128 PRINCIPLES OF STRATIGRAPHY 



lake behind it grew to a length of four miles before it overflowed, 

 a year after the slide. "The flood occurred at midnight, August 

 26-27, 1894. In four hours about 400,000,000 cubic yards of water 

 were discharged, cutting down the dam nearly 400 feet, flooding 

 the valley to a depth of from 100 to 170 feet, and rushing forward 

 with a velocity of 20 miles an hour. . . . Every vestige of habita- 

 tion was destroyed in villages along the Ganges above Hardwar. 

 But so well was the notice of danger given that only one man lost 

 his life, and that because he would not heed the warning." (Davis- 

 11:182, 18^.) (2) Dune sand barriers may give rise' to marshes 

 and small ponds, and more rarely to lakes of some size. These 

 will, however, rarely be deep, for seepage will keep down the 

 level. 



(e) Detrital dams built by organisms. Dams built by organisms 

 of detritus are typically represented by beaver dams, and by the 

 artificial dams built by man. Neither of these requires more than 

 a passing notice. 



Classification of Lakes as a WJiole. 



From a bionomic and stratigraphic point of view, lakes must be 

 classified irrespective of the origin of the basin in which they are 

 held. The most natural division from such a viewpoint should be 

 based on the character of the water, for, since this is the medium 

 in which life exists or sediments are deposited, its composition 

 exercises a more or less controlling influence. Waters may be 

 classed as fresh, salt or alkaline. Fresh waters do not on the aver- 

 age carry much over 0.2 part per thousand of dissolved mineral 

 matter, of which only about 2 per cent, is sodium chloride. (See 

 tables in Chapter IV.) With an increase in the percentage of 

 sodium chloride, the water becomes brackish and then salt. Alkaline 

 waters contain a large percentage of alkali carbonates. Lakes may 

 next be divided into deep lakes, i. e., those in which an abyssal as 

 well as a littoral region is distinguishable, and shallotv lakes, those 

 in which the abyssal district is wanting. The shallow lakes include 

 permanent and temporary ones, the latter sometimes being ex- 

 tremely shallow, as in the case of playa lakes, which may not 

 exceed six inches in depth. The presence or absence of an outlet 

 or effluent is also of significance, and its permanent or intermittent 

 character, when marked, must be considered. These features may 

 be tabulated as follows : 



