^72 R. A. Daly — Mechanics of Igneous Intrusion. 



and exceptional nature, to show that granitic magmas, if they 

 have " made their own way " at all, have usually done so in 

 some manner different from merely assimilating the invaded 

 formations on molar contacts. 



In a limited fashion, both theories may be accepted as giving 

 the required explanation of actual occurrences of igneous rocks. 

 But what is to be the final conclusion regarding the much 

 greater family of granitic bodies where the objections to both 

 the laccolithic and assimilation theories are felt in full force ? 

 They form a riddle for which the writer has been able to find 

 no solution in the existing literature of intrusion. As a con- 

 tribution to that old and difficult theme, he offers, in the 

 following pages, a brief outline of certain suggestions which 

 have come to him as a result of field study in New England 

 and Canadian areas of granitic formations.* For the sake of 

 brevity, the hypothesis to be proposed will assume a more gen- 

 eral character than that warranted by the discussion of any one 

 area, wherein local facts would have to be related with unneces- 

 sary detail. 



The Hypothesis of Overhead Stopinyf by Deep-seated Magmas. 



Believing that assimilation by magmatic action of some kind 

 is responsible for practically all the chambers occupied by those 

 intrusives with which he is more or less intimately acquainted, 

 the writer has sought for field evidence as to whether any 

 other sort of assimilation is possible than that by caustic or sol- 

 vent action of a magma on its roof and walls. Such informa- 

 tion is found in the same internal contact-belt where the 

 general failure to prove solutional absorption of the country 

 rock has been so often reported. Within that belt it is the 

 rule to find often very numerous blocks of the invaded rocks. 

 These have usually the following characteristics : varying size ; 

 angular or subangular outlines against the eruptive rock, which 

 is essentially unmodified even close to the contact with each 

 block ; sharp contacts with the eruptive, in which the blocks 

 are completely immersed ; a normally high crystallinity and 

 increased density as a result of contact metamorphism. Very 

 often they show that they have moved but short distances 

 from the niches they once occupied in wall or roof. The molar 

 contact is similarly sharp. It may preserve, with exceeding 

 definiteness, the sharp corners left when the blocks were rifted 

 off. Passing, now, inwards, it is an equally normal thing to find 



* A few paragraphs in the paper are identical with passages in a forth- 

 coming bulletin (No. 209) of the United States Geological Survey on the 

 " Geology of Ascutney Mountain, Vermont." 



f A technical mining term meaning to excavate upwards or sideways to 

 remove ore. 



