270 A CENTURY OF GEOLOGY. 



t 

 earth forces which are the most important to study. And among 

 these etfects the most fundamentaliy important of all is the formation 

 of those greatest features — the ocean basins and continental arches. 

 The most probable view is that they are formed by unequal radial 

 contraction in the secidar cooling of the earth. The earth was cer- 

 tainly at one time an incandescent!}' hot mass, which gradually cooled 

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

 perfectly homogeneous l)oth in density and in conductivity in all parts, 

 then, cooling and contracting equally in ever}^ part, it would retain its 

 symmetric oblate-spheroid form, though diminishing 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, con- 

 tracting 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 l)asin and the continental 

 arches of the lithosphere. The same causes which ])roduced would 

 continue to increase them, and thus the ocean basins woidd increase in 

 depth and the continents in height. 



The hydrosphere is still to be added. In the l)eginning of this 

 process douljtless 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 partl}^ or wholly 

 cover the earth — whether partly or wholly would depend on the 

 amoiuit of precipitated water and the amount of inequalit}^ which had 

 already taken place. The amount of water, as we know, is sufficient, 

 if the inequalities were removed, to cover the whole surface 2^ miles 

 deep. Inasmuch as the forming of the inequalities is progressive, and 

 still going on, it seems improbable that the ine(|ualities had become 

 sufficiently great, at the time of precipitation, to hold the waters. If 

 this be so, then the primeval ocean was universal and the future con- 

 tinents existed only as continental lianks in the universal ocean. 



However this may have been, there seems little doul)t that the same 

 cause Avhich produced the inequalities continued to operate to increase 

 them. The ocean basins, so far as these causes are concerned, must 

 have become deeper and deeper, and the continents larger and larger. 

 In spite of man}' oscillations producing changes mostly on the mar- 

 gins, but sometimes extending oxer wide areas in the interior of the 

 continent, this, on the whole, seems to be in accordance with the 

 known geological history of the earth. If so, then the oceanic basins 

 have always been oceanic basins, and the places of the continents have 

 always been substantially the same. This introduces a subject on 

 which there has been much discussion recently, viz: 



The j)€rnianeii<;ii of ocean haslmi. — Closely associated with the Lyel- 

 lian uniformitarianism was the doctrine of extreme instability of earth 

 features, especially the forms and places of sea and land. Crust move- 

 ments were irregularly oscillating to such a degree that in the course 



