270 7?. A. Daly — Mechanics of Igneous Intrusion. 



with magma during the strong dislocation. Encircling faults 

 leading to the foundering of large blocks of the crust, or to the 

 upward thrust of others, are conceived as affording possible 

 modes of intrusion.* Or, finally, as illustrated in the well- 

 known conclusions of Brdgger on the Christiania region, colos- 

 sal masses of granite have been explained as true, deep-seated 

 laccoliths, parting heavy strata after the manner of the trachyte 

 of the Henry Mountains, f Yet it is clear from a survey of 

 geological literature, that the field evidence for such a view is 

 but negative in the great majority of stocks and so-called 

 " batholiths." Most of them are not true laccoliths, as they 

 characteristically occur in regions of great structural complexity, 

 where igneous contacts have none but the most remote sympathy 

 with the structural planes of any one bedded series. Many are 

 much too large and irregular in form to be explained as the 

 result of single faults or single zones of faulting ; and the 

 imagined intersecting faults of the " bysmalith " or of the sub- 

 merged graben-block have been generally sought for in vain 

 about those greatest of all granitic massifs. For the latter no 

 other interpretation seems possible by the theory outlined. On 

 the other hand, every observer who has even a small acquaint- 

 ance with crystalline terranes of the sort, is now and again 

 struck with the evidences that the granitic magmas represented 

 in his field of study were far from being passive in the hands 

 of the eruptive Titan. Their general defiance of structure and 

 composition in the invaded formations, the irregular ground- 

 plans, and the huge finger-like projections sent into the country- 

 rocks,;); which are undisturbed either in dip or strike, are among 

 the familiar phenomena indicating that such magmas actively, 

 aggressively, " made their Way in the world " by the irregular 

 removal of the invaded formations. The latter look as if they 

 had been, as it were, corroded on a huge scale. 



The " marginal assimilation " theory of plutonic intrusion. 

 — Emphasis has, therefore, been laid by a second school of geol- 

 ogists on a hypothesis of slow caustic action by magmas that 

 have advanced into the overlying earth-crust by their own 

 energetic solvent action on their walls and roofs. Additional 

 evidence for the truth of this contention is sought in the facts 

 of the internal contacts, at which magmas are sometimes seen 

 to be modified by the incorporation of the country-rock. This 



*W. C. Brogger, Die Eruptivgest. des Kristianiagebietes, vol. ii, 1895, p. 

 148 ; J. P. Iddings, U. S. Geol. Surv., Monograph xxxii, Part 2, 1899, p. 16. 



fOp. cit., p. 152. 



% The author will use this term throughout the paper as a synonym for 

 " invaded formation," meaning the rocks cut by, and in contact with, an 

 intrusive. Mining terminology also gives " roof" and " wall" for the upper 

 and lateral surfaces of contact between intrusive body and invaded formation. 

 This temporary appropriation from a kindred science will prove of conven- 

 ience in the following discussion. 



