Proceedings of the Ohio State Academy of Science 39 



signs the differences between continents and ocean basins to 

 unequal radial contraction of the earth in its secular cooling but 

 does not suggest that conditions producing and maintaining con- 

 tinents may also be responsible for mountains. Geological theory 

 today makes this contrast of continent and ocean basin the funda- 

 mental fact of earth structure. Successive continental uplifts 

 are due to the crowding of the continental segments between the 

 heavier oceanic segments. Folded mountains are due to the vield- 

 ing to the accompanying lateral thrust. Wliile plateaus, plateau- 

 like folds, and areas of sinking within the continent are due to re- 

 adjustment between blocks of a second order which make up the 

 continent. 



The Plancicsiuial Hypothesis. 



In our school days, as in those of our fathers, the Nebular 

 Theory of La Place held unchallenged sway. This hypothesis is 

 so familiar that I need not state it. Objections to this hypothesis, 

 mechanical, geological and astronomical, have seriously shaken 

 confidence in it. Within the last twenty years a substitute for it 

 has been proposed by Chamberlin. the planetesimal hypothesis. 

 Like the La Placean hypothesis it is a nebular hvoothesis. But in 

 so far as it concerns the growth of the earth the two are almost 

 antipodal. On the La Placean hypothesis the earth reached its 

 present size by the cooling and shrinking of a vastly larger hot 

 gaseous mass, and on George Darwin's modification of the theory 

 (the meteoric hypothesis) it is probable that the meteorites 

 would be vaporized early in the earth's growth by collisions; on 

 the new, it grew to a maximum by the in-fall of planetesimals, 

 cold bodies of various size moving in elliptical orbits about the 

 sun, and added to the growing mass of the young earth. Long 

 before the beginning of the Paleozoic the heavens had been 

 cleared in the region of the earth's orbit, of these scattered 

 masses, and the earth had reached its maximum size. Today it is 

 condensing, perhaps as a result of loss of heat, perhaps under the 

 attraction of gravity on its mass. The interior heat of the earth, 

 on the newer theory, is generated by condensation, and is not an 

 inheritance from the nebula. 



