THE MOON’S FACE. 
249 
geologic periods were doubtless different from those de¬ 
termining the size of the craters we can examine. 
(3) The material of the moon may differ from that of the 
earth’s crust in such way as to affect the size of volcanic 
craters. 
In vertical dimensions there is no important discrepancy. 
Lunar craters of the first rank range from 8,000 to 15,000 
feet in depth; terrestrial, probably from 2,000 to 4,000. 
Dividing the lunar measures, as before, by six, we obtain 
2 : 3 as the ratio of lunar depth to terrestrial; but as few 
terrestrial craters have been measured, this result cannot 
claim high precision.* 
The contrasts as to form are of greater importance. To 
set them forth fully it is necessary to give separate con¬ 
sideration to several types of terrestrial craters. These may 
be called the ordinary or Vesuvian, the Hawaiian, and the 
maar types. Craters of the Vesuvian type—and these in¬ 
clude nineteen-twentieths of all terrestrial volcanoes—are 
formed of lavas containing a considerable amount of water, 
and usually result from extravasation and explosion in 
alternation. As the lava rises in its conduit the contained 
water is converted into steam, by which the lava is torn to 
fragments and thrown into the air. That which falls back 
into the vent is again thrown upward, and that which falls 
outside the vent builds the crater rim. From time to time 
drier lava wells up and overflows the rim, or else forces a 
way to the surface at some lower level. In this manner 
there is accumulated a conical mountain with a funnel- 
shaped cavity at the top. Eruption is not continuous, but 
is interrupted by periods of quiescence, and sometimes, after 
a long interval of quiet, operations are again initiated by a 
great explosion of steam, the upper portion of the cone 
being blown out and an immense cavity left in its place. 
* The greatest measured depth of a terrestrial crater with which I am 
acquainted is that of the cup holding Crater lake, Oregon—3,000 feet. 
Pichincha has been estimated at 3,000 to 4,000 feet. The general depth 
of the lunar crater Theophilus is 15,300 feet (Ebert). 
