CLASTIC CONSTANTS OF ROCKS. 55 
GREEN GABBRO, NEW GLASGOW, PROVINCE OF QUEBEC, CANADA. 
This rock forms a large dyke* cutting the anorthosite from the locality 
described above. It is a rock which is darker in color than the anorthosite, 
owing to a much higher content of iron-magnesia constituents, but which, like 
that rock, is quarried and used for paving sets. 
Under the microscope this rock is seen to differ entirely in structure from 
the other igneous rocks examined. It is composed of a very pale green aug- 
ite, a rhombic pyroxene of the same color, and plagioclase, the two former 
minerals being present in about equal amount, and the plagioclase not form- 
ing more than about one-quarter of the rock; there is also present a small 
amount of a pale green spinel. 
The rock is seen to have been crushed in a most extraordinary manner and 
to present a most striking cataclastic structure. The plagioclase occurs in 
groups of individuals which are well twinned, and are frequently very much 
bent and twisted one individual being bent through an angle of 65. The 
mineral is also filled with very minute rounded inclusions, which give to it a 
green color. These plagioclase grains, quite irregular in form, lie embedded 
in a mass of little irregular-shaped grains of augite and rhombic pyroxene. 
These vary somewhat in size. The two pyroxenes are sometimes intimately 
intermixed and at other times separated into groups of grains of, their 
respective species, which are distinguished from one another by the different 
values of their double refraction and by the fact that one has parallel and 
the other inclined extinction. The spinel is associated with this minutely 
granulated pyroxene. 
The original structure of the rock has been entirely broken down, and it 
now presents an assemblage of grains of the minerals varying in size and 
differing in arrangement from place to place in the slide. The pyroxenes are 
granulated, the plagioclase twisted, and the whole presents a perfect cataclas_ 
tic appearance, differing entirely in this respect from that of the anorthosite 
just described. This cataclastic structure is combined in some specimens 
of the rock with a more or less distinct parallel arrangement of the constit- 
uent minerals, although this is not very distinct in the specimen shown in 
the color-process photograph of a polished surface (Plate XIV A) . 
To this irregularity in structure may be attributed the irregularities in the 
elastic deportment of the rock. 
A photomicrograph of a thin section of the rock taken between crossed 
nicols in polarized light and magnified 30 diameters is given in Plate XIV B. 
It is found that satisfactory measurements of the elastic constants could not 
be made in the case of this rock, the same specimen giving a great variation 
* Adams F D. Op. cit., p. 121. 
