DIABASE DYKES OF RAINY LAKE. 



183 



Fig. h. 

 Polysomatic g r a i n of 

 augite — Grassy Narrows dyke 

 Rainy Lake, a and 6 are twins 

 — the other granules are of 

 diverse optical orientation. 

 X28. 



in most cases. The augite occurs both in simple individuals and in 

 polysomatic masses. It exhibits the usual 

 marginal alteration to hornblende and there 

 is besides a certain amount of chlorite. 

 Original magnetite is frequently surround- 

 ed by a margin of secondary biotite. Micro- 

 pegmatitic quartz is abundant. It is often 

 intimately intergrown with the feldspar, and 

 as the latter is much decomposed, woiild seem 

 to replace it as a partial pseudomorph, but 

 apatite needles of the same aspect as those 

 which occur as inclusions in feldspar, aug- 

 ite, and quartz, are often seen to be inclosed 

 partly in a feldspar and partly in quartz grain. The primary origin 

 of the quartz in spite of its niicropegmatitic character, is however, 

 not beyond doubt. It is to be noted that were the quartz original 

 we would hardly expect to find it in such close association vvith 

 the feldspar. The plagioclase of these rocks affords unmistakable 

 evidence in its idiomorphic character of its having first crystallized 

 from the magma. The augite crystallized next, enclosing the lath- 

 shaped plagioclase ; and the quartz, which would be the last to 

 crystallise, we would expect to find separate from the plagioclase by 

 the augite, i.e., to fill in the interstices between the augite. Again 

 although single apatites are often found extending from a quartz, 

 grain to a feldspar grain, a condition of things favoring the notion 

 of a common primary origin of both the latter minerals, yet such a 

 phenomenon is not incompatable with a secondary origin for the 

 quartz, since the replacement of feldspar by quartz must necessarily 

 be a slow operation and proceed particle by particle. Fui-ther, if 

 the quartz wei'e original we should hardly expect to find in it 

 inclusions of crystals of the first generation like apatite, which 

 would be liable to be enclosed for the most part in the earlier 

 secretions like feldspar and augite, rather than in the residual silica 

 of the magma. The non-existence, 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 primaiy and how much secondary in an old diabase is a 



