492 Professor John Joly [Feb. 24, 



extremes do not sensibly affect solvent denudation. Atmospheric 

 circulation, being largely conditioned by the earth's rotation and the 

 distribution of solar heat, cannot have varied to any effective extent. 



(b) That a considerable percentage of the existing land area 

 being rainless, changes in continental area cannot greatly affect the 

 amount of denudation : the belt undergoing denudation being merely 

 displaced outwards or inwards. The evidence derived from palaeo- 

 graphy and from the extent of sedimentary deposits in all ages shows 

 that the present land area is not greatly different from the past mean 

 area. 



(c) That the minor factors affecting solvent and detrital denuda- 

 tion being very many and of very different character are unlikely to 

 combine at any time, and for any long period, in one direction, so as 

 to create a considerable departure from the mean. 



Time will not permit a discussion of these statements. I shall 

 refer but briefly to the methods by which the statistics of solvent 

 and detrital denudation are used to afford the age of the ocean. 



(1) The chemistry of the ocean and of the rocks is the key to 

 our position. As the result of a comparative study of the primary 

 or igneous rocks and the secondary or sedimentary rocks, we find 

 that, say, n grams of sodium are shed into the ocean for each ton 

 of igneous rock converted into sedimentary rock, and in the ocean 

 we find N grams of sodium. The total denudation over geological 

 time has, therefore, been N/w expressed in tons of denuded igneous 

 rock. Our study also tells us the average total loss attending the 

 conversion of the primary rock into sediment, and so we get the total 

 of the secondary rocks in tons. We now go to the principal rivers 

 of the world, and availing ourselves of estimates which have been 

 made of the amounts of sediment — i.e. of secondary rock material — 

 which they transport from the land in a year, we calculate the 

 number of years it would take to lay down in the ocean the great 

 mass of sediment generated in the past ages. After certain allow- 

 ances this comes out at about 100 million years. 



(2) Again the total of oceanic sodium may give us the age in 

 another and more direct way. We know that by far the greater part 

 of this sodium was carried into it by the rivers during geological 

 time. We turn to the analyses of river water and estimate the total 

 annual supply of this element to the ocean. Dividing the latter 

 into the former and making certain allowances we find an age which 

 is about 100 million years. 



(3) A third and more difficult method is independent of our 

 knowledge of chemical denudation. We estimate the maximum 

 thickness of the integral sedimentary deposits, and knowing the 

 burthen of sediment conveyed per annum by the rivers, we estimate 

 the maximum thickness of deposit annually derived from the same ; 

 we divide the latter into the former and find an age which, again, is 

 about 100 million years. 



