110 GEOLOGY. 



into the water-basins, and other influences modified the results, but 

 the deepening and spreading of the water-basins is believed to have 

 been a markedly dominant process during the earth's growth. After 

 growth ceased and modern processes became dominant, a more nearly 

 balanced relation of sea and land is thought to have ensued, with a 

 close approximation to constancy. 



Configuration of basins accidental or systematic? — The amount of 

 the original depression of the areas occupied by the water is assumed 

 to have been slight, and, we prefer to think, accidental, so to speak. 

 There may have been systematic causes that determined the relative 

 depression of certain broad tracts and the relative elevation of others, 

 such as some systematic difference in the infall, or some rotational 

 change, or some inherent tendency to shrinking in certain particular 

 ways, as for example that held by advocates of a tetrahedral earth; 1 

 but it is not clear that the actual distribution of depressions and eleva- 

 tions points to such systematic agencies. The elevated and depressed 

 tracts of the moon seem to have a distribution quite unlike those of 

 the earth, and those of Mars, if the lighter and darker areas are cor- 

 rectly interpreted as elevated and depressed tracts, are quite different 

 from those of either earth or moon. Each seems to be a law unto 

 itself, if such irregular distributions can be styled a law at all. Our 

 hypothesis requires nothing more than the inevitable slight differences 

 of growth, of volcanic activity, of compression, and their joint effects. 

 Starting with only such slight differences as were sufficient to give 

 preponderance in large tracts in favor of the water or of the land, the 

 selective and self -propagating nature of the process may have done 

 the rest. 



Smallness of the differentiating action. — If it be assumed that the 

 earth's growing hydrosphere appeared at the surface when our planet 

 had attained the mass of Mars, whose radius is about 2100 miles, the 

 subsequent growth would form a shell about 1900 miles thick. It 

 is not altogether certain that Mars bears water-bodies on its surface, 

 but the areas of greenish shade, environed by a surface generally ruddy, 

 the polar white-caps (" snow-caps ") that come and go with the seasons, 

 and the apparent occasional presence of clouds, not to appeal to the 

 evidence of aqueous absorption-lines in the spectrum reported by some 

 good observers, but unconfirmed by others, lend some support to the 



1 Emerson, Presidential Address. Geol. Soc. Am., Vol. II, pp. 61-106, 1900. 



