278 AN ESTIMATK OF THE GEOLOGICAL AGE OF THE EARTH. 



that of the varying ratios of land and sea areas of the past and the 

 amount of rainfall received upon the latter. The fact that paleonto- 

 logicall}' similar deposits in the various parts of the world are not 

 necessarily contemporaneous, but homotaxial, debars the geologist 

 from mapping the sediments of any horizon (even were these fully 

 known) as forming part simultaneously of the oceanic area. Could 

 he even claim full assurance here, the land areas supplying the sedi- 

 ments must still remain unknown. 



In this difficulty indirect inferences onlj^ can be resorted to. 



Those who accept the stability of the continents and oceans as a 

 whole can not well admit that the balance of land and water was ever 

 ver}^ seriously interfered with. Sir J. Murray^ has calculated that if 

 the present land of the globe were reduced to the sea level by being 

 removed to and piled up in the shallow waters of the ocean its extent 

 would be altered from the present 55x10® to 80x10*^ square miles, the 

 ocean simultaneously changing from 137.2X10* to 113x10*^ square 

 miles. The mean height of land, which is at present 2,250 feet, 

 would become 0, while the mean depth of the ocean, at present 2,080 

 fathoms, would increase to 3 miles, 23.45 X 10® cubic miles of material 

 being transported into the sea. 



If the earth's crust were rigid and neither subsidence or elevation 

 ever took place, such a calculation would mark the extreme distribu- 

 tion of the existing subaerial material which would be possible under 

 the action of denuding agencies. It could only be brought about by 

 an infinitely prolonged denudation and quiescence of the crust. 



As a matter of fact, however, we know that over the continental 

 areas there have been frequent depressions and elevations, and these 

 acting alternateh^ again and again over the same area. The Uniformi- 

 tarian, we assume, regards this shifting balance of land and water as 

 confined mainly to the area indicated above, the 80,000,000 square miles 

 marking out the elevated plateaux of the glol:)e. The dry land of 

 to-day occupies some 08 per cent of this area. It can not be supposed 

 to have ever occupied 100 per cent of it, for then sediments must have 

 been laid down in the present ocean troughs. That such sedimenta- 

 tion, again, as we see in the great formations could have been effected 

 without large areas of exposed land is impossible. These rocks infal- 

 libly assert the existence of dry land proportional to their own magni- 

 tude and complementary to their own submergence. The sedimentary 

 deposits themselves suggest, then, from the necessities of their 

 supply, a limit on the other side; that is, to the reduction of land area 

 in past times. ^ 



^Scottish Geological Magazine, 1888, pp. 1 et seq. 



^See Wallace's Island Life, Chap. VI. See also Green's Physical Geology, 1892, 

 pp. 687 et seq., and Three Cruises of the Blake, by A. Agassiz, 1888, pp. 126 and 166. 

 The question of the permanence of continents and oceans has been so much discussed 

 that further reference here is unnecessary. 



