JR. A. Daly — Mechanics of Igneous Intrusion. 269 



Art. XXIX. — The Mechanics of Igneous Intrusion; by 

 Keginald A. Daly. 



CONTENTS. 



Introduction 269 



The ' ' laccolithic theory ' ' of plutonic intrusion 269 



The ' ' marginal assimilation " theory of plutonic intrusion. 270 



The Hypothesis of Overhead Stoping by Deep-seated Magmas _ 272 



The relative densities of holocrystalline, glassy, and molten rock 



at ordinary atmospheric pressure 273 



Influence of plutonic pressures on rock density 278 



Relative densities of solid and molten rocks under plutonic con- 

 ditions . 279 



Fluidity of plutonic magmas 281 



Enclosures of foreign rocks in the endomorphic zones of intrusives 280 



Overhead stoping in magmas of high fluidity. 282 



The opening of the magma chamber _ 282 



Tests of the Hypothesis of Overhead Stoping 285 



Testimony of laccoliths 285 



Abyssal assimilation _ 287 



Formation of compound magmas by abyssal assimilation 290 



Differentiation of compound magmas 290 



The eruptive sequence _ 292 



Summary _ 295 



General Summary on the Mechanics of Igneous Intrusion 297 



The problem of the origin of the igneous rocks is intimately 

 associated with the need for clear definition of all the possible 

 methods according to which chambers now occupied by intru- 

 sive bodies may have been prepared to receive their respective 

 magmas. Then the recognition of the relative importance of 

 these methods, in explaining the vast number of such chambers 

 now exposed to the light, becomes a second step in the direc- 

 tion of solving the genetic problem. However, those rock 

 bodies — dikes, sheets or sills and true laccoliths — about the 

 intrusion mechanics of which there is a fair consensus of opinion 

 among geologists, are certainly the least notable as to volume 

 among the intrusive masses of the earth. The enormously more 

 important stocks and allied greater granitic bodies to this day 

 form the subject of controversy which, though well regulated 

 by fact, is as vigorous as in the days of Hutton and Werner. 



The " laccolithic " theory of plutonic intrusion. — One school 

 of geologists would extend the laccolithic idea to many, if not 

 most, granitic intrusions. Accordingly, the chambers filled 

 with such igneous masses are interpreted as the products of 

 crustal displacement. The planes of single great faults may, 

 in this way, become the locus of the subterranean eruption of 

 magmas, wedging their way along by hydrostatic or other pres- 

 sure. The well-known "failure to match " of the heaved and 

 thrown sides permits of the existence of potential cavities filled 



