Theories of the Earth's Origin — Upham. 205 
that the rings are composed of Rights of disconnected satel- 
Htes, so small and so closely packed that, at the immense 
distance to which Saturn is removed, they appear to form 
a continuous mass." In other words, the Saturnian rings 
are made up of myriads of separately moving small masses, 
which are doubtless similar to the stony meteorites that 
fall rarely on the earth. 
Again, the origin of the hundreds of asteroids, or 
minor planets, mostly no more than a few miles in diam- 
eter, but including several from 100 to perhaps about 300 
miles in diameter, seems very readily explained under this 
modification of the nebular theory. 
Professor Young well says :* "The meteoric theory 
of a nebula does not in the least invalidate, or even to any 
great extent modify, the reasoning of Laplace in respect 
to the development of suns and systems from a i^aseous 
nebula. The old hypothesis has no quarrel with the new." 
Another theory, which differs more widely from that 
of Laplace, has been very recently proposed by Prof. T. C. 
Chamberlin, of the University of Chicago, who names it 
the Planetesimal Hypothesis. His studies in this direc- 
tion have been in progress about five years, with publica- 
tion of preliminarv papers.'^ pre])aring the way for the new 
hypothesis ; but its first somewhat detailed statement in 
print has appeared since the beginning of the present 
year.t In this latest paper. Professor Chamberlin gives 
the following principal outlines of his researches for a new 
and more applicable nel)ular theory, especially having in 
view its relation to the origin of the earth. 
Under the typical form of the planetesimal hypothesis it is 
assumed that the parent nebula of the solar system consisted of 
innumerable small bodies, planetesimals [infinitesimal plane- 
toids], revolving about a central gaseous mass, somewhat as 
do the planets to-day. The hypothesis, therefore, postulates no 
fundamental change in the system of dynamics after the nebula 
was once formed, but only an assemblage of the scattered material. 
* Text-Book of General Astronomy, p. 526. 
•;■ An Attempt to Test the Nebular Hypothesis by the Relations of 
Masses and Momenta, in the Journal of Geolog-y. Chicago, vol. viii. pp. 
5>i-7.3, Jan. -Feb., 1900. On a Possible Function of Disruptive Approacli 
in the Formation of Meteorites, Comets, and Nebulae. Journal of Geol- 
ogy, vol. ix. pp. 369-392, July-August, 1001. 
t Fundamental Problems of Geolos.v. in the Year BooVc No. 3. foi- 
1904, of the Carnegie Institution of Wa.shington, published in Januarv, 
1905, pp. 195-258. 
