Geography of Ohio 369 



with the radiation of further heat, this crust shriveled about 

 the contracting center. In this manner mountains were made. 

 Later, with the appearance of an atmosphere and hydrosphere, the 

 agents of weathering with which we are acquainted became oper- 

 atiA^'e. 



Since the earhest rocks formed were considered frozen or cooled 

 molten material, they were called plutonic rocks. So the basal 

 formations, according to this theory, were looked upon as igneous; 

 and the later weathering of the igneous rocks produced material 

 that formed the sedimentary horizons. This is the Nebular 

 theory of earth-origin, omitting, of course, all details that concern 

 the origin of other members of our planetary system.^ 



Astronomers of the present generation have made wonderful 

 discoveries; their predecessors had neither such powerful tele- 

 scopes, nor the other observatory equipment with which the mod- 

 ern astronomer does his work. The marvelous facts of the hea\'- 

 — *«ns revealed by the great observatories in recent times have caused 

 doubts concerning the time-honored Nebular hj'pothesis, which 

 had its origin in very incomplete data at a time when observa- 

 tions and calculations could not be accurately made. 



Planetesimal hypothesis. Quite opposed to this, is a theory 

 which has been formulated within the last decade. In accordance 

 with this hypothesis our earth has not always been a contracting 

 body; it was not a molten mass to start with, but instead a grad- 

 ually increasing sphere possessing a moderate degree of heat. 

 This theory conceives our planetary system as having evolved 

 from a nebulous body like the present nebulas, the units of which 

 are called planetesimals. The large modern telescopes show the 

 prevailing form of nebula to be disc-like, not spheroidal, in out- 

 line, and to be spiral in structure; to have a denser center from 

 which generally two arms extend giving the nebula its spiral 

 structure, and to have apparent motion about this center; fur- 

 thermore, there appear dense masses or nuclei along the arms, and 

 in the interval between the nuclei, as well as between the arms, 

 ''finely divided and nebulous material." 



In their revolution about the center, it has been suggested, 

 the orbits cross and collisions between planetesimals take place, 



^For a full discussion, see Moulton's Introduction to Astronomy, pp. 440-447, 

 1907. 



