436 POPULAR SCIENCE MONTHLY. 



the former is the original and fundamental kind. These deter- 

 mine earth forms, while the other only modify them; these de- 

 termine the great features, the other only the lesser features; 

 the former rough-hews the earth features, the latter shapes 

 them. It is the effects of these interior earth forces which are the 

 most important to study. And among these effects the most fun- 

 damentally important of all is the formation of those greatest fea- 

 tures — the ocean basins and continental arches. The most prob- 

 able view is that they are formed by unequal radial contraction 

 in the secular cooling of the earth. The earth was certainly at 

 one time an incandescently hot mass, which gradually cooled and 

 contracted to its present temperature and size. ISTow, if it were 

 perfectly homogeneous both in density and in conductivity in all 

 j)arts, then, cooling and contracting equally in every part, it 

 would retain its symmetric oblate-spheroid form, though diminish- 

 ing in size. But if there were any, the least, heterogeneity either 

 in density or especially in conductivity over large areas, then the 

 more conductive areas, contracting more rapidly toward the center 

 radially, would form hollows or basins, and the less conductive areas 

 would stand out as higher arches. Thus were formed the oceanic 

 basin and the continental arches of the lithosphere. The same 

 causes which produced would continue to increase them, and thus the 

 ocean basins would increase in depth and the continents in height. 



The hydrosphere is still to be added. In the beginning of this 

 process doubtless the lithosphere was hot enough to maintain all 

 the water in the form of vapor in the atmosphere. But when the 

 surface was cool enough the water would precipitate and partly or 

 wholly cover the earth — whether partly or wholly would depend 

 on the amount of precipitated water and the amount of inequality 

 which had already taken place. The amount of water, as we know, 

 is sufficient, if the inequalities were removed, to cover the whole 

 surface two and a half miles deep. Inasmuch as the forming of 

 the inequalities is progressive and still going on, it seems improb» 

 able that the inequalities had become sufficiently great, at the time 

 of precipitation, to hold the waters. If this be so, then the prime- 

 val ocean was universal and the future continents existed only as 

 continental banks in the universal ocean. 



However this may have been, there seems little doubt that the 

 same cause which produced the inequalities continued to operate 

 to increase them. The ocean basins, so far as these causes are con- 

 cerned, must have become deeper and deeper, and the continents 

 larger and larger. In spite of many oscillations producing changes 

 mostly on the margins, but sometimes extending over wide areas 

 in the interior of the continent, this, on the whole, seems to be in 



