246 
GILBERT. 
Usually the small craters are far more numerous than those 
of medium size, but in certain districts well covered by cra¬ 
ters those of small size are less abundant. The craters over¬ 
lap one another with every conceivable relation, except that 
the overlapping is never reciprocal. It is in every case pos¬ 
sible to distinguish the newer from the older, the older being 
partially effaced by the newer. Small craters occur on all 
parts of larger ones, not excepting the wreath and the steep 
inner slope (see Plate 3). 
Volcanic Theory .—By the majority of writers the craters 
are assumed to be volcanic, and as they differ in size, abun¬ 
dance, and form from terrestrial volcanoes, it is thought that 
they represent some special type of volcanism determined 
by physical conditions peculiar to the moon. Let us com¬ 
pare the lunar and terrestrial craters and see how far their 
differences can be explained as dependent on differences of 
physical condition. 
Take first the difference in abundance. Faye estimates 
the number of visible lunar craters, of all dimensions, at 
20,000 to 30,000. There is no equivalent area on the earth 
so well explored as to admit of a close estimate, but a general 
idea may be derived from our knowledge of North America. 
From personal observation of the principal volcanic districts 
in Utah, Nevada, Arizona, and New Mexico, I estimate the 
number of craters and ruins of craters in those States and 
Territories at 1,000. In the remainder of our western 
mountain region there are probably 500 more; 500 may 
safely be ascribed to the districts of similar geologic type in 
Alaska and British Columbia, and 1,000 to Mexico and Cen¬ 
tral America; giving as an estimate for the continent 3,000 
craters, or one-tenth of Faye’s larger estimate for a lunar 
area of similar extent. Our estimate includes only the 
craters formed at so recent a date that the processes of erosion 
and deposition have neither demolished nor buried them; 
but the geologic record shows that there have been volcanic 
eruptions in all ages, and indicates as probable that every 
district has been at one time or another a field of volcanic 
