414 0. Fisher — Rival Theories of Cosmogony. 



Art. XXXIV. — On Rival Theories of Cosmogony /* by the 

 Eev. O. Fisher, M.A., F.G.S. 



The nebular hypothesis of the origin of the earth has 

 of late been brought into competition with a meteoric theory, 

 which supposes that our globe has been built up out of the 

 conglomeration of a swarm of meteorites gradually falling in ; 

 that it was never melted at its surface, and that it owes its 

 internal high temperature to gravitational action upon the 

 materials of which it is composed. Professor Chamberlin of 

 Chicagof is of opinion that these meteorites formed "a swarm 

 or belt revolving about the sun in the general neighborhood of 

 the present orbit of the earth." If that was the case they did 

 not belong to the class of meteorites which we still encounter 

 in the shape of " stones" and "irons" that reach, us from 

 regions, as it is believed, beyond the solar system, although 

 they may be supposed to have been petrol ogically similar. 



Probably physicists would feel no difficulty in admitting 

 that an earth so formed would continually assume the sphe- 

 roidal shape appropriate to the speed of rotation, so that any 

 objection on that score may be passed over. 



It is not intended either to advocate or oppose the meteoric 

 theory, but it may be useful to draw attention to some consid- 

 erations which may help us to form an opinion upon its merits, 

 as contrasted with the nebular hypothesis. 



In the first place, if the meteorites were similarly consti- 

 tuted to those which sporadically reach us now, a difficulty, 

 perhaps more apparent than real, arises in limine / because, 

 although no elements occur in these which do not also occur in 

 our rocks, nevertheless they differ greatly in the arrangement 

 of these elements. For instance, free silica, which is one of 

 the most abundant constituents of terrestrial rocks, is not 

 known in that form in meteorites.^: It may perhaps be con- 

 ceded that a swarm of meteors, being as it were thrown into 

 hotchpot, the minerals might emerge rearranged in such com- 

 binations as we find in the earth's crust, and that the deeper 

 strata, which are concealed from us, might become proportion- 

 ally poor in those common minerals, having by loss of them 

 enriched the more superficial rocks which we can examine. 

 The possibility of such a process involves considerations upon 

 which we shall enter further on. 



Ignorant as we are about the internal constitution of the 



* Received from the Author. 



f Journal of Geology, "On hypotheses bearing on climatic changes," 1891. 

 Science, June 30th, 1899, "On Lord Kelvin's estimate of the age of the world." 

 % Sir Norman Lockyer, "The Meteoric Hypothesis," p. 23. 



