624 EXTERNAL FORM, STRUCTURE, AND ORIGIN [Oh. XXIX. 



immense chasm, 1000 feet deep, and its onter circuit no less than 

 from two to three miles in diameter. Lava is usually seen to boil 

 up at the bottom in a lake, the level of which varies continually, for 

 the liquid rises and falls several hundred feet according to the active 

 or quiescent state of the volcano. But instead of overflowing* the 

 rim of the crater, as commonly happens in other vents, the column of 

 melted rock, when its pressure becomes excessive, forces a passage 

 through some subterranean galleries or rents leading towards the sea. 

 Mr. Coan, an American missionary, has described an eruption which 

 took place in June 1840, when the lava which had risen high in the 

 great chasm began to escape from it. Its direction was first recog- 

 nized by the emission of a vivid light from the bottom of an ancient 

 wooded crater, called Arare, 400 feet deep and 6 miles to the east- 

 ward of Kilauea. The connection of this light with the discharge or 

 tapping of the great reservoir was proved by a change in the level Of 

 the lava in Kilauea, which sank gradually for three weeks, or until the 

 eruption ceased, when the lake stood 400 feet lower than at the com- 

 mencement of the outbreak. The passage, therefore, of the fluid 

 matter from Kilauea to Arare was underground, and it is supposed by 

 Mr. Coan to have been at its first outflow 1000 feet deep below the 

 surface. The next indication of the subterranean progress of the same 

 lava was observed a mile or two from Arare, where the fiery flood 

 broke out and spread itself superficially over 50 acres of land, and 

 then again found its way underground for several miles farther towards 

 the sea, to reappear at the bottom of a second ancient and wooded 

 crater, which it partly filled up. The course of the fluid then became 

 again invisible for several miles, until it broke out for the last time at 

 a point ascertained by Captain Wilkes to be 1244 feet above the sea, 

 and 27 miles distant from Kilauea. From thence it poured along for 

 12 miles in the open air, and then leapt over a cliff 50 feet high, and 

 ran for three weeks into the sea. Its termination was at a place about 

 40 miles distant from Kilauea. The crust of the earth overlying the 

 subterranean course of the lava was often traversed by innumerable 

 fissures, which emitted steam, and in some places the incumbent rocks 

 were uplifted 20 or 30 feet. 



Thus in the same volcano examples are afforded of the overflowing 

 of lava from the summit of a cone 2-J- miles high, and of the under- 

 flowing of melted matter. Whether this last has formed sheets inter- 

 calated between the stratified products of. previous eruptions, or 

 whether it has. penetrated through oblique or vertical fissures, cannot 

 be determined. In one instance, however, for a certain space, it is 

 said to have spread laterally, uplifting the incumbent soil. 



The annexed section of the crater of Kilauea, as given by Mr. Dana, 

 follows the line of its shortest diameter, a, 6, which is about 7500 feet 

 long. The boundary cliffs, a, c, and b, d, are for the most part quite 

 vertical and 650 feet high. They are composed of compact rock in 

 layers, not divided by scoriae, some a few inches, others 30 feet in 



