OF PRE-CAMBRIAN ROCKS 213 
site mass, and especially in the narrow extension running off from 
the southern part of the area and representing the stalk of the clover 
leaf, the rock becomes perfectly white in color, so closely resembling 
a saccharoidal marble that the two can be distinguished only on 
making a trial of their hardness. 
A detailed stratigraphical study of this mass proved undoubtedly 
that it was an igneous intrusion which had broken through the Gren- 
ville series, cutting off the hmestone bands of the latter instead of 
covering them up, and which had been subsequently granulated by 
great pressure brought to bear upon the whole region from the east, 
seeing that it is on that side that the mass is most intensely granulated, 
while on the west granulation is confined to the immediate border 
of the mass. 
With regard to the petrographic character of this anorthosite, 
it may be stated that it is composed almost exclusively of labradorite; 
augite, hypersthene, and a little iron ore being the other constituents. 
It is identical in character with most of the other anorthosite masses 
in the Laurentian of Canada. Hunt, in his early investigations of 
these rocks, stated that three-quarters of the anorthosite ooccurrences 
in Canada did not hold over 5 per cent. of minerals other than plagio- 
clase. 
It is to be noted that the foliation of the anorthosite, which is a 
secondary structure due to pressure, was already completed in pre- 
Potsdam times, for the perfectly foliated rock is overlain by horizontal 
beds of the sandstone in question. 
In addition to this great intrusion of anorthosite, there are twelve 
smaller intrusions similar in character occuring in the area mapped 
by Adams. 
In addition to these anorthosite intrusions, the sedimentary series, 
in the original area and in its extension to the east, is penetrated by 
several intrusions of more acid rocks. One of these consists of a 
mass of syenite which occurs in the township of Grenville and has 
an area of about thirty square miles, which is in its turn penetrated 
by a small body of feldspar porphyry, sometimes holding a small 
amount of quartz. 
Another acid intrusion is represented by the great body of granite, 
in places developed in the form of coarse augen gneiss, which occurs 
