10 GEOLOGY. 



molten rock has been extruded from it at various times throughout 

 the known geologic ages, and that the earliest known rocks are largely 

 igneous or igneous derivatives. In its favor also has been cited a 

 long list of beautiful harmonies in the solar system, whose features of 

 symmetry have been thought to tally well with the orderly separa- 

 tion of its members as successive rings. 1 



Objections to the Laplacian hypothesis. — Notwithstanding the 

 very general acceptance of the Laplacian hypothesis during the past 

 century, certain objections have all along been urged against it, and 

 recently certain new ones of a cogent nature have been pressed upon 

 consideration. 2 



(1) As already noted, it has been urged that definite rings might 

 not be formed, but that the equatorial matter would more probably 

 separate particle by particle. 



(2) It has been questioned whether the rock substance of the earth 

 in the attenuated condition it must have had in the supposed earth- 

 moon ring would not have cooled to solid particles long before it could 

 have collected into a spheroid. 



(3) By mathematical investigation, Moulton has recently shown 

 that there are grave mechanical difficulties in the contraction of a 

 ring into a spheroid as simply and promptly as supposed by Laplace, 

 and this gives new point and force to the preceding objection. 



(4) It has been urged that in so highly heated a condition as the 

 hypothesis assumes, the molecular velocities of the lighter gases of 

 the ring would be so great that they could not be held together by 

 the attraction of the ring, and perhaps could not be controlled even 

 by the gravity of the supposed gaseous spheroid. 



(5) It is a most singular fact that Phobos, the inner satellite of 

 Mars, revolves in less than a third of the time of the planet's rota- 

 tion, whereas, according to the Laplacian theory, the planet's rate 

 of rotation should have kept on increasing after the ring that formed 



1 A full list of these is given in World Life by Alex. Winchell. 



2 A Group of Hypotheses bearing on Climatic Changes, Jour. Geol., Vol. V, No. 7, 

 1897, pp. 653-83; An Attempt to Test the Nebular Hypothesis by the Relations of 

 Masses and Momenta, Jour. Geol., Vol. VIII, No. I, January-February, 1900, pp. 

 58-73; Certain Recent Attempts to Test the Nebular Hypothesis, Science, Vol. XII, 

 August 10, 1900, T. C. Chamberlin; An Attempt to Test the Nebular Hypothesis 

 by an Appeal to the Laws of Dynamics, Astrophys. Jour., Vol. XI, No. 2, March, 

 1900, pp. 103-130, F. R. Moulton. 



