[dresser] serpentine SERIES OF QUEBEC 9 



was ordinarily exposed at a time. Granite dykes appeared and 

 disappeared in the progress of mining, and when lost were supposed to 

 be faulted into unmined ground. This doubtless occurs, and perhaps 

 frequently, but the dykes are rarely, if ever, traceable by any means 

 available beyond a few hundred feet in length, and usually for much 

 shorter distances. But the adoption of underground methods of min- 

 ing, with the development of large areas at different levels in advance 

 of actual mining, affords a better view. It is thus made possible to 

 observe on all sides — above, below and all around — many masses of 

 granite and to be assured that they are not connected with one another 

 nor with any parent mass.' 



True dyke forms, which actually and frequently occur, seem 

 satisfactorily accounted for as the filling of contraction cracks in the 

 earlier cooled peridotite, by the more acid and still liquid granitic 

 residue. These dykes, it may be repeated, are local in extent, are 

 irregular in distribution, have not been observed to cross one another, 

 and are confined to the peridotite-serpentine area. 



Contact Features: The contact between the granites and the 

 enclosing serpentine is generally sharp, rarely if ever gradational. 

 This is somewhat in contrast to the relations existing between other 

 rocks of the series, and has been frequently cited as evidence of the 

 intrusive character of the granite. The smaller dykes and masses 

 are apt to be aplitic in character and to show little variation through- 

 out. But in the larger of the isolated bodies, the tendency seems to be 

 to develop pegmatite along the border of the granite near the contact. 



Individual Bodies: The largest single body of granite in the mining 

 area is an elliptical boss, three-quarters of a mile long and about a 

 quarter of a mile in its greatest width. The actual contact is drift- 

 covered except for intervals of a few hundred feet. The central and 

 greater part of this area is composed of coarsely porphyritic biotite- 

 hornblende granite. Towards the margin, in exposed areas at least, 

 biotite disappears and the rock becomes the ordinary hornblende 

 granite of the series. Bordering this phase, and separating it from 

 the serpentine there is a distinct zone of pegmatite, perhaps 2 or 3 feet 

 in width. From this pegmatite border small dykes or veins, also of 

 pegmatite, are occasionally found running into the granite for dis- 

 tances of 40 or 50 feet. They seem to become more acid the farther 

 they penetrate the granite, and end in or near stringers of quartz. No 

 invasion of the serpentine by pegmatite was observed in this locality, 



^The writer is indebted to the managers of the Bell and the Jacobs mines, the 

 Hon. G. R. Smith and Mr. N. R. Fisher, for facilities in visiting their underground 

 workings at various times. 



