386 The American Geologist. December, t90S. 
Grenville series? (2) lias there been an intrusion of later 
igneous material identical with the "fundamental gneiss"? and 
(3) Is the "fundamental gneiss" as a whole younger than the 
Grenville series, although hitherto supposed to be older? The 
author entertains only the first two of these, but to the reviewer 
the third query is equally worthy of consideration. 
The author seems to presume that the field evidences are 
not conclusive, and that a microscopical examination is nec- 
essary, coupled with chemical analyses, in order to confirm 
the field conclusions. The writer has elsewhere discussed the 
comparative value of field and microscopical evidence in the 
deduction of such general conclusions.* He is disposed to 
follow the field evidence, correctly observed, rather than the 
microscopical, when they seem to collide. But when, as in 
this case, they confirm each other, there is no reason for fur- 
ther investigation on the points involved, and, it may be added, 
the field structures that have been seen and the inferences de- 
rived from them, are probably equally valid for other similar 
cases of Archean geology. There is no question but the strat- 
igraphical methods of field geology have been reinforced in 
this instance by the careful microscopical examination of nu- 
merous thin sections. 
The specific object of the paper is two-fold: viz., (1) to 
determine the origin of the basic contact rocks by petrograph- 
ical methods, ( 2) to learn the true character of the seeming in- 
clusions by the saifie methods. As already stated, Dr. Ad- 
ams had already answered both these question by field studies; 
and, as also already stated, the author's results confirm the 
conclusions reached in the field. "These limestones are the 
same white or pinkish crystalline limestones or marbles every- 
where characteristic of the Grenville series, which have been 
described at length by Dr. T. Sterry Hunt and various other 
writers. * * :;: * Away from the intrusions before 
described the limestones are comparatively pure, though they 
sometimes contain bands of very dark hornblendic rocks or 
amphybolytes ; but approaching the igneous rocks they are 
found to contain little rounded grains of pyroxene and other 
lime-rich minerals, and in many cases to pass into banded basic 
rocks which warrant the field name of pyroxene gneiss. These 
* Fourth Annual Report of the Minnesota Survey (for 1891), pp. 18-22. 
