2i8 The American Geologist. Ap.ii.19u5 
The planets undoubtedly tended in some degree toward 
the same intensely hot condition which is reached by the 
sun and stars in the concentration of originally nebulous 
matter. 
But another explanation of the origin of the very 
abundant small and large crateriform features of the moon 
has been advocated by G. K. Gilbert, of the United States 
Geological Survey. This very remarkable and ingenious 
explanation seems largely identical with the later plane- 
tcsimal hypothesis of Chamberlin, so far as that hypothesis 
deals with the segregation of the originally nebulous mat- 
ter to form planets and satellites. Mr. Gilbert writes :* 
* * * It is my hypothesis that before our moon came into 
existence the earth was siuTounded by a ring similar to the 
Saturnian ring: that the small bodies constituting this ring after- 
ward gradually coalesced, gathering first around a large number 
of nuclei, and finally all uniting in a single sphere, the moon. Un- 
der this hypothesis the lunar craters are the scars produced by 
the collision of those minor aggregations, or moonlets, which last 
surrendered their individuality. 
* * * The introduction of the hypothesis of a Saturnian 
ring thus accomplishes much toward the reconciliation of the im- 
pact theory with the circular outline of the lunar craters. * * * 
In fine, the hypothesis of the Saturnian ring, by restricting 
the colliding bodies to a single plane, by substituting a low ini- 
tial velocity and thus rendering the moon's attraction the domi- 
nant influence, and by introducing a system of directions con- 
trolling, and therefore adjusted to, the moon's rotation, relieves 
the meteoric theory of its most formidable difficulty. It also ex- 
plains in a simple way the abundance of colliding bodies of a 
different order of magnitude from ordinary meteorites and aero- 
lites. * * * 
The velocity of impact, depending chiefly on the moon's at- 
traction, must be supposed to have increased gradually as the 
■ * The Moon's Face, a Study of the Origin of its Features. Address as 
Retiring President, delivered December 10, 1892, Bulletin of the Philoso- 
phical Society of Washington. D. C, vol. xii, pp. 241-292, with one plate 
and 14 figures in the text; published April, IS.'tS. 
Referring to early suggestions of meteoric accumulation of the moon, 
and of other cosmic bodies. Mr. Gilbert said in this paper (1892"): "I have 
discovered no published statement of meteoric theories more than twenty 
years old. but the idea is older and various obscure allusions indicate 
that it was earlier in print. Proctor makes a meteoric suggestion in 1873 
rThe Moon. p. 34G). and advocates it in 1878 (Belgravia, vol. 36. p. 153). 
A meteoric theory is said to be contain- d in Die Fhysiognomie des Mondes 
bv 'Asterios.' Nordlingen. 1879. A. Meydenbauer advances another in 
Sirius. for February, 1882." 
With these publications, compare The Meteoritic Hypothesis. 1S9<1. by 
Lockyer. before cited, and a most important paper by Prof. George H. 
Darwin, "On the Mechanical Conditions of Swarms of Meteorites iind 
on Theories of Cosmogony," Phil. Trans. Royal Society, 1888. 
