It can be shown that a column of liquid lava 500 feet high, will exert a pressure on the walls of the crater of about 625 pounds to the square inch. Therefore, in very high volcanic mountains, long before the lava can reach the edge of the crater and overflow, the pressure becomes so great, that cracks or fissures are made in the sides of the mountain, through which the lava is quietly discharged; when, of course, the level of the lava in the crater falls considerably. In volcanoes of the explosive type, no matter what may be the condition of lava, should a large quantity of water suddenly find an entrance to a large body of molten lava at some distance below the surface, the lava would be suddenly thrown explosively into the air, where being chilled, it would afterwards descend in showers of ashes, cinders, or volcanic dust. In some volcanic mountains such as Mt. Loa, the crater, instead of being situated at the top of a conical pass of ashes or other material, consists of a pit-like depression, generally occupying a level tract or plain at the top of the mountain. This pit is known as a _caldera_, or _caldron_, or what you might, perhaps, call a huge kettle or boiler. The pit has more or less vertical sides that extend downwards for unknown depths to the place from which the lava comes. The vertical walls of the caldera are not, however, smooth, but exhibit numerous horizontal ledges, that mark places where portions of the floor of the caldera were situated at different times. At the bottom of the large pit or caldera on the summit of Mt. Loa can be seen the level floor formed of hardened lava. This floor is surrounded by vertical walls on which can be seen the broken edges of the old lake bed. Captain Dutton, in a paper on Hawaiian volcanoes, prepared for the United States Geological Survey, and published in its Fourth Annual Report for 1882-83, thus describes the appearance at the great crater as it was in 1882.