2IO Lawson on diabase dykes of ihe (Rainy lake region. 
rather than in the residual silica of the magma. The non-exis- 
tence, however, of quartz in some diabases which are very much 
decomposed, and its presence in fresh ones, militates against the 
theory of the secondary origin of the quartz in these rocks, so 
that the question of how much of the quartz is j^rimary and how 
much secondary in an old diabase is a question that as yet does 
not appear susceptible of definite settlement. 
About a mile to the west of this dyke where it crosses Grassy 
Narrows island is another nearly parallel dyke converging on the 
former at a small angle towards the south. The rock is a urai- 
itic quartz diabase and in its coarser portions, near the middle, 
the texture is more granular than that of typical diabase. The 
plagioclase is cloudy with decomposition products and quartz is 
abundant. The augite is entirely replaced by compact green 
hornblende, the only indication of the augite that remains being 
the light colored character of the central portion of the hornblende 
and the abundance of magnetite granules that have separated 
out in the process of alteration. Apatite occurs in slender hex- 
agonal needles mostly in the quartz but also in the feldspar and 
hornblende; and a number were obscved which were common 
to both feldspar and quartz. A. few zircons showing par- 
allel extinction, deejD black border and brilliant polarization 
colors also occur. A few colorless, rounded, isotropic grains, 
probably garnets were observed. Nearer the contact where 
the rock is much finer grained the tvpical diabase structure is 
much better developed, the feldspar having its usual lath-shaped 
character with augite in allotriomorphic structure around it, 
although the character of the latter is obscured by its extensive 
alteration into hornblende. The augite so far as It is revealed 
in the cores of the hornblende occurs both in simple individuals 
and in polysomatic masses, and it is interesting to note that the 
hornblende derived from a polysomatic aggregate of augite is of 
uniform orientation throughout. TSlagnetite or titanic Iron with 
associated leucoxenc is generally distributed. The quartz is in 
small grains proportioned to the finer grained texture of the 
rock. I.n the central part of the dyke the quartz is In large 
grains commensurate with the increased size of the feldspar and 
augite. In neither case does it occur In the mosaics which are 
so characteristic of the secondary or vein quartz. In addition to 
