196 The American Geologist. September, i90i. 
tribution of the force that gave origin to the general metamor- 
phism. 
The southeastern flanks of the Mont Blanc massif differ 
from the western. Here is found a great mass of acid schists, 
characterized by satiny lustre, and by porphyritic quartz. 
From the description given this group is plainly quite like cer- 
tain Archean light colored sericitic schists which have been 
described in several places in Canada, and in northern Minne- 
sota. They vary from exceedingly fragile to firm and dense 
rocks. They are often distinctly bedded by sedimentary action, 
and they also form large bands and bosses that rise into ridges 
or hills of considerable size and endurance. They frequently 
become strewn with quartz crystals and by ortho-clase. They 
then resemble quartz porphyry, and have sometimes received 
that name.* They pass, however, to a more green porphyritic 
rock, more basic, but still a part of the schist series, and also 
become a fine mica-schist more or less felsitic. In general this 
series is described by the authors under the term "porphyres 
quartziferes," but they distinctly take note of the intercalations 
of schistose rock and of granulyte (p. 89). 
These schists and porphyries are allied to the schistose 
rocks of the Mont Blanc region rather than to the granite, 
and between them and the granite there is no known gradual 
passage. Yet the authors describe them from the view point of 
eruptive origin, and their schistose features they ascribe to 
dynamo-metamorphic crushing. They never contain vitreous 
matter, but quartz is developed as spongy grains embracing the 
other elements, as vermicular quartz and as micropegmatyte by 
entering into the feldspars. Of this secondary quartz there is 
a copious development. According to the authors these rocks 
under dynamic action take on such characters that they cannot 
be distinguished from certain horn schists, since the mica is 
drawn out and disposed in parallel trains, the quartzes are 
broken and the parts lie adjacent. In other cases the feldspars 
are similarly broken, and the rock has the aspect of a sericitic 
schist. 
The chemical similarity of these rocks with the granitic 
veins, which cut the granite, suggests to the authors (p 109) 
that they are from the same magma, but consolidated in two 
*A notable instance of this is discussed by the writer in Vol. iv of the 
final report of the Minnesota Geological Survey, pp. 525-539, 
