JoLY — An Estimate of the Geological Age of the Earth. 41 



by 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 land-locked 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 claim "rainless" conditions to finish tlie depo- 

 sition of salts in the enclosed area. 



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

 Sea), the waters, as a whole, are not so saline as those of the Mediterra- 

 nean. Ultimately evaporation must, however, lead to extensive salt deposits 

 in this sea. But these will only to a fractional extent be derived from the 

 sea. "Salt lakes of oceanic origin are comparatively 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 ordinary history of the Rock-Salt deposit is undoubtedly 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 from the rain to such an 

 inland basin indeed diminishes as the basin fills up, and the evaporation corres- 

 pondingly increases. When the latter balances 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 Amei-ica belong to 

 Upper Silurian, Carboniferous, Triassic, or Tertiary ages, and vary in thickness 

 from a mere film to upwards of 1200 feet," and are ascribed, by Mr. C P. Merrill, 

 to the evaporation of water in inclosed lakes and seas.f 



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 considered in the estimate 

 of river supply to the ocean furnished by Sir J. Murray, which takes into account 

 only that falling directly into the ocean. The drainage of the "rainless" regions 

 of the Earth — regions where the rainfall is less usually than 1 1 inches per annum, 

 and which do not drain into the sea — is excluded. As in the present so in the 

 past, we conclude that such regions existed scattered over the land surface at 

 various periods of the Earth's history ; and we find no better confirmation of the 

 preservation of present climatic conditions than exists in these tell-tale beds of 



* Sir A. Geikie's "Text-Book of Geology," 3rd ed., p. 410. 



f G. P. Merrill's "Treatise ou Kocks and Eock Weathering and Soils," p. 120, 1897. 



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