43S W. T. BRIGHAM ON THE VOLCANIC PHENOMENA 



the deeper it burns into the subjacent rock, acting, if I may use the comparison, like a 

 soldering iron which may often be held against a stick of solder for some time without 

 melting it, when the motion of rubbing would liquefy the metal at once. Examples have 

 been mentioned where this action of a rapidly moving stream has melted into the lava beds 

 over which it passed, to the depth of even two hundred feet. This was the case in the long- 

 continued flow of 1855 from Mauna Loa, and the apparent fissures are often as much the 

 result of this action as of a forcing apart of the mountain side, and they often mark the site 

 of a future valley. 



Each overflow from the central vent will form a rib or buttress covering a segment of the 

 mountain more or less extensive, imparting a vast strength to the walls and permitting the 

 rise of the lava within, until its pressure breaks through the walls and escapes in a lateral 

 eruption. These side outflows are of frequent occurrence in various volcanic domes, and 

 may arise from a rending of the whole mountain, — or the opening of a small fissure; in 

 the former case strengthening the wall by an interlacing dyke, in the latter usually render- 

 ing it cavernous. All the recent discharges from Mauna Loa have been of the latter kind, 

 and occur at nearly the same elevation. A small fissure opens, which gradually widens as 

 the flow progresses, until the stream leaps forth in its full strength. It is a noticeable fact, 

 that never has the rending of the mountain been perceived by earthquakes or tremblings. 

 " A small beacon fire " announces the opening of a small crack, which opens as gently as the 

 cracks in drying clay. 



The eruption of 1840 from Kilauea made a more extensive crack, and no subsequent 

 eruption has taken the same direction. 



Within the mountain the tube of lava which supplies the summit crater, protected in 

 great measure from a loss of heat by the outer wall, melts during its periods of activity 

 into the sides of the mountain, becoming a conical reservoir of lava which will have a 

 greater or less diameter at its base in proportion to its activity. 



It is probable that a mass of melted lava, fluid and comparatively quiet, will experience a 

 change in composition at different levels, by a separation of the different minerals mechan- 

 ically united in its mass, according to their different specific gravities. Thus trachyte and 

 the feldspar lavas, will, owing to their low specific gravities, rise to the top, and the heavier 

 augitic lavas fall towards the bottom. The specific gravity of trachyte is 2.4 - 2.62, while 

 that of augite is 3.23-3.5, a difference quite sufficient to cause a separation if other 

 circumstances admit. 1 That the top of the cone will be all trachyte and the bottom all 

 basalt must not be supposed, for the constant currents prevent a complete separation, and 

 all varieties and all proportions of admixture will be found at different heights. This is the 

 view taken by Mr. Darwin, 2 as well as by Prof. Jukes and Mr. Scrope. 



After a separation of tins nature, which doubtless requires time, if the lavas rise and over- 

 flow the edge, trachytic streams will be poured out ; and in support of this view we find that 

 all the summits of the Hawaiian volcanoes are more or less trachytic, or at least feldspathic. 



1 See Scrope on Volcanos, p. 110. Mica (hexagonal) . . .2.7 — 3.1. 



The specific gravity of volcanic minerals is as follows : — Hornblende . . . . 2.9 — 3.4. 



Orthoclase^ . 2.4 —2.62. Augite 3.23 —3.5. 



Albite [-Feldspar . . 2.59 -2.65. Garnet 3.15 —4.3. 



Oligoclase) . . . 2.58 —2.69. Olivine 3.33 —3.5. 



Leucite 2.483 — 2.49. Haematite .... 4.5 — 5.3. 



Quartz .... 2.5 —2.8. 2 Volcanic Islands, pp. 118-124. 



