Lawson on diabase dykes of the (Rainy lake region. 205 
statite also occurs in variety of the allied rock diabase porphyrite 
from Schaumberge, which has been described by Laspeyres and 
Streng under the name Palatinite. This enstatite was not ob- 
served in the coarser parts of the dyke but occurs, as will be 
noted, in the still finer grained diabase at the contact. 
At the immediate contact the dyke assumes microscopically 
the characters of a very compact grayish black aphanitic rock 
in which can be occasionally detected minute glistening facets 
of porphyritic crystals. With low powers of the microscope 
the matrix is not resolvable but appears as a uniformly yellow- 
ish to greenish gray ground thickly dotted with grains of mag- 
netite. Under the higher powers this is seen to be made up, in 
addition to magnetite, of a fine felt-work of minute lath-shaped 
crystals of plagioclase imbedded in a hazy, somewhat yellowish 
green flocculent chlorite substance derived presumably from the 
alteration of the augite, since that mineral cannot with certainty 
be identified in the base. The porphyritic character of this part 
of the dyke is well marked, though the imbedded crystals are 
small. These are augite in small irregular polysomatic masses, 
with a hazy margin or fringe of greenish decomposition prod- 
uct, and long lath-shaped plagioclase and occasionally stouter 
broken fragments. Besides these there are porphyritic crystals 
of enstatite much more altered and less plentiful than at 6 feet 
from the contact. Neither quartz nor garnets are observable in 
the contact. 
Considering then the dyke with reference to its variation in 
structure and mineral composition the points of interest to be 
noted are: The passage of the coarse grained central portions 
of tlie dyke to compact aphanitic rock at the contact ; the absence 
of porphyritic structure in the middle of the dyke as contrasted 
with the well marked development of the same as the rock 
becomes finer grained towards the dyke walls; the absence of 
the characteristic chloritic substance of diabase in the centre of 
the dyke and its abundance towards the contact; the presence of 
quartz in greater quantity in the coarse grained middle portions 
than at the sides; the presence of garnets in the coarsest parts 
of the dyke, their abundance in the medium grained parts and 
their rarity or total absence in the neighborhood of the contact; 
the presence of the rhombic pyroxene enstatite in ty2:>ical idio- 
