490 EXTERNAL FORM, STRUCTURE, AND ORIGIN [Ch. XXIX. 



Fie. 640. 



Mount Loa, in the Sandwich Islands. (Dana.) 



a. Crater at the summit. b. The lateral crater of Kilauea. 



The dotted lines indicate a supposed column of solid rock caused by the lava consolidating after 



eruptions. 



on an average from 4 degrees to 8 degrees ; but in some places consider- 

 ably steeper. Sometimes deep rents are formed on the sides of these 

 conical mountains, which are afterwards filled from above by streams of 

 lava passing over them, the liquid matter in such cases consolidating in 

 the fissures and forming dikes. 



The lateral crater of Kilauea, 6, fig. 640, is 39*70 feet above the level 

 of the sea, or about the same height as Vesuvius. It is an immense 

 chasm, 1000 feet deep, and its outer 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 alters 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 recognized by the emission of a vivid light from the 

 bottom of an ancient crater, called Arare, 400 feet deep and 6 miles to 

 the eastward 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 21 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. 



