THE MOON’S FACE. 
289 
(excepting only the rill pits), correlating size variation with 
form variation in a rational way. Specialized by the assump¬ 
tion of an antecedent ring of moonlets, it accounts also for 
the great size of many craters. It brings to light the history 
of a great cataclysm, whose results include the remodeling 
of vast areas, the flooding of crater cups, the formation of 
irregular maria, and the conversion of mere cracks to rills 
with flat bottoms. It explains the straight valleys and the 
white streaks. In fine, it unites and organizes as a rational 
and coherent whole the varied strange appearances whose 
assemblage on our neighbor’s face cannot have been fortu¬ 
itous. 
Growth of the Moon .—In an incidental way there has 
sprung from this investigation of the moon’s craters a theory 
as to the building of the moon itself. An attempt to develop 
that theory would lead far afield, but it is due to the crater 
discussion that its implications as to the moon’s history be 
brought together, so that their coherency may be judged. 
In the breaking up of the postulated pre-lunar ring there 
were at first many centers of aggregation,—were the moon 
the only center, the scars of impact would all be small. So 
long as the masses were small the process of aggregation de¬ 
veloped little heat, for the heat of impact depended almost 
wholly on velocities created by mutual attractions. That 
particular moonlet which became the nucleus of the moon 
may therefore be conceived as cold, or at least as sufficiently 
cool to be solid. As the moon’s mass grew, the blows it re¬ 
ceived were progressively harder, and for a time their fre¬ 
quency also increased. The rate of heating probably reached 
and passed its maximum while the mass was materially less 
than now. During the whole period of growth the surface 
lost heat by radiation, but the process of growth cannot have 
been slow enough to permit the concurrent dissipation of all 
the impact heat. On the one hand, there should have been 
some storage of heat in the interior, and, on the other hand, 
the stored heat can never have sufficed for the liquefaction 
of the nucleus. Toward the close of the process, when blows 
37—Ball. Phil. Soc., Wash., Vol. 12. 
