184 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



question that as yet does not appear susceptible of definite 

 settlement. 



About a mile to the west of this dyke where it crosses Grassy 

 Na.rows Island is another nearly parallel dyke converging on the 

 former at a small angle towards the south. The rock is an uralitic 

 quartz diabase and in its coarser portions, near the middle, the 

 texture is moi'e granular than that of typical, diabase. The pla- 

 gioclase is cloudy with decomposition products and quartz is 

 abundant. The augite is entirely replaced by compact green horn- 

 Vjlende the only indication of the augite that remains being the 

 light colored character of tlie central portion of the hornblende and 

 the abundance of magnetite granules that have separated out in 

 the process of alteration. Apatite occurs in slender hexagonal 

 needles mostly in the quartz, but also in the feldspar and hoi-nblende ; 

 and a number were observed which were common to both feldspar 

 and quartz. A few zircons showing parallel extinction, deep black 

 border and brilliant polarization colors also occur. A few colorless, 

 I'ounded, isotropic grains probably garnets were observed. Nearer 

 the contact where the rock is much finer grained the typical diabase 

 structure is much better developed, the feldspar haying its usual 

 lath-shaped character with augite in allotriomorphic structure around 

 it, although the chai-acter 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 occui'S both in simple individuals and 

 in polysomatic masses and. it is interesting to note that the horn- 

 blende derived from a polysomatic aggregate of augite is of 

 uniform orientation throughout. Magnetite or titanic iron with 

 associated leucoxene is generally distributed. The quartz is in small 

 grains proportioned to the finer grained texture of the rock. In 

 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 quai-tz. In addition to the minerals enumerated 

 in this part of the dyke, there is in prominent porphyritic develop- 

 ment an altered rhombic pyroxene. The alteration has proceeded 

 very far and the mineral is now represented only by a mass of 

 yellowish green serpentine with perhaps some of the intermediate 

 alteration product bastite. The cleavage is, however, well defined 



