TOURMALINE-BEARING ROCKS, MT. HEEMSKIRK. 151 



the adjoining wall rock. When the nodules are absent 

 from the wall rock, they are also absent from the vein rock. 

 It is quite evident that the quartz tourmaline nodules, 

 which consist of the same material as was contained in the 

 solutions, were not attacked by them, but remained unaltered 

 while the surrounding felspar of the granite or aplite was 

 replaced. 



Microscopic examination of the vein rock entirely con- 

 firms these conclusions. Two slides were prepared from a 

 vein-stone poor in tourmaline, replacing gi^anite, which 

 contains quartz tourmaline nodules. The vein stone also 

 contains nodules, but these were not sliced. Both slices are 

 almost entirely made up of quartz, which is present in two 

 forms. It occurs either as large grains, having the usual 

 aspect of the quartz of the tourmaline granites (containing 

 no microscopic rods of tourmaline), and as very small grains 

 confusedly arranged, so as to produce a mosaic structure, 

 A small amount of pleochroic hypidioniorphic tourmaline, 

 enclosing small grains of quartz also occurs either entirely 

 within or nearly surrounded by the mosaic quartz. The 

 tourmaline and mosaic quartz are evidently replacements of 

 the felspar of the granite. There is a tendency of the 

 mosaic quartz to extinguish simultaneously over fairly 

 well-defined areas. The junction of two such areas 

 may be a straight line or an irregular line, suggesting 

 strongly that the orientation of the quartz grains was con- 

 ditioned by the position of the original felspar grains of the 

 granite, the straight lines representing the contact planes 

 of two adjacent members of a polysynthetically-twinned 

 felspar. 



Three sections were cut of quartz tourmaline rock replacing 

 normal granite in which quartz tourmaline nodules were 

 absent. These slides are composed of quartz, tourmaline, 

 and a small amount of opaque matter, disseminated through 

 the former mineral. The two types of quartz which have 

 abeady been described as occurring in the quartz rock are 

 also present in these slides. The large grains contain a little 

 opaque matter and fluid pores arranged somewhat in linear 

 fashion, and opaque rods of tourmaline showing radiant 

 structure of exactly the same nature as those described 

 as occurring in the quartz of the normal granite. There 

 can be no doubt that these grains represent the original 

 quartz of the granite. The mosaic or secondary quartz 

 is of the same nature as that described in the quartz rock, 

 but is less abundant, and in one of the slides is absent. 

 The tourmaline occurs as idiomorphic crystals of prismatic 

 habit in the secondary quartz, and as confused, ragged 



