172 PICKERING. — LUNAR AND HAWAIIAN PHYSICAL FEATURES COMPARED. 
is no summit crater. It is probably a solid block fallen from a neighboring cliff, that 
had been undermined by the liquid flow, and after floating awhile and being trans- 
ported, was now frozen in, in its present position. Such a block is shown in Figure 
27. It is twenty-five feet in height, and was found upon the floor of Haleakala. A 
second one is shown in the distance. Similar objects are of frequent occurrence upon 
the various maria of the Moon. Doubtless they are often formed as above described, 
* 
but in many instances it is evident that they have been left in their original positions, 
while the objects formerly surrounding them have been destroyed by the flood of 
molten lava. Innumerable pinnacles are found upon the Mare Imbrium. A num- 
ber of them are shown in Figure 28. The large crater in the photograph, near the 
left-hand edge, is Euler. Its diameter is nineteen miles. 
A curious feature in Hawaii is the very extensive series of caves that penetrate 
the lava, especially the flows from Mauna Loa. Indeed, so many of them have been 
found that it has been suggested that they make up an appreciable part of the bulk 
of the mountain. A very accessible cave is situated a few miles above the town of 
Hilo. It is said to extend two miles up and two miles down the mountain from the 
entrance, which is a place where the roof happened to fall in, disclosing the cavity. 
The breadth of the cave is about thirty feet. Its height varies in the portion that 
we traversed from three to ten feet. Larger caves are found in other places, some- 
times, according to Dutton, being as much as sixty or eighty feet in height, and wide 
in proportion. Their origin is due to the fact that the surface of the lava hardens 
first, and that the lower portions meanwhile flow away, leaving the cavity. Small 
the formation seems to be d 
o 
of gases under sufficient pressure 
hold back the lava until it has had time to solidify. Sometimes lava channels form 
without any roof. Some well marked channels and caves are found two or three 
miles nortli of Huehue on the Kona coast. A lava channel was noted not far from 
the summit of Hualalai, where the path crosses an open lava field. Another channel 
was found in Kilauea near Halemaumau. 
At first it was thought that these lava channels were analogous to the broad 
grooves found upon the Moon, of which the valleys of the Alps and of Rheita are 
the most conspicuous examples. The Valley of Rheita is shown in Figure 31. 
190 miles long by 15 miles wide. Several parallel valleys similar to these are found 
to the southwest of Pallas, and a less well marked series to the southeast of Sinus 
Indum. The great range of the Altai Mountains in the southwestern quadrant of 
the Moon seems to form one side of such a valley constructed upon a very large 
It 
the floors of Kilauea and Haleakala where, since the floors are level, 
