PICKERING. LUNAR AND HAWAIIAN PHYSICAL FEATURES COMPARI I). 150 
and solidify. From a height of 500 feet the phenomenon presented little of interest 
compared to what had been seen in the last century. The crater is gradually filling 
up from a subterranean inlet. The depth of the pit in 1902 was estimated at 1,000 
feet. The lower portion, which has now been filled was then conical in shape. 
Turning now to the third subdivision of the lava craters, the crat 
begin by a study of what is believed to be their internal structure, 
- 
internal structure, as exhibited in 
Figure 11. This photograph represents a vertical section of a small ring crater formed 
naturally in cooling iron slag. When the slag is drawn off from the furnace it is 
allowed to solidify in conical moulds four or five feet in diameter, and about a foot in 
depth at the centre. Unless interfered with, a crater three or four inches in diameter 
is invariably formed as soon as the surface has fairly hardened. On breaking up the 
slag considerable cavities are always found beneath the crater. These are well shown 
in the figure, as are also the cracks connecting them with one another and with the 
central peak, which it will be noted is also hollow. The large pear-shaped cavity 
beneath the peak was in the present instance filled up from below with melted iron. 
It will be noted that the inner walls are very steep, while the outer ones slope more 
gradually. During the process of formation the crater sometimes fills to the brim 
and overflows, building up the walls ; later the interior fluid withdraws, forming the 
crater floor. 
Besides these larger craters other ones are often formed, which, while retaining a 
base of perhaps two inches in diameter, frequently build up to the height of several 
inches, forming vertical tubes or spiracles. Sometimes these tubes are closed at the 
top and sometimes they are left open. For these facts, and for the specimen from 
which Figure 11 is taken, I am indebted to Mr. J. A. Brashear. 
Halemaumau, known also as " the pit," is the centre of volcanic activity in Kilauea. 
No eruption has ever been known to overflow the walls of the latter, although lava 
has sometimes been emitted from cracks located high up on its sides. When the pit 
of Halemaumau is emptied, it is always through some subterranean passage, occasion- 
ally reaching the surface, but usually either filling some subterranean cavity, or else 
discharging beneath the sea. These eruptions, though often accompanied by slight 
earthquake shocks, have in only one instance caused serious damage and loss of life. 
In this case it is thought that the active agency was really Mauna Loa, whose erup- 
tion took place at the same time. 
When Halemaumau is really active the sight is said to be grand beyond description, 
especially at night. Lakes of liquid lava occur both within and without it. Numer- 
ous fire fountains from ten to fifty feet in height play over the surface of these lakes. 
