264 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



estimated at 42 X 10' tons per annum. Hence the river suppl}- is now 

 reduced to 15,307x10* tons. The quotient of 13,655 XlO'^ by 

 15,307x10* is 89.2 XlO^ To this number of years may be added the 

 decimal O.lXlO® years as the period approximately required to effect 

 the denudation of the primitive rocks to the extent of fixintr the free 

 hydrochloric acid, giving-, finally, as the estimate of the duration of 

 denudation, 89.3x10' years. 



It must not be understood from the foreo-oing that we claim a degree 

 of accuracy for our estimate approximating to so small a time interval 

 as 100,000 years. The period is onlv taken into account as arising 

 from our figures. It will be seen later on that a far larger margin of 

 error is of necessit}' assumed. 



It will conduce to clearness to sununarize here a statement of the 

 corrections: 



Basis of culfiilatiim. 



1. If no free acid existed in the primeval atmosphere and the total river supply of 



sodium be assumed as derived, at a uniform rate, from the rocks 



2. As 1; but assuming that free acid in the original atmosphere, to the extent calcu- 



lated from the chlorine now in the sea, less that subsequently supplied by rivers, 

 attacked the original rocks and became neutralized in negligible time 



3. As 2; but allowing for a period of acid denudation at a rate five times the average 



rate of present subaerial denudation 



4. As 3; and assuming 10 per cent of the sodium chloride in the river discharge to be 



derived from the ocean 



Duration, in 

 millions of 

 years, of geo- 

 liigicnl time 

 since con- 

 tlensiition of 

 water I III the 

 globe. 



86.9 



87 



89.3 



Of these estimates, No. 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, hardl v admit of numerical approx- 

 imation. 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. 



ly. — THE SALINE DEPOSITS. 



Very considerable deposits of rock salt, etc., occur among bedded 

 rocks of various ages— even those of early Paleozoic times— as the 

 salt range of. the Punjab, which dates back to Cambrian age.' 

 That these in the aggregate represent a very considerable mass of 

 sodium chloride can not be doubted, although their local character and 



^Sir A. Geikie's Text-Book of Geology, third edition, p. 737. 



