296 



KNOWLEDGE. 



August, 1913. 



Looking for a ready means of computing the 

 total effect produced by the multifarious denudation 

 agencies, he naturally turned to rivers, as being the 

 vehicles for the removal of detritus, and exercised 

 himself to find some constituent which, while 

 capable of sufficiently accurate measurement, should 

 be non-cyclic in character, that is to say, should 

 accumulate in the sea without possibility of return. 

 The well-known fact that the sea, and inland lakes 

 in particular, are slowly growing salter and Salter 

 led Joly to select sodium — in the combined state, of 

 course — as the element best satisfying the necessary 

 conditions. The broad outline of the method is 

 now easy to understand. The volume of the ocean 

 has first to be estimated and then, from a knowledge 

 of its average chemical composition, the total 

 quantity of sodium at present in it can be obtained. 

 This sodium must have accumulated there ever 

 since denudation began, and if, therefore, we divide 

 its total by the calculated amount of it which enters 

 the ocean annually from the rivers, an uncorrected 

 estimate of the earth's age will result. 



The amount of sodium in the sea at present is 

 known to within a few per cent., but the quantity 

 discharged per annum by all the rivers in the world 

 is not so well authenticated, being much more 

 difficult to determine. The sodium content of rivers 

 is so variable a quantity that the water of a large 

 number must be analysed before any reliable data be- 

 come available ; a difficulty, of course, which time will 

 entirely remove. The magnitude of the quantities 

 in question will be realised from the statement that 

 if all the salt at present in the ocean were con- 

 solidated into a uniform layer of rock salt upon 

 the land area of the globe the earth would be 

 covered to a depth of about four hundred feet. 



Without going into numerical details the fact 

 may be stated that the estimate of the earth's age 

 derived from the above-mentioned data amounts to 

 practically one hundred million years. 



To this number several corrections must be 

 applied. The most important of these refers to 

 what is known as " wind-borne sodium," i.e., to 

 those impalpable particles of salt which are carried 

 inland by air currents. This factor slightly disturbs 

 the non-cyclical character of the process, but only 

 requires a small correction (six to ten per cent.). 

 Relatively insignificant, too, is a correction which 

 takes into account the land deposits of sea-salts. 

 The only other conceivable corrections are inherent 

 in the method itself, or, more precisely, in the 

 assumptions which lie at the basis of the method. 

 There is, for instance, the possibility that in the 

 remote past the conditions which determined the 

 rate of denudation were far different from those 

 which prevail to-day. The sun may have been much 

 less powerful, or, on the other hand, the elements 

 may have been more active, and therefore more 

 destructive. Again, there is every reason to believe 

 that the ratio of land to sea has been far from 

 constant during the long ages of the past. But if 

 uniformity of the past and present conditions be 



assumed, as indeed seems most likely, and the other 

 uncertainties be duly allowed for, the time which 

 has elapsed since the "waters under the heavens were 

 gathered together in one place, and the dry land 

 appeared," runs to between eighty and ninety 

 million years. 



A couple of years ago the fundamental assump- 

 tion underlying Joly's method was challenged by 

 Dr. Becker, who put forward some very interesting 

 views. He pointed out that the sodium of the 

 ocean is derived mainly from igneous rocks, and 

 that when denudation began these rocks were the 

 sole constituents of the earth's crust. A very 

 different state of affairs obtains nowadays. Sedi- 

 mentary rocks are the obvious features of surface 

 strata, and the igneous rock lies buried far beneath 

 them. Does not this imply that the rate of decom- 

 position has changed considerably since the early 

 days of the earth's youth ? For, whereas the neces- 

 sary rocks were then freely exposed to denudation 

 influences, the upper layer of decomposition pro- 

 ducts now arrests decay. In other words, the rate 

 of decomposition, being dependent upon the area of 

 exposure, has diminished throughout the ages in the 

 same way as a sum of money would diminish if sub- 

 jected to a certain percentage deduction annually, 

 each deduction being calculated on the sum still 

 remaining at the beginning of the year. That is to 

 say, it has varied according to an exponential law. 

 The estimate of the earth's age, worked out on this 

 formula, gives a somewhat smaller value than by 

 Joly's method, viz., from fifty to seventy million 

 years. 



However, the two results are of the same order of 

 magnitude, preference perhaps being given at present 

 to Joly's value. Let us see if this is borne out by 

 the next computation. 



III. — Stratigraphy. 



The reverse of the preceding method would 

 obviously consist in determining the rate at which 

 the strata are being deposited ; and then, from a 

 knowledge of the maximum depth of the stratified 

 rocks, an estimate of the earth's age readily follows. 

 Sir A. Geikie, even before Lord Kelvin had assaulted 

 the geological stronghold, held that one hundred 

 million years would suffice for the history of the 

 sedimentary rocks. More recently Professor Sollas, 

 Professor Joly and others have discussed the 

 question. 



The uncertainties attached to the method, how- 

 ever, render it practically impossible to obtain any 

 reliable result. From the observations of geologists 

 in different parts the maximum thickness of the 

 deposits laid down during the various geological 

 epochs has been estimated, but the accuracy of the 

 measurements decreases with the increasing 

 antiquity of the formation concerned, until, when 

 pre-Cambrian times are reached, anything but the 

 merest guesswork becomes impossible. Even were 

 this difficulty removed, the intermediate lapses of 

 time which are represented by unconformities and 



