B. A. Daly — Mechanics of Igneous Intrusion. 297 



degree by such facts, than the theory recognizing caustic assimi- 

 lation as occurring merely or chiefly on main vault contacts. 



4. The hypothesis is supported by a survey of the facts of 

 magmatic differentiation. It is believed, further, to give the 

 best explanation of the petrogenic cycles yet found illustrated 

 in nature, and to explain with special force the normal eruptive 

 sequence. At the same time, the hypothesis is not weakened 

 by the known cases of the reversal, or other modification, of 

 that order of eruption. 



5. The hypothesis accords with the facts known with regard 

 to other kinds of igneous intrusion. Even in the case of 

 those great granitic massifs organically associated with master- 

 lines or zones of dislocation (e. g., the tonalite and the " Judi- 

 carienlinie " of the Tyrol), the magma chamber may have been 

 principally opened by overhead stoping. The same process 

 may similarly enlarge the deep-seated cross-section of a vol- 

 canic neck. Yet no one can deny its practical insignificance in 

 the intrusion of sheets or dikes, nor, for obvious reasons, does 

 that fact injure the strength of the proposed hypothesis when 

 dealing with much larger igneous bodies. The latter must be 

 longer molten by reason of their size and direct communica- 

 tion by means of convection and other currents with the earth's 

 interior. The same remark made concerning dikes and sheets 

 applies also to the proved laccoliths, although it is probable 

 that, in a very limited way, laccolithic magmas may carry on 

 stoping and therewith assimilation in their hot interiors. Dikes, 

 sheets and laccoliths are but offshoots of the greater magma- 

 basins with which the hypothesis is concerned. 



Among the further tests needed comes first, perhaps, that 

 for further experimental investigation of molten rock magmas 

 under high pressures. It should be particularly determined 

 whether the carbon and silicon compounds show analogous 

 behavior under high pressures, as they certainly do at ordinary 

 pressure and at temperatures below, at, and above their respec- 

 tive melting-points. Fusion experiments additional to those of 

 Barus and covering a range of igneous rock types and at vary- 

 ing high pressures, would be still more valuable than the indi- 

 rect method of comparison between hydrocarbons and silicates. 



General Summary on the Mechanics of Intrusion. 



In the foregoing pages, the writer has briefly compared the 

 hypothesis of overhead stoping in the formation of magma 

 chambers with the " laccolithic " theory of crustal displace- 

 ment and with the theory of marginal assimilation of invaded 

 formations. Since the question is intimately connected with 

 that of the origin of the igneous rocks, it might be considered 



