40 Joty—An Estimate of the Geological Age of the Earth. 
It will conduce to clearness to summarise here a statement of the correc- 
tions. 
Duration, in millions of years, of 
Basis of Calculation. Geological Time since Conden- 
sation of Water on the Globe. 
. Ifno free acid existed in the primeval atmosphere and 
the total river supply of sodium be assumed as de- 
rived, at a uniform rate, from the rocks, : j 99-4 
_ 
2. As 1; but assuming that free acid in the original 
atmosphere, to the extent calculated from the 
chlorine now in the sea, less that subsequently 
supplied by rivers, attacked the original rocks, and 
became neutralised in negligible time, . 86:9 
3. As 2; but allowing for a period of acid-denudation 
at arate 5 times the average rate of present sub- 
aerial denudation, 87 
4. As 8; and ‘assuming 10 per cent. of the sodium 
chloride in the river discharge to be derived from 
89°3 
the ocean, 
Of these estimates number 4 is based on the most complete estimate of 
probabilities. 
We have still to consider known or possible sources of disturbance which, with 
our present knowledge, hardly admit of numerical approximation. We hope to 
show, however, that the resultant of their often opposed effects was probably 
subtractive, and must be included in an allowance of about 10 per cent. 
IV.—The Saline Deposits. 
Very considerable deposits of Rock Salt, &c., occur among bedded rocks of various 
ages—even those of early Palaeozoic times—as the Salt Range of the Punjaub, 
which dates back to Cambrian age.* That these in the aggregate represent 
a very considerable mass of sodium chloride cannot be doubted, although their 
local character and limited extent reduces this amount probably to but a small 
fraction of that contained in the sea. 
It is believed by some geologists that such beds were derived from the sea by 
enclosure of bays, &c., and evaporation to dryness of the land-locked 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 rainfall. In the hypothetical bays a bar must occur 
* Sir A. Geikie’s ‘“‘ Text-Book of Geology,” 3rd ed., p. 737. 
