1884.] The Crystalline Rocks of the Northwest. 989 
generally consists largely of orthoclase, and in several instances 
passes imperceptibly into red felsite. It contains also quartz and 
hornblende, the latter generally changed by decay. The gabbro 
when unaffected by proximity to the red rock, consists of the 
three essential ingredients, labradorite, diallage and magnetite, 
with some necessary products of alteration, but in the vi- 
cinity of contact with the red rock it also holds orthoclase and 
quartz, 
Il. Below this granite and gabbro group is a series of strata 
that may be designated by the general term mica schist group. 
This is the principal, but not the only, horizon in which mica 
schist exists. This division is penetrated by veins and masses of 
red biotite-granite which appear to be intrusive in somewhat the 
same manner as the red granite in the gabbro overlying. How- 
‘ver, whether this granite is exotic, or can be referred to aqueo- 
igneous fusion and transmission of the sedimentaries in a plastic 
State through fissures in the adjacent formations, is a question 
which still is a matter of earnest investigation. The existence of the 
great associated igneous gabbro is suggestive, if not demonstrative, 
of the presence of an adequate agent for such a metamorphism— 
unless it be claimed, indeed,that such an extravasation of molten 
rock could take place without any marked and traceable effect on 
the contiguous formations. These granite veins penetrate only 
through the overlying gabbro and this underlying mica schist. 
They are wanting or comparatively rare throughout the rest of 
the crystalline rocks. On the other hand there is an abundance 
of diabase and other doleritic rock, in the form of dykes, through- 
Out all the crystalline strata. This points to the mere local nature 
of the Origination of these granitic veins, and hence to the meta- 
Morphic nature of the granitic mass with which they are con- 
nected, It has been shown by Dana that granite suffers a change 
to mica schist in Western Massachusetts; Brooks as well as Em- 
Mons has shown it interstratified with limestone in St. Lawrence 
county, New York. They both also state that the Potsdam sand- 
stone becomes gneissic. The same has been affirmed in Ver- 
mont by Dr. Hitchcock, and by Dr. Frazer in Pennsylvania, 
Hence there is no impropriety in supposing that some great 
change has passed over the sedimentary strata of this horizon 
throughout a wide extent of country reaching from the Atlantic 
to Lake Superior, and that in the emergences of upheaval and 
