62 Adams—Contribution to our knowledge of the Laurentian. 
In view, however, of the continuously banded character of these 
rocks over very wide areas and the well-defined character of 
the individual bands this is an extreme supposition and one 
which as will be shown is rendered yet more untenable by the 
chemical composition and structure of the rocks themselves. 
The fact, moreover, that in the movements induced by pressure 
and resulting in the foliation of the complex, the more basic 
and less plastic bands are continually observed to be torn apart 
and resolved into a series of disconnected fragments, shows 
that if the banding was produced by pressure in the first case, 
subsequent pressure acting in the same direction tended to 
destroy it. 3 
Petrography.—Our knowledge of the petrography of this 
area is based on careful studies in the field, as well as upon a 
careful microscopical study of one hundred and sixty typieal 
specimens representing so far as possible all varieties of the 
rocks occurring in the district. . 
Jt is found that the rocks fall into four classes: 
1. Anorthosites and granites, of igneous origin. 
2. Augen gneisses, leaf gneisses, granulites and foliated 
northosites, genetically connected with the last group and 
largely if not exclusively of igneous origin also. The structure 
characteristic of this class is the cataclastic or granulated struc- 
ture, formed by the mechanical breaking down of the web of 
the rock under movements induced by great pressure, which 
movements produce in the rock a foliation more or less distinct 
according to their intensity. By leaf gneisses are understood 
very finely foliated gneisses very rich in orthoclase and contain- 
ing numerous thin leaves of quartz—they are usually almost free 
from iron-magnesia constituents. 
3. A series of crystalline limestones and quartzites, together 
With certain gneisses usually found associated with them, and 
which are probably wholly or in part of sedimentary origin. In 
‘these rocks the granulated structure is very subordinate or 
entirely absent. They are characterised by a very extensive 
recrystallization with the development of new minerals, they also 
differ from the rocks of classes 1 and 2 in chemical composition. 
4, Pyroxene gneisses, pyroxene granulites, and allied rock 
whose origin is as yet doubtful. : . 
It appears, therefore, that the great pressure which has 
squeezed and foliated these rocks has acted in two ways, it has 
granulated one class and recrystallized another. 
The granulated rocks of class 2 are prevailing poor in iron- 
magnesia constituents, being composed either of plagioclase on 
the one hand or of quartz and orthoclase on the other, that is to 
say being either granulated anorthosites, or granulated granites 
poor in mica or hornblende. These sometimes occur as inde- 
