Ldivson on diabase dykes of the (Rainy lake region. 201 
does not appear to have been altered perceptibly towards the 
contact. Specimens for microscopic examination were taken 
from different parts of the dyke, viz., at 60 feet, 20 feet, and 6 
feet from the contact, and at the contact. At 60 feet from the 
contact, the rock is a coarse-grained mottled gray rock in which 
dirty white feldspar and black pyroxene are the prominent con- 
stituents. Under the microscope it presents the characters of a 
coarse-grained, comparatively fresh diabase. Aiigite of a pale 
mauve tinted gray colour is abundant and often occurs in masses 
that fill the field of the microscope when low powers are used- 
Sometimes these plates of augite are individual crystals. For 
the most part however, they are not single individuals. When 
examined between crossed nicols the plate of augite is seen at 
once to be resolved into an intimately interlocking mosaic of ir- 
regularly shaped grains of diverse optical orientation. In or- 
dinary light the boundaries between the different members of 
these "polysomatic"^ masses of augite are traceable only with 
difficulty and uncertainty. There is no interstitial matter what- 
ever, the different grains being as intimately associated as in the 
case of interpenetration twins of feldspar. That they are not 
twins is shown by the fact that there are often as many as 
half-a-dozen grains all of different orientation thus combined 
in the same mass. The cleavage, by its lack of continuity 
over the field, of course indicates a difference of orientation 
in different parts of it, but the cleavage traces are not strongly 
marked, and attention is only directed to the discordance 
of the cleavage after the polysomatic character of the mass 
has been rendered prominent by the analyzer of the mi- 
croscope. This polysomatic structure of augite does not ap- 
pear to be common. Rosenbusch does not mention it in his 
last comprehensive summary of the present state of petrograph- 
ical knowledge.^ The nearest approach to this structure that is 
at all well known is the polysomatic character of some chondri of 
olivine in certain meteorites such as are figured by Tschermak^ 
' Adapted from Tschermak's use of this word as applied to a similar 
structure in the olivine of certain meteorites. — V. Die Mikr. Beschaff . der 
Meteor. Stuttgart, 1885. 
* Mikr. Phys. der Mineralien und Gesteine; Stuttgart, 1886. 
3 Die Mikr. Beschaff. der Meteor. Stuttgart, 1S85, Taf. xv., Figs, i and 2. 
