IN THE NEIGHBOUEHOOD OF SUDBTJKY, CANADA. 37 



the slides as an original constituent, all of it, on applying the polar- 

 izing apparatus, breaks up into the above-described mosaic of separate 

 crystalline grains. The change of the felspar is uot always as above 

 described ; in some cases it is replaced by microscopic white mica 

 and occasional interstitial quartz in the more usual manner. Dark 

 brown mica occurs in the slide, occasionally in isolated flakelets, but 

 commonly in aggregated patches of these, and there is, as usual, a 

 little iron-oxide. It can, I think, hardly be doubted that these 

 fragments were once a fairly coarse granite or granitoid gneiss, and 

 that their present aspect is due to subsequent change. If so, it is 

 interesting as indicating a very considerable freedom of molecular 

 movement during the change, for the position of the constituents 

 must sometimes have been altered by quite -01". I cannot but 

 conclude that spots of quartz -025" in diameter have formed in the 

 heart of felspar crystals, so that some of the constituents have been 

 pushed aside for half that distance, and I believe that some, at least, 

 of the larger patches have been thus formed. If I am right in 

 inferring that formerly the quartz and the brown mica were in grains 

 sometimes approaching -1" in diameter (and this the size of the 

 felspar crystals, still occasionally discernible, appears to justify), then 

 it would seem as if all the constituents had undergone a crystalline 

 modiJfication so as to form aggregated groups of smaller crystals *. 



Was, then, the alteration of the fragments anterior or posterior to 

 their inclusion in this conglomerate ? As the matrix of the latter is 

 altered, we might reasonably expect some marked change to have 

 occurred in the fragments; but inasmuch as the matrix contains 

 numerous small fragments (associations of from 3 to 6 or 7 granules) 

 of the " mosaic " quartz, and two or three fragments, still recognizable, 

 of the modified felspar, there can be no doubt that these changes 

 occurred anterior to the formation of the conglomerate, so that we 

 have here a case of " Pre-Huronian " alteration. 



With this group I include a fine-grained schistose rock (occurring 

 just west of Sudbury Station), which, though quartzose, is evidently 

 less rich in that mineral than the rocks belonging to the great 

 series already described. Assuming these to be altered sands and 

 gravels, this would represent a more silty or earthy stratum. 

 When examined microscopically, the latter rock is found to consist 

 almost wholly of granules of quartz seldom exceeding '002" in 

 diameter, and flakes of a micaceous mineral, commonly slightly 

 larger, sometimes approaching near -01". Some of the quartz grains 

 look as if they were fragments, but I expect the majority have been 

 formed in situ. There are a few granules of magnetite scattered 

 about. There is a slight, but only a slight tendency to banding in 

 the arrangement of the constituents, and the flakes of the micaceous 



* I have sought to calculate the proportion of silica which would be set free 

 in converting a crystal of average orthoclase into quartz and potash-mica. 

 Supposing none of the constituents removed, the quantity of silica set free 

 would be nearly ^^, so the result would be about ^% free quartz, and yo mica ; 

 but in reality a considerable percentage of the alkali is not needed to form an 

 ordinary mica, and has probably been removed, so that the actual percentage of 

 free quartz in the resulting compound would be rather higher. 



