Ontario is a warped river 

 valley which is partly closed by drift deposits. 4. Great basins are 

 produced when mountains arise all around a less disturbed area by 

 combined folding, faulting, and warping, and thus leave this area 

 surrounded by a rim, and in condition to become a lake. The size 

 of this lake, when below the maximum possible for the lowest 



