154 roURMAIJNK-nRAKINd ROCKS, M'l'. II I] KM SK I R K. 



metamorphism may have been produced by vapours which 

 traversed the fissure, prior to the introduction of the quartz- 

 tourmaline magma, but it seems more probable that it was 

 caused by emanations from the dyke-rock itself. 



Two interesting instances of variation in composition were 

 noted in the dykes traversing the Silurian strata at the 

 Gentle Annie Rise, to the east of Mount Heemskirk. In 

 one case of a dyke 2h inches in width, a portion of the 

 tourmaline had segregated towards the centre of the dyke 

 and formed a central seam about ^' inch in width, the 

 remainder of the dyke being composed of the normal 

 quartz-tourmaline mixture. In ^.nother case of a some- 

 what larger dyke, the quartz had segregated, and for a 

 foot or fifteen inches the dyke channel was filled with white 

 quartz almost free from tourmaline. The latter instance 

 seems to point to the fact that the quartz-tourmaline dykes 

 may change . to quartz veins as well as the quartz 

 tourmaline veins. 



Several observations were made at the Gentle Annie 

 Rise of small quartz and quartz-tourmaline veins being 

 cut through or faulted by quartz-tourmaline dykes. 

 These observations are perhaps not yet sufficiently numerous 

 to establish the relative ages of the two kinds of reefs 

 definitely, but it may be said, that in some cases at least, 

 the dykes are younger than the veins. This conclusion is 

 of great importance, for it proves that the quartz and 

 quartz-tourmaline veins were formed before the close of 

 the period of eruptive action of the granite. 



Conclusions. 



We may now state briefly the conclusions which we 

 think are justified by the facts which were observed in 

 this district with regard to the origin of these tourmaline 

 rocks and the associated tin-bearing veins. 



It appears perfectly evident that the tourmaline is an 

 original constitutent of the granite magma, since in one 

 form or another it is contained in all the rocks which have 

 been examined. There appears, however, to have been a 

 continued tendency for -the tourmaline to segregate to- 

 gether and separate itself from the rest of the magma. In 

 the normal granite, it occurs only in microscopic rods in 

 some of the quartz, and must represent an extremely small 

 percentage of the total composition of the rock. But even 

 here there is a tendency for the tourmaline to come to- 

 gether and form minute bundles of interlacing or radiating 

 rods. In the tourmaline granites, the tourmaline is much 



