Ch. XXIX.] OF VOLCANIC MOUNTAINS. 491 



Thus in the same volcano examples are afforded of the overflowing of 

 lava from the summit of a cone 2^ miles high, and of the underflowing of 

 melted matter. Whether this last has formed sheets intercalated between 

 the stratified products of previous eruptions, or whether it has penetrated 

 through oblique or vertical fissures, cannot be determined. In one in- 

 stance, however, for a certain space, it is said to have spread laterally, 

 uphfting the incumbent soil. 



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

 follows the line of its shorter diameter, a, b, which is about 7500 feet 



Fig. 641. 





e 



/ 



7> 



, 



(A 





b 



M 





Section of the crater of Kilauea in the Sandwich Islands. (Dana.) 



a, h. External boundaries of the chasm in the line of its shortest diameter. 

 c, e, f, d. Black ledge. g, h. Lake of lava. 



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 scorise, some a few ioches, othei's 30 feet in 

 thickness, and nearly horizontal. Below this, we come to what is 

 called the " black ledge," c, e and /, d, composed of similar stratified 

 materials. This ledge is 342 feet in height above the lake of lava, g, A, 

 which it encircles. The chasm, a, 5, and its walls have probably been 

 due to a former sinking down of the incumbent rocks, undermined for 

 a space by the fusion of their foundations. The lower ledge, c, e and 

 /, c?, may consist in part of the mass which sank vertically, but 

 part of it at least must be made up of layers of lava, which have been 

 seen to pour one after the other over the " black ledge." If at any 

 future period the heated fluid, ascending from the volcanic focus to 

 the bottom of the great chasm, should augment in volume, and, before 

 it can obtain relief, should spread itself subterraneously, it may melt 

 still farther the subjacent masses, and, causing a failure of support, 

 may enlarge still more the limits of the amphitheatre of Kilauea. 

 There are distinct signs of subsidences, from 100 to 200 feet perpen- 

 dicular, which have occurred in the neighborhood of Kilauea at various 

 points, and they are each bounded by vertical walls. If all of them were 

 united, they w^ould constitute a sunken area equal to eight square 

 miles, or twice the extent of Kilauea itself Similar accidents are also 

 likely to occur near the summit of a dome like Mount Loa, for the 

 hydrostatic pressure of the lava, after it has risen to the edge or lip 

 of the highest crater, «, fig. 640, must be great and must create a ten- 

 dency to lateral fissiu'ing, in which case lava wnll be injected into every 

 opening, and may begin to undermine. If, then, some of the melted 

 matter be drawn ofl:' by escaping at a lower level, where the pressure 



