THE MOON’S FACE. 
251 
Craters of the Hawaiian type are produced by lavas con¬ 
taining so little moisture that its conversion into steam does 
not cause violent explosions. Successive eruptions, flowing 
from the orifice in different directions, build by their con¬ 
gelation a massive cone with crater at top. In the intervals 
between eruptions the lava stands in the crater as a pool or 
lake, the liquidity of which is maintained by a circulation, 
through the conduit, of super-heated lava from below. If 
the circulation slackens, a crust forms over the lake, giving 
the crater an inner plain like those of the moon. If the 
current is more active, the molten lava remelts part of the 
lava of the cone, undermining the w^alls of the crater and 
causing them to fall in, whereby the cavity and the lake are 
enlarged. Partially fallen fragments of the crater wall con¬ 
stitute terraces of the land-slip 
type. Sometimes the lava re¬ 
treats downward after crusting 
over, and fragments of the crust, 
adhering to the crater walls, form 
terraces of another type. Craters 
of this sort are somewhat rare, 
but their rarity does not affect their value as interpreters of 
extra-telluric phenomena. As long ago pointed out by Dana, 
they resemble the moon’s craters much more closely than do 
those of ordinary volcanoes. The}^ agree with lunar craters 
in the possession of inner plains, and to a certain extent in the 
terracing of their inner walls. They differ in the fact that 
they occupy the tops of mountains; in the absence of the 
wreath; in the absence of the central hill, and usually in the 
presence of level terraces due to the formation of successive 
crusts. In my judgment the differences far outweigh the 
resemblances, and I have not succeeded in imagining such 
peculiarities of local condition as might account for the 
divergence in form. 
The maars are of still rarer occurrence, and represent the 
antithetic phase of volcanism. The process of their forma¬ 
tion includes no eruption of lava, but merely an explosion 
Fig. 8.—Crater of Hawaiian type. 
Features due to erosion are omitted. 
