40 



JoLY — A)i Estimate of the Geological Age of the Earth. 



It will conduce to clearness to summarise here a statement of the correc- 

 tions. 



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 

 * Sii- A. Geikie's " Text-Book of Geology," 3rd ed., p. 737. 



