Ie REGINALD ALDWORTH DALY 
of the constituents of the older rock will become mobile and 
will tend to assume the same structure as those of the second 
period of consolidation in the younger rock. 
McMahon pointed out how difficult it was to explain the pres- 
ence of zones of transition about the Dalhousie granite of the 
Himalayas in some places and their absence in others on the 
old metamorphic theory of the central gneiss. He favored 
the opinion that, whether a zone of passage characterizes the 
contact or not, depends on the closeness of mineralogical sim1- 
larity between the Dalhousie granite and the invaded strata. 
Only in places where the latter had been regionally metamor- 
phosed did he find the appearance of gradual change from the 
granite into the country rock. The transition zone described 
by Lawson between the Laurentian gneiss and hornblende-schist 
in Rainy Lake region is so similar to the zones of the porphyritic 
granite that it will be well to read his own words on the subject: 
“Within the hornblende-schist, distinctly recognizable as such, 
there may occasionally be detected large crystals of red feldspar, 
which is quite foreign to these rocks, as if the feldspathic magmas 
had penetrated within the schist and crystallized there in the 
same large crystals in which they are wont to appear in the 
coarse gneiss.” 
These authentic determinations of transgressive junctions 
between plainly eruptive rocks and their respective country 
rocks, coupled with the expectation that they should appear in 
contacts of that nature, lead us to follow Gregory ? in concluding 
that they may form a useful criterion for a decision on an 
eruptive origin for massive rocks. So far as this principle is 
concerned then, the porphyritic granite can be eruptive. 
The foregoing description of contact phenomena seems to 
us to indicate that the porphyritic granite has been intruded into 
the schistose rocks which have been grouped together in the 
terrane of the Lake Winnipiseogee gneiss by the survey. This 
conclusion is forced upon one as well in the study of the northern 
tAnn. Rep. Ceol. Surv. Canada, 1887-8, Part. F. p. 33. 
JOM Geos SocamSo4nps2o2% 
