F'UNDAMENTAL PROBLEMS OF GEOLOGY. 247 



growth ceased and modern processes became dominant a more nearly 

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

 closer approximation to constauc}^ 



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 tendenc}^ to 

 shrinking in certain particular waj-s, as, for example, that held by 

 advocates of a tetrahedral earth, but it is not clear that the actual 

 distribution of depressions and elevations 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 correctly 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 laws at all. My hypothesis requires 

 nothing more than the inevitable slight differences of growth, of 

 volcanic activity, of compression, and their joint effects. Start- 

 ing with only such slight differences as were sufficient to give pre- 

 ponderance 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. 



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 2,100 miles, the subsequent growth w'ould form a 

 shell about 1,900 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 occa- 

 sional presence of clouds, not to appeal to the evidence of aqueous 

 absorption lines in the spectrum reported by some good obsen-ers, but 

 unconfirmed by others, lend some support to the opinion that water 

 is present, though perhaps not in the form of definite water bodies. 



It has been inferred from the almost complete, and sometimes total, 

 disappearance of the polar white caps in summer, and from other 

 phenomena, that the climate of Mars is phenomenally mild , consider- 

 ing its distance from the sun. This has been regarded as all the 

 more puzzling because of the scantiness of the Martian atmosphere, 

 but is what might be expected if Mars' atmosphere is like that 



