378 _ EEPORT— 1900. 



the volume of the ocean, its mass is 1-539 x 10^^ tonnes. This would raise 

 our result by nearly 6 per cent. 



We sum up the results of our inquiry, then, in the statement that the 

 probable age of the earth, estimated from solvent denudation, is between 

 ninety and one hundred millions of years. 



Sock-salt Deposits from the Ocean negligible. 



The amount of chloride of sodium in the ocean is sufficient to cover 

 the entire land area with a layer of solid salt 122 metres deep. Compared 

 with so great a mass the rock-salt deposits on the land are negligible. 

 They are, moreover, only in part derived from the ocean, the circumstances 

 leading to abstraction of salt from the ocean and its retention upon the 

 land being exceptional. Likeness in chemical composition is no proof 

 that bedded salts were derived from the ocean. Thus the proportions of 

 salts in the Great Salt Lake are much the same as in the sea.^ 



So far as these deposits are derived from the denudation of ' rainless ' 

 regions, and are being gradually conveyed by rivers to the ocean, they 

 constitute part of the normal supply of sodium to the sea. 



Unifonnity of sub-aerial Denudation. 



The uniformitarianism involved in the present mode of calculating the 

 age of the earth is broadly restricted to the approximate persistence 

 throughout the past of the present sub-aerial association of water and 

 rock. 



The rate of solution of the rock -forming silicates is so slow that the 

 abundance of the solvent or its rate of renewal is a relatively unimportant 

 factor compared with the surface area exposed. Thus within certain 

 limits climate will not seriously affect the question. 



The existence of a rainless area, amounting to one-fifth the land 

 surface, subject to extreme conditions of dryness secures that subsidence 

 or elevation of land does not necessarily involve corresponding changes in 

 the area of active solvent denudation, In the first case the more active 

 margin moves inwards, in the second case it moves outwards. 



The argument that the surface materials of the land areas must have 

 been growing poorer in alkalies throughout geological time is met by the 

 fact of the less resistent nature of sedimentary rocks, involving soils richer 

 in soluble constituents. These are, in fact, more rapidly formed and 

 removed. Observation shows that on comparing soils from the most 

 diverse kinds of rocks the rapidly concentrated soils of limestones, or 

 those derived from sandstones, very generally exceed in percentage of 

 alkalies soils derived from igneous rocks.- It is within the soils that the 

 chief work of solvent denudation is accomplished. 



Confirmation in the Soda-content of the Igneous and Sedimentary HocJcs. 



We assume in our present argument necessarily that the sedimentary 

 rocks were derived from the igneous in the process of denudation, a certain 

 loss, representing matter gone into solution, occurring. Taking this loss 

 into account, we may recover from the estimated mass of the siliceous 

 detrital sedimentaries on the earth's surface the approximate total mass 

 of the parent igneous rock. This represents a certain mass of soda 



' Nature, December 28, 1899, p. 204. 



-' E. G. Merrill, Rocks, Rocli-weatherbuj, and Soils (Macmillan), 1897, pp. 305, 

 306, 358, and 359. 



