306 PROCEEDINGS OF THE AMERICAN ACADEMY. 



part, at least, solidified as a vitreous rock, should have so altered the 

 granite-porphyry as to reproduce in it the characteristic contact 

 structures which it now possesses. 



An alternative hypothesis to the one proposed above, viz. that the 

 aporhyolite represents engulfed blocks of the original chilled roof, 

 might be that, the aporhyolite was first intruded, at a distinctly earlier 

 period into the relatively cold slates relatively near the surface, in the 

 form of great dikes, or more irregular masses, or even was extruded, 

 and that later followed the greater invasion of the main mass. In such 

 case the magma would be expected to have enveloped much of the 

 rocks first invaded by the rhyolite along with it, which is certainly 

 not the case. It must be frankly admitted that the view that it is 

 an earlier formed rock is not free from serious difficulties and the con- 

 clusion finally reached in any case is perhaps determined by a " choice 

 of evils", and the writer has chosen what appears to him to be the 

 lesser one, namely those which make the aporhyolite an earlier con- 

 solidated part of the alkaline intrusion. 



Cotisolidation of the Magma. — From the relationships previously 

 set forth we may conceive of the consolidation of the batholith having 

 taken place somewhat as follows : — at the highest levels, near the 

 surface, sudden chilling resulted in the formation of considerable 

 masses of highly vitreous rock; a little lower down, in the denser 

 forms of the feldspar-quartz-porphyry. These earliest consolidated 

 rocks were fractured and broken off by movements of the fluid mass 

 beneath, large blocks of them became immersed in the magma, per- 

 haps sunk, and against them the magma consolidated with sharp but 

 tightly sealed contacts showing a variable amount of local chilling. 

 Further cooling of the magma, still at relatively high levels, resulted 

 in the formation of a thick zone of feldspar-quartz-porphyry and gran- 

 ite-porphyry. This thick cover of still relatively hot rock, being a 

 poor conductor of heat, acted as an effective blanket, protecting the 

 magma underneath and permitting it to solidify with sufiicient slow- 

 ness to develop a truly granitoid texture. In places, perhaps very 

 generally, the magma beneath this porphyry-cover moved relatively, 

 and sometimes broke through, forming the granite dikes like those 

 noted as occurring in and about Slide Notch. As these dikes contain 

 numerous cognate xenoliths, — rhoml^enporphyry and fine-granite 

 types — and as these are also found in the granite directly underneath 

 the porphyry, as at Rattlesnake Hill, it appears that the magma, 

 wherever it remained hot and sufficiently fluid for some time, dift'eren- 

 tiated to a small extent yielding more l^asic phases, which were more 



