R. A. Daly — Mechanics of Igneous Intrusion. 293 



basalt. On the oceanic areas, the staple visible eruptive is the 

 basic, and necessarily volcanic, basalt or allied augite andesite. 

 These facts of relative volume in igneous output agree with 

 the views derived from chemical considerations long ago by 

 Bunsen,* and more recently by Michel Levyf and others. 

 Two fundamental magmatic types, the alkaline granitic and 

 the ferromagnesian, dominate among the world's igneous rock- 

 types. The fact that they do exist is independent of theory. 

 Certain other facts point to the conclusion that the supply of 

 the ferromagnesian magma available for eruptive purposes is 

 much greater than that of the alkaline magma. 



In those conduits where the escape of igneous magmas from 

 the earth's interior to the surface takes place to such an extent 

 as to build large volcanoes, we should expect the sequence of 

 eruption to be completed by effusions of lava more nearly 

 representing the original or primary magma than the antece- 

 dent flows. The reasons for this are : first, that assimilation in 

 the immediate vicinity of the vent would, in that late stage in 

 the development of the volcano, have progressed so far as to 

 have enlarged the conduit to a size suitable to a large cone ; 

 secondly, that the vent would, by the long continuance of the 

 volcano's activity, have become freed from the products of the 

 assimilation ; and, thirdly, that the latest flows would be 

 derived from the original magma practically unaffected by 

 assimilation. Now, it is a significant fact that the latest 

 extrusive product of the great majority of the largest volcanoes, 

 such as Etna, Fusiyama, Chimborazo, Cotopaxi, Kilimanjaro, 

 etc., is, so far as known, either basalt or augite andesite. 



Not less important is the equally indisputable fact that the 

 greater fissure-eruptions of the globe give birth to only one kind 

 of lava, again basaltic. The familiar examples in Iceland, 

 northwestern Europe, India, the northwestern United States 

 of America, and the Hawaiian archipelago, tell no uncertain 

 story concerning the nature of the vast reservoirs from which 

 they have derived their enormous volumes of lava. The more 

 acid flows which occur in any of these regions are insignificant 

 in bulk compared with the total basic output. The question 

 is quite open whether the former are not the product of differ- 

 entiation acting on the primary basaltic magma influenced by 

 the assimilation of the continental rocks, which are characteris- 

 tically more acid. Further, we should expect assimilation to 

 be less active in determining the composition of fissure eruptives 

 than in preparing the secondary magmas erupted in volcanic 

 cones or injected in intrusive forms. From the nature of the 

 geological dynamics rendering possible the rapid expulsion of 

 *Pogg. Annalen, lxxxiii, 197. f Op. eit., p. 368. 



