CONSTRUCTIONAL LAKE BASINS 121 



feet above sea-level, owing to artificial drainage. The lake decreases 

 to a few inches in the woods surrounding it. Although originally 

 believed to have started in a depression in the underlying stratum, 

 it is now entirely enclosed by, and owes its continuance to, a rim of 

 vegetal material, and so is typical of this class. Lakes and ponds 

 of this type due wholly to the growth of vegetation, abound in the 

 tundra of Alaska, where they occur even on hillsides, which other- 

 wise would be freely drained. (Russell-3o:/i'6'.) b. Lake basins 

 of animal or zoogenic origin are less common, but are represented 

 by coral island lagoons, when their connection with the sea ceases 

 or is reduced to a minimum. An elevation of the coral island or 

 atoll would result in complete separation of the basin from the 

 ocean, and a freshening of the water. Lakes apparently of this 

 type have been reported from the coral islands of the Pacific. 



4. Detrital (clastic) basins. These are built up by detrital 

 material, which, though it is the product of rock destruction else- 

 where, nevertheless produces constructional basins similar to the 

 volcanic and the chemical, the material for the building of which is 

 obtained from within the earth, and to the organic, the material for 

 which is obtained from the air or the water. They may also be 

 called reconstructional, since the material of old land forms is 

 reconstructed into the new. 



a. Marine detrital basins are depressions on the subcoastal 

 plain, due to irregularity of arrangement of material. On emergence 

 these will carry shallow lakes. Such are the lakes on the coastal 

 plain of Buenos Ayres. (Davis-7 rj/i'.) b. Lacustrine detrital basins 

 are those left in the irregular detrital deposits on the floors of large 

 extinct lakes. Here belong many lakes remaining in the Great 

 Basins, such as the successors of Lake Lahontan and Lake Bonne- 

 ville and others, c. Fluviatile detrital basins comprise ( i ) those 

 formed on river flood plains by irregular deposition, though these 

 are always complicated by erosion, as in the case of oxbow lakes ; 

 (2) those formed by irregular deposition of deltas; and (3) those 

 formed by irregular deposition on alluvial fans. Lake Pontchar- 

 train. near the mouth of the Mississippi, is of the delta-lake type. 

 It has a surface of twenty by forty miles, but a depth of only 2y 

 feet. Others are found on the Mississippi delta, the deltas of the 

 Rhine, and elsewhere. (Davis-7.) Oxbows and other flood-plain 

 lakes occur on most large rivers, while delta-fan basins, though 

 small, are not uncommon. A combination of flood plain and delta 

 basin is seen in the lakes of the great alluvial plains of China, 

 d. Morainal and drift kettle holes are common in the region aflfected 

 by the Pleistocenic ice sheet. They are mostly small, though often 



