AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 265 



limited extent reduce this amount probably to but a .small fraction of 

 that contained in the sea. 



It is believed b}" some geologists that such beds were deriA'ed from 

 the sea by inclosure of bays, etc. , and evaporation to dryness of the 

 landlocked water. There are, however, many arguments for believing 

 that such occurrences must have been rare, and for the support of the 

 opposed view that they represent the deposits of areas deficient in rain- 

 fall. In the hypothetical bays a bar must occur b}" crust elevation in 

 such a position as to cut this off and imprison the water. This must 

 be effected sufficiently rapidly to overtake the tidal scour which proves 

 the more effective in preserving the channel of communication with 

 the sea the more narrow the channel becomes. But this is not all. 

 The landlocked bay is very unlikely to contain the salts adequate to 

 account for the thickness of the beds and periodic variations formed 

 in the deposit. Fresh influx of sea water must be therefore obtained, 

 or the advocates of this view must now join hands with the advocates 

 of the rival theory and claun "rainless" conditions to finish the depo- 

 sition of salts in the inclosed area. 



In the best example known of a salt lake of marine origin (the Cas- 

 pian Sea) the waters as a whole are not so saline as those of the 

 Mediterranean. Ultimately evaporation must, however, lead to exten- 

 sive salt deposits in this sea. But these will only to a fractional extent 

 be derived from the sea. "Salt lakes of oceanic origin are compara- 

 tively few in number;"^ and we see by this example of one that it by 

 no means follows that the salt deposits so derived ever formed part of 

 the original ocean save to a small extent. 



The ordinar}" history of the rock-salt deposit is undoubtedW that of 

 the majority of the present salt lakes of the world. The formation of 

 such deposits is, indeed, inevitable wherever a depression and rainfall 

 below the amount required to flood the depression to repletion exist. 

 The inflow of the rain to such an island basin, indeed, diminishes as the 

 basin fills up, and the evaporation correspondingly increases. When 

 the latter lialances the rain supply, the waters continue to grow in 

 salinity, till a salt lake — derived from denudation within the water- 

 shed — is formed. Such have been formed in all ages at periods even 

 older than the Silurian, Thus "some of the more important beds of 

 America belong to Upper Silurian, Carboniferous, Triassic, or Ter- 

 tiary ages, and vary in thickness from a mere film to upward of 1,200 

 feet," and are ascriljed l)y Mr. G. P. Merrill to the evaporation of 

 water in inclosed lakes and seas.^ 



So far as our present theory is concerned, there is no need to take 

 these into consideration; for, in point of fact, they are already consid- 



^Sir A. Geikie's Text-Book of Geology, third edition, p. 410. 



^G. P. Merrill's Treatise on Rocks and Rock Weathering and Soils, p. 120, 1897. 



