304 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and immersion of, the broken blocks in tlie yet unconsolidated magma 

 beneath. The writer is strongly inclined to believe that the masses of 

 aporhyolite in like manner represent masses of the first consolidated 

 portions of tlie invading magma, broken up and partially or perhaps 

 wholly immersed in their own magma. R. A. Daly, as a result of his 

 acquaintance with the region in question, has suggested tentatively that 

 these masses of aporhyolite may be foundered blocks of the original 

 roof rocks. ^^ The fuller knowledge of the characters and relationships 

 of the rocks of the region which we now ha\-e, supports the view^ tluit 

 the aporhyolite represents portions of the invading magma originally 

 consolidated very near the surface, at least. This view of the origin 

 of the aporhyolite brings us into direct opposition to Professor Crosby's 

 conclusions and the question of its relative age must be examined 

 more in detail. The characteristics of the porphyry at the contact 

 with the aporhyolite have been described and shown to be everywhere 

 indicative of a chilled contact and this fact was fully recognized by 

 Crosby. He offered two hypotheses as to the nature of the aporhyo- 

 lite. First, that it is in the main truly intrusive through successive 

 zones of the batholith and interformational with reference to the slate 

 and contact rocks; and secondly, that it is truly effusive. The first 

 as stated by Crosby is as follows: ®° " Hence we seemed forced to the 

 conclusion that after the development of the contact zone of quartz- 

 porphyry in the usual manner the extrusion of the magma now 

 represented by the felsite" (aporhyolite) "took place in such a way 

 as to form, not great dikes extending up through the Cambrian strata, 

 possibly to the surface, but rather a laccolithic accumulation between 

 the contact zone and the Cambrian cover, with a bentling down or 

 falling in of the edges of the contact or porphyry zone sufficiently 

 marked to account for the great width of the dike, for the narrowness 

 of the porphyry zone, for the fact that in spite of very imequal erosion 

 the felsite is nowhere found in contact with the granite, and for the 

 high incHnation of the felsite-porphyry contact. ..." "A sill ^^ ex- 

 tending south from a laccolithic trough between the contact zone 

 of the batholith and its Cambrian cover would, perhaps, best express 

 the idea." 



The second hypothesis, and the one favored by Crosby, is stated by 

 him as follows : ^^ — . . . , " the felsite is truly effusi^-e, post-dating .... 



59 These Proceedings, 47, No. 3, p. 62 (June, 1911). 



60 op. cit., p. 386. 



61 op. cit., p. 387. 



62 op. cit., p. 387. 



