168 PICKERING. — LUNAR AND HAWAIIAN PHYSICAL FEATURES COMPARED. 
number of the lunar craters, such as Fabricius, Hercules, Manilius, Reinhold, and 
Bullialdus (see Figure 5). 
Some of the lava pits occur very near together, the intermediate wall being only 
a few yards in thickness, but we saw none which actually intersected. When the 
large summit crater was formed a smaller one near it was partially destroyed, and 
filled by the erupted material from the larger crater. 
We have no description of the method of formation of a crater bowl, but a study 
of the various sections on p. 171 will show how they have probably been formed, 
Starting with a crater ring such as is shown in Figure 19, and in section at h, p. 171, 
when the lava retreats a crater pit will be formed within it. If the pit is very deep 
relatively to the diameter of the ring, the latter will be destroyed, and we shall 
have simply an ordinary pit, I If the pit is not so deep relatively to the diameter 
of the rim the latter may be preserved, n. The floor of the pit may be flat, or it may 
sink towards the centre, m. If the pit is on a large scale the sides are less liable 
to be vertical, and moreover a talus will collect at their base, o. This will 
gradually become rounded as at p, or if the crater ring is still left, as at /. The 
flattened floors of the larger craters on the Moon are illustrated by g, while the 
terrace and central peak are shown at Tc. 
With the exception of the crater pits, nearly all the smaller depressions upon the 
Moon are crater bowls, and they outnumbe 
times all the other depres 
sions put together. The smallest crater rings are about five miles in diameter. 
One of the largest and best situated crater bowls is Triesnecker, 14 miles in diameter. 
It has an inconspicuous central peak. In the smaller bowls this feature seems to be 
lacking. A well-graduated series of bowls is shown in the interior of Clavius, 
Figure 16. The process which converts a pit I into a bowl^ upon the Earth is due 
chiefly to the action of water. Upon the Moon, even in former times, water was 
probably scarce, but owing to the extremely rarefied atmosphere the extremes of 
temperature are excessive. A range of 300° C. or 540° F. occurs every fortnight, 
and it seems likely that a considerable destruction of ridges and filling of hollows 
would be due to this cause by itself. 
Another explanation of crater bowls is given by Gilbert in his dissertation on 
The Moon's Face," Philosophical Society of Washington, Bulletin XII, 251, where 
^ ts that they may be due to a single explosion of steam, like the terrestrial 
maars." This seems improbable, since volcanic features due to steam are notably 
absent from the Moon. On Hualalai the crater bowls have smooth lava walls which 
are not at all fragmentary, as would be the case were they due to an explosion. 
he siiscu: 
