THE MQOX’s FACE. 
255 
lated normal stresses are measured by lava columns from 
5,000 to 15,000 feet in height, and the tangential strains re¬ 
sulting from the greater of these stresses would rend a crust 
of granite 100 miles thick.* Yet, again, there are numerous 
craters of small or medium size occupying slopes of the 
greater crater rims, and the initiation of these by tidal pro¬ 
cess seems impossible. Whatever lava escaped from an 
orifice on a slope would flow down the slope instead of being 
drawn back. 
Snoiv Theory .—Another theory assumes that the moon is 
covered with snow or ice. The site of each crater was once 
occupied by a pool of water which by heat from below was 
vaporized. The vapor was quickly converted into snow, 
part of which fell back in the pool to be vaporized again, 
and all of which was eventually accumulated in an annular 
ridge.f 
There is some reason to question the existence of water 
and ice on the moon’s surface, but as this subject will pres¬ 
ently be considered in another connection, the point will be 
waived here and attention restricted to questions of form 
and relation. If the rim were built up by the quiet fall of 
an infinitude of ice particles or snow flakes, its configuration 
should be smooth and regular instead of exhibiting the 
rugosity actually observed. The postulated heat of the 
central area might render the inner slope steep and even 
produce the inner cliff and terraces, but the theory affords 
no explanation of the wreath nor of the central hill. It fails 
likewise to account for the small craters formed on the rims 
and slopes of the larger, for the bottoms of these are far 
above the assumed rock plain of the moon through which 
* The computation on which this statement is based assumes 2.75 as the 
density of the lunar lava and 11,000 pounds per square inch as the tensile 
strength of granite. It assumes also that all parts of a vertical section of 
crust are subjected to the same strain, but any tidal deformation of the 
crust would make the distribution of strain unequal. 
f John Ericsson : Nature, vol. 34 (1886), p. 248. S. E. Peal: Nature, vol. 
35 (1886), p. 100; English Mechanic and World of Science, vol. 47 (1888), 
p. 477. 
