72 SECTIONAL ADDRESSES. 



water is returned to the ocean, but the whole of the salt it contained is with- 

 held in the land, as we see in the New York salt-wells, or the shores of 

 the Dead Sea. Landlocked seas have provided a great volume of our sedi- 

 mentary rocks ; but, even when formed on a sinking continental shelf, a 

 sandstone would take down from the ocean enough sea-water in its inter- 

 stices to leave it with "25 per cent, of sodium when the water was evaporated. 

 If we call the sodium in the sea and in the land above sea-level the ' avai lable ' 

 sodium, of this quantity 82 per cent, is in the sea, about 8^-per cent, has been 

 in the sea and is now in sedimentary rocks, and only about 10 per cent, is in 

 igneous rocks above sea-level, and has presumably never been in the sea. 

 Geologists tell us that we are at the climax of a period of maximum elevation ; 

 therefore much salt has been imprisoned in the land and the present 

 salinity of the sea must be a minimum. If we take the extreme hypothesis 

 that, at the next period of maximum denudation, a quantity equal to 

 one-sixth of the mass of the present land will be denuded without com- 

 pensation, and all its sodium dissolved in the sea, it would only increase 

 the specific gravity of sea-water from I -026 to l - 027. If it be true 

 that in 1500 million years igneous rocks have supplied eight-ninths of 

 the salt of the ocean, a mass equal to all the igneous rocks now above 

 sea-level must on the average have been erupted and dissolved every 

 200 million years, which is about the age of the Permian. Yet most of 

 our volcanic rocks are far older than the Permian. 



From Murray's calculations (1888, Scott. Oeog. Mag. iv, \,fide Mill, 1892, ' Physio- 

 graphy,' p. 191) the volume of the ocean is 14 times that of the volume of the 

 land above sea-level. Taking the specific gravity of the land as 2-6 times that of 

 the ocean, the mass of the land in tons is therefore ^.^ that of the ocean. Holmes 

 (I.e. p. 37) estimates ' igneous and other crystalline rocks ' as roughly forming a 

 quarter of the land. From descriptions of Canadian and Scandinavian geology one 

 would guess that half of these are sedimentary, but to give Professor Joly the largest 

 figure possible, we will assume that a quarter of the mass of the land is volcanic. 

 Taking Holmes's figures for the percentage of sodium, we find therefore : — 



Therefore to provide eight-ninths of the sodium in the sea would require 7 J times the 

 mass of volcanic rocks now above sea-level. Also a sixth of the whole existing land would 

 yield 3-8 per cent, of the sodium now in the sea ; and, supposing that the sodium 

 yielded were accompanied by its proportional chlorine (on which see Jeffreys, I.e. p. 71), 

 magnesium, lime salts, &c, since the existing sea of sp. gr. 1'026 contains 

 3-4 per cent, of salts, these would be increased to 3-53, and the specific gravity to 

 1-0270. 



As to included sea-water, we may take from Molesworth's Engineering ' Pocket- 

 Book ' (1917, p. 99) the interstices of pounded sandstone or of gravel as 34 per cent. 

 of the total volume. Therefore in a cubic metre of this the sandstone would weigh 

 1500 kilogrammes, and the sea-water 350 kilogrammes, containing 3-8 kilogrammes 

 of sodium, which would amount to -25 per cent, of the weight of the consolidated 

 rock when the water had evaporated from the warm lower strata. 



[Schuchert, in his 1924 edition, p. 133, calculates that igneous rocks sufficient to 

 supply the salt of the ocean would cover all continents to the height of one or two 

 miles. Possibly four-fifths of its salt reached the ocean in Pre-Palaeozoie erosion.] 



