Joty—An Estimate of the Geological Age of the Earth. 4] 
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 the depo- 
sition of salts in the enclosed area. 
In the best example known of a salt lake of marime 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 America 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. 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 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 Karth—regions where the rainfall is less usually than 11 inches per annum, 
and which do not drain into the sea—is excluded. Asin the present so in the 
past, we conclude that such regions existed scattered over the land surface at 
various periods of the Harth’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,” 8rd ed., p. 410. 
+ G. P. Merrill’s ‘‘ Treatise on Rocks and Rock Weathering and Soils,” p. 120, 1897, 
Jal 
