164 PICKERING. — LUNAR AND HAWAIIAN PHYSICAL FEATURES COMPARED. 
some of the maria, as in the case of Sinus Iridum, Figure 15. The walls of Mokuaweo- 
weo or of Kilauea, Figures 9 and 10, furnish excellent illustrations of these forma- 
tions. Examples of the crater rings themselves, however, are rare. The best preserved 
one that we were able to visit was found on Hualalai, at an elevation of 400 feet 
below the summit of the mountain, and was reached a few minutes after passing the 
so-called Bottomless Pit. It was located on the floor of a crater some 500 feet in 
diameter by 100 feet in depth, g p. 171. The diameter of the crater ring was 120 
feet, its internal depth was six feet and the height of its outside wall twelve to sixteen 
feet. It is shown upon a larger scale at h p. 171. Near the centre of the outer crater, 
and outside of the crater ring, was a low peak. What appeared to be another crater 
ring was seen from the distance of a mile on the northern slopes of Mauna Loa, and 
will be described later. A portion of one of the crater rings formed by Halemaumau 
is still to be seen on the rim near the view-point where visitors look down into the 
interior. 
The 
o 
numerous upon the Moon and so rare 
upon the Earth is apparently that the terrestrial 
generally permanent 
Figure 16, or they contain one or more sm 
general not central, like 
The smaller craters on the Moon do not take this form, and if some of them did exist 
formerly they have since been destroyed. The reason of this is that when the lava 
recedes into the bottom of the pit, the depth is so great in proportion to the diameter, 
that the walls cave in, destroying the ring, — as usually happens at Halemaumau. 
On the Moon no crater is known whose depth exceeds five miles, and two miles is the 
usual depth for large craters. This distance compared to a diameter of twenty to 
sixty miles is so slight that the ring remains uninjured. The outer walls of Haleakala 
may in part be the remains of an old crater ring of very elliptical shape. They have 
been breached and totally destroyed in two places. 
The floors of the craters on the Moon are of three kinds, either they are furnished 
with a central peak, like Tycho, the large crater in the lower left hand corner of 
Clavius, in the same figure ; or they are without conspicuous detail, like Kies, Fig 
6. ^ In the last two cases the floor is often of a later origin than the walls, as indicated 
by its color and smoothness, the original floor having been melted by a flood of dark 
colored lava from below, which dissolved all the lowermost portions of the solid crust 
with which it came in contact. .This seems to be the case with Longomontanus, the 
large crater northeast of Clavius. Its diameter is ninety-on* miles. Often, however, 
the floor is bright, and not perfectly smooth, as in Tycho, showing it to be part of the 
original formation. Central peaks are occasionally, but by no means universally, 
