36 HISTORICAL GEOLOGY 



Some of the well-known facts which any hypothesis of the 

 origin of the solar system must explain are as follows: (1) The 

 planet orbits are all elliptical, but nearly circular; (2) the orbits 

 lie in nearly the same plane; (3) all planets revolve about the sun 

 in the same direction; (4) the sun's direction of rotation is the same 

 as that of the planets' revolution; (5) the planes of the planets' 

 rotation nearly coincide with the planes of their orbits (except 

 Uranus and Neptune); (6) the direction of the planets' rotation 

 is the same as that of their revolution; and (7) the satellites re- 

 volve in the direction of rotation of their planets (two or three 

 exceptions) . 



Hypotheses of Earth Origin 



Nebular or Ring Hypothesis. — In 1796 Laplace published a 

 remarkable work on astronomy, and in its last chapter he put forth 

 his now well-known hypothesis regarding the origin of the solar 

 system. He postulated a spheroidal mass of very highly heated, 

 incandescent gas or nebula greater in diameter than the present 

 solar system, this whole mass rotating in the direction of the revolu- 

 tion of the existing planets. Due to loss of heat by radiation, this 

 mass contracted and its shrinkage necessarily made it rotate more 

 rapidly upon its axis, at the same time causing the centrifugal force 

 on its outside to become stronger and stronger. Finally the centrif- 

 ugal force at the equator became equal to the force of gravity and 

 the equatorial portion was left off (not thrown off) as a ring sur- 

 rounding the contracting remainder. The materials of the ring 

 condensed to form the outermost planet. By continued contrac- 

 tion of the rotating nebula, the other rings and planets were formed. 

 The satellites were produced in a similar manner by rings left off 

 by the shrinking planets. 



Briefly, according to this hypothesis, the earth was originally 

 highly heated and much larger than now. During its cooling and 

 contraction, its original hot and dense atmosphere, which contained 

 all the earth's water in the form of vapor, gradually became thinner 

 due to absorption by the earth. When the conditions of pressure 

 and temperature were favorable, water vapor condensed to form 

 the hydrosphere. The oldest rocks must have been igneous, that 

 is they were portions of the original crust formed b}^ cooling of the 

 molten globe. 



For over a hundred years the Laplacian hypothesis has exerted 



