50 AN INVESTIGATION INTO THE 
BASIC PLUTONIC ROCKS. 
ANORTHOSITE, NEW GLASGOW, PROVINCE OF QUEBEC, CANADA. 
This rock is from the great Morin anorthosite intrusion which occupies 
an area of 990 square miles on the border of the Laurentian protaxis, 
some 30 miles north of the city of Montreal.* The specimen is from the 
margin of the intrusion, where the mass has undergone extensive movement 
of the nature of rockflow, which movement has been brought about by 
pressure exerted upon the earth's crust in this district. The flow has 
taken place through a granulation of the larger individuals of the original 
rock, combined with a movement of this granulated material under the 
influence of the pressure, giving rise to a rude banding in the rock. This 
granulation has not, however, been accompanied by any loss of strength, for 
the rock is a hard and exceedingly tough one, being used as paving sets in 
some of the streets in the city of Montreal, where there is an especially heavy 
traffic. 
Most of the Morin intrusion consists almost exclusively of plagioclase 
feldspar, which has the composition of labradorite, with only a very small 
portion of iron-magnesia constituents, and hence the rock is properly termed 
''anorthosite." 
The specimen used for the determination of the elastic constants of the 
rock was cut from a paving set which was richer than usual in the iron- 
magnesia constituents and which consequently might be more properly 
referred to as gabbro, although it is merely a part of the anorthosite locally 
richer in these darker constituents. It has a rudely streaked structure, as 
seen in the accompanying color-process photograph of a polished specimen, 
Plate XII A. This structure crossed the vertical face of the test piece diag- 
onally, so that if there be a variation in the values of the elastic constants 
dependent on the direction of the streaking, the readings attained will 
represent a mean, or at any rate an intermediate value. 
Under the microscope the rock is seen to be composed chiefly of plagioclase, 
associated with which is a pale green augite, a deep green hornblende, with a 
few grains of ilmenite, and an occasional individual of hypersthene, now 
altered to serpentine, and of pyrite. 
The plagioclase forms a mosaic of well-twinned grains, through which are 
distributed the other constituents in little irregular- shaped grains of rounded 
or subrounded outline. Of these the augite in the most abundant. With 
the exception of the alteration which has overtaken the few hypersthene grains 
*Adams, F. D. Report on the Geology of a Portion of the Laurentian Area lying to the 
North of the Island of Montreal. Annual Report of the Geological Survey of Canada, 
Part J, vol. vni, 1896, p. in. 
