I 
THE moon’s face. 253 
well to bring together the results of our inquiry into the 
adequacy of the volcanic. The comparative abundance of 
lunar craters is readily accounted for without prejudice to 
the theory. Their greater maximum width, though partly 
referable to a gravitational factor, constitutes a real diffi¬ 
culty, especially as volcanoes appear to have a definite size 
limit, while lunar craters do not. Form differences effectu¬ 
ally bar from consideration all volcanic action involving the 
extensive eruption of lavas, whether dry or saturated with 
water. They also exclude the maar process (single explo¬ 
sion) as an explanation of medium and large craters, but 
not as an explanation of small craters. The volcanic theory, 
as a whole, is therefore rejected, but a limited use may be 
found for the maar phase of volcanic action in case no other 
theory proves broad enough for all the phenomena. 
Tidal Theory .—Of other theories, a few are somewhat re¬ 
lated to the volcanic. It is hardly profitable to discuss the 
suggestion that the greater walls were formed about the 
vortices of a primeval liquid moon,* nor the suggestion, 
albeit advanced independently by several authors, that the 
vast circling cliffs of the moon are remnants of Cyclopean 
bubbles that have burst.f But an ingenious tidal theory, 
which appears to have sprung up independently in France, 
Germany, and England,! merits careful examination. It 
postulates a time when the moon was liquid, with the excep¬ 
tion of a thin crust. The moon then rotated more rapidly 
than now, and great tides, excited by the earth’s attraction, 
racked and cracked its crust and here and there squeezed 
out a portion of the liquid nucleus, which flowed back again 
when the tidal wave had passed; but congelation caught 
* Rozet: Memoire sur la Selenologie. Comptes rendus, vol. 22 (1846), p* 
470. 
tRobert Hooke: Micrographia (1667) [not seen]. Jules Bergeron: 
L’Astronomie (1882), p. 346; (1883), p. 112. A. St. Clair Humphreys: 
Jour. Brit. Astr. Ass., Dec., 1891, p. 132. 
JFaye: Rev. Scientiflque, 27 (18811, p. 130. H. Ebert: Ann. Physik 
und Chemie, vol. 41 (1890), p. 351. J. B. Hannay: Nature, vol. 47 (1892), 
p.7. 
