Russell — Massive-Solid Volcanic Eruptions. 263 



the table, the rocks are arranged according to their degree of 

 fusibility ; the most refractory being the rhyolite, and the most 

 fusible the basalt. The known massive-solid eruptions 1, 2 and 

 3 are thus shown to consist of refractory lavas. On the other 

 hand, basalt, which so far as known has not been extruded in. 

 a massive-solid condition, is more readily fusible, and in the 

 case of the example cited, as recorded elsewhere,* was poured 

 out in a highly fluid condition. From the data in hand it 

 appears, therefore, that one of the characteristics of the rocks 

 known to occur in massive-solid extrusions is their refractory 

 nature. That this is always a characteristic condition, however, 

 cannot as yet be definitely affirmed. Another related condi- 

 tion is the amount of water-vapor present, since aqueo-igneous 

 fusion is known to require much less heat than dry fusion. 

 The bearing of this principle on the occurrence of massive- 

 solid volcanic eruptions, although seemingly of fundamental 

 importance, cannot be discussed at this time. Assuming, how- 

 ever, that the influence of water- vapor on the fusion of acid 

 and basic lavas is the same, it is evident, from the data given 

 above, that the former should occur more commonly than the 

 latter, in massive-solid extrusions. 



In addition to the presence of a plug of rigid lava in the 

 summit portion of a volcanic conduit, in order to bring about 

 a massive-solid extrusion, it is evident that there must also be 

 sufficient pressure on the base of the plug to force it out. Such 

 pressure, as we know, is present during the eruptions of many 

 and we presume all volcanoes which discharge lava. The critical 

 or determining conditions, which lead to massive-solid eruptions, 

 seem to be that the lava in the summit portion of a volcano in 

 action shall become solid. The lavas most apt to solidify at 

 such times are evidently those which are refractory and on cool- 

 ing pass quickly from a fluid to a rigid condition ; that is, the 

 lavas rich in silica and alumina and relatively poor in basic 

 substances. Hence as it seems, it is to be expected that massive- 

 solid extrusions will consist of rocks like the rhyolites, trachytes, 

 andesites and phonolites, rather than basalts or still more basic 

 lavas. 



Judging from the heated conditions of the material extruded 

 during a massive-solid eruption at the time it rises into the air, 

 and reasoning also from the known variations in the physical 

 and mineraiogical features of igneous rocks which depend on 

 the conditions under which they solidify, we should expect the 

 lavas extruded in a massive-solid condition to present at least 

 three leading physical characteristics : they should be (1) com- 

 pact, or at most but moderately vesicular, and not scoriaceous, 

 (2) granular or perhaps finely crystalline but not glassy, and 



* Israel C. Bussell, U. S. Geological Survey. Bulletin No. 199, Washington, 

 1902, p. 88. 



