370 MR. J. B. SCRITENOR ON THE [Dec. I914, 



Pleochroic halos are abundant in the dark mica, and some of the 

 inclusions bear a strong resemblance to zircon-crystals. A quantity 

 of the mica was separated from the rock by an electro-magnet, and 

 treated with the object of isolating the included minerals. Minute 

 olive-green prisms of tourmaline were found, but it is not certain 

 that these were surrounded by halos. A few well-formed zircon- 

 cr} r stals were obtained, and there can be no doubt that in some 

 cases the halos do surround zircons, as in the biotite of granite. 



(3) The Border of the Quartz-Topaz Veins without 

 Dark Zinnwaldite. 



The junction of the quartz-topaz veins with the weathered 

 porphyritic granite is clearly defined, and where there is no dark 

 zinnwaldite there is almost invariably a dark border to the quartz- 

 topaz rock, varying in thickness from half an inch to 8 inches, that 

 appears to be part of the vein-rock. This border is equally distinct 

 in the thickest and in the thinnest veins, or perhaps it would be 

 more correct to say that it is more distinct in the very thin veins, 

 measuring only an inch across — because sometimes, in such cases, 

 the dark borders on each side of the vein as seen in section are 

 thicker than the vein itself. In the bigger veins the lines of 

 division between the border and the granite and between the border 

 and the quartz-topaz rock are both distinct, but in some of the 

 smaller veins the dark border can be seen gradually passing into 

 granite. 



That this dark border of the quartz-topaz veins is a result of 

 reaction between media that came off: from the vein -rock and the 

 porphyritic granite, and not a portion of the original vein-rock, is 

 evident from the examination of sections, and therefore I propose 

 to call it the reaction -border. The evidence is as follows: — 

 Zircons, which are abundant in the granite, but extremely rare in 

 the quartz-topaz rock, are common in the reaction -border. This 

 was proved by separating the minerals in several crushed specimens. 

 Some of the original biotite of the granite, bleached, but showing 

 halos, and with secondary brown tourmaline formed in it, is found 

 in the border. The colour of the border is dark, because of the 

 abundance of tourmaline, which is nearly all brown. There is, 

 however, some blue tourmaline. In addition to the tourmaline 

 masses of fine-flaked muscovite occur, which suggest the alteration 

 of the felspar of the granite ; and, in this connexion, it is interesting 

 to note that separations of the powdered rock afforded very little 

 topaz. In one section a little topaz was seen in a mass of muscovite. 

 No topaz was found in other sections. The presence of cassiterite 

 could not be proved in the border, but in crushed rock one grain 

 was found which may be that mineral. 



The base of the reaction -border is quartz, and there is no felspar. 

 The grain of the border is finer than that of the veins. 



