R. A. Daly — Mechanics of Igneous Intrusion. 49 



iar as the comparative rarity of quartz-veins and pegmatites 

 about gabbroid masses and the comparative feebleness of the 

 cuntact-metamorphism produced by gabbros. The abundant 

 water found in obsidian and rhyolite is, in this view, largely 

 or wholly of secondary origin. Volcanic gases may similarly 

 be largely "resurgent*' rather than "juvenile." In no case, 

 however, would one class of emanations be represented to the 

 exclusion of the other. For post-Archean granites the emana- 

 tions are dominantly " resurgent" : for gabbros the emanations 

 are largely or dominantly - juvenile." 



Conclusion. — The first two papers of this series were writ- 

 ten in the light of experimental results bearing on the methods 

 of igneous intrusion. Since 1903 a number of additional 

 leading experiments according to refined method- have been 

 carried out by Doelter and his colleagues, by the Geophysical 

 Laboratory staff at Washington, by Bran, Gautier. Hall. Doug- 

 las. Ladenburg and others. These later investigations, like 

 those of Deville, Bischof. A. Becker. Lehmann, Fouque. 

 Michel Levy. Cossa, Thoulet, Barns, Oetling. Hofman, Tarn- 

 mann. Morozevricz. Forbes. Joly. Mallet, Eeade. Cusack, 

 ^Veber. Akerman, Yogt, Bartoli, Jamin. Lagorio, and others. 

 seem to show that the physical conditions and processes 

 involved in the stoping hypothesis have been in the main 

 correctly stated. 



It is obvious that further laboratory study of rocks on the 

 physico-chemical side is highly desirable, but the accordance 

 of independent experimental results now on record appears to 

 have demonstrated : first, the enormous efficiency of thermal 

 expansion in causing shattering stresses in solid rock : secondly. 

 the fact that the average xenolith must sink in molten granite, 

 syenite, diorite and acid gabbro when these magmas are under 

 ordinary plutonic conditions : thirdly, that the sunken xeno- 

 liths must melt or become dissolved in the depths of plutonic 

 magma, forming syntectic magma ; and fourthly, that, if the 

 primary magma is basic, the average syntectic must rise through 

 it and thus collect at the top of the magmatic chamber. 



The attempt has been made, in using the experimental 

 results of Barns, Boberts-Austen and Biicker. Weber, Bartoli. 

 Akerman and Vogt, to estimate the amount of average crust- 

 rock (gneiss) which may be dissolved in one volume of super- 

 heated primary magma (basalt). The sources of superheat in 

 plutonic magma and a rough quantitative analysis of abyssal 

 assimilation have been discussed. The result points to an 

 explanation of the fact that magmatic stoping has not destroyed 

 the roofs of post-Archean batholiths. The more general prob- 

 lem of the stability of batholithic covers which, on any the- 

 ory of magmatic intrusion, seem to be in danger of foundering 



Am. Jour. Sci. — Fourth Series. Vol. XXVI. No. 151.— July. 1908. 

 4 



