J. D. Dana — History of the Changes in Kilauea. 29 



dominant feldspar, and Trachyte and Rhyolyte volcanoes in 

 which orthoclase is a chief constituent ; and, besides these, 

 there are also intermediate grades or kinds. 



The differences in form and action among these kinds of 

 volcanoes depend chiefly on the physical quality of fusibility, 

 but partly on that of specific gravity. 



Neither of these qualities, it is to be noted, has any relation 

 to the acidic or basic character of the feldspar or rock, that is 

 to the amount of silica present. The distinction of basic and 

 acidic, of great interest mineralogically and chemically, has in 

 fact little importance in the science of volcanoes, while that 

 of fusibility is fundamental. The most basic of all the felds- 

 pars, anorthite, is as little fusible as the most " acidic" of 

 feldspars, orthoclase, and more so than the equally ""acidic", 

 albite.* It is plain therefore that the quality of being basic, 

 does not explain the fusibility of the lavas. Neither does it 

 explain any other of the physical characteristics on which the 

 peculiarities of the volcano depend. 



It is also true that the chrysolite (or olivine), the ultra-basic 

 constituent of the lavas, has little influence on their physical 

 characters except through its high specific gravity — which is 

 about 3 - 3 to 34. The mineral chrysolite is infusible, and 

 cannot increase the mobility of the lavas ; and there is 

 commonly not enough of it in the Kilauea rocks to diminish 

 the mobility; for a large part of the lava contains less than 5 

 per cent, and much of it less than - 1 per cent. Chrysolite, is 

 ultra-basic ; but this quality has little volcanic importance. 

 It is not the little amount of silica in it that is influential 

 volcanically but the much iron, the ingredient that gives it its 

 high density or specific gravity. The presence of much 

 chrysolite may affect the distribution of the lavas in the 

 conduit, or of the out-fiows from the conduit, on account of 

 their high density ; but it does not accomplish this through the 

 ultra-basicity of chrysolite, but through its ultra-ferriferous 

 character, and the conditions under which it is formed. 



3, The degree of mobility is dependent also on temperature. 

 — It is probable, that at the temperature of fusion, or better 

 a little above it, all the feldspars, the least and the most fusible, 

 are nearly alike in mobility. But the lower the degree of 

 fusibility the .less likely is the heat to be deficient, or below 

 that required for complete fusion and mobility ; and here 

 comes in the great difference among them as regards lavas and 

 volcanoes. 



The basalt-volcano has special advantage over all others in 

 this respect, as the copious Mount Loa lava-streams and the 



* In my Manual of Mineralogy and Petrography, page 436, I point out further 

 that the distinction of alkali-bearing and not alkali-bearing among the silicates is 

 of much more geological importance than the much used one of acidic and basic. 



