TOUEMALINE-BEAEING ROCKS, MI. HEEMSKIRK. 153 



case where the veins traverse the sedimentary rocks. The 

 tourmaline appears to gradually decrease as the distance 

 from the granite increases, until finally only quartz veins 

 are to be seen. With the disappearance of the tourmaline, 

 disappears also the metamorphism of the wall rock, which 

 is so characteristic of the veins, both in the granite and in 

 its immediate vicinity. 



Quartz-tourmaline Dykes. 



These interesting dykes were, we believe, first observed 

 by Mr. F. J. Ernst. They are found traversing both the 

 granite and the sedimentary rocks, and are distinguished 

 from the veins by the fact that their structure is not 

 dependent upon the character of the wall rock, since they 

 traverse granites, aplites. and sedimentary rocks without 

 undergoing change. They have no central fissure, and the 

 walls are generally sharp and clear-cut, even" more so than 

 the aplite dykes. They are usually of uniform composition 

 throughout, consisting of a somewhat finely granular 

 mixture of quartz and tourmaline, with sometimes a little 

 felspar. When they traverse sedimentary rocks, they 

 often contain included angular fragments of the country- 

 rock. In the slides of this rock which were prepared the 

 only minerals observed were quartz and tourmaline. The 

 former mineral occurs as irregular grains, containing a very 

 small amount of minute opaque specks. Tourmaline is 

 present in well-formed crystals of prismatic habit, as grains, 

 and as clusters of grains and crystals. The larger crystals 

 of prismatic habit have a marked pleochroism, but as the 

 crystals dwindle in size the pleochroism gradually 

 diminishes, and cannot be detected in the smallest crystals. 

 The tourmaline has preceded the quartz in order of 

 crystallisation ; radiating prisms of small size frequently 

 penetrate the quartz grains, the centre of radiation being 

 often one of the larger tourmaline crystals. The tourma- 

 line rods previously alluded to are quite absent from the 

 quartz of this rock. 



In many cases the wall rock of the dykes has suffered 

 metamorphism in a similar manner to that of the tourma- 

 line veins, but to a lesser extent. In several instances, 

 where the dykes occurred in granite, it was observed that 

 the latter had been converted into quartz-rock or quartz- 

 tourmaline rock, for a short distance (generally not more 

 than a few inches) on either side of the dyke. In other 

 cases, even along the same dyke, the wall rock had remained 

 apparently unaltered. It is conceivable that this 



