TOUKMALINE- HEARING HOCKS, MT. HEEMSKIRK. 149 



is, however, now generally conceded that it is impossible to 

 draw any sharp line of division between these two types, 

 and our investigation of the quartz tourmaline rocks of the 

 Heemskirk district affords evidence of this fact. We find 

 that there are both quartz tourmaline dykes and quartz 

 tourmaJine veins, and also types which occupy an inter- 

 mediate position. As it is often difficult to decide at once 

 to which class any given deposit belongs, it is evident that 

 a general term is desirable, and the term '' reef " appears to 

 us to be quite suitable. 



Quartz Tourmaline V eina. 



It is a well-recognised fact that fissure veins are often 

 not merely "fissures filled with mineral matter;" the 

 material forming the vein may have been either deposited 

 in an open cavity formed by the fissure, or it may have 

 been deposited as a replacement of the wall-rock, or of some 

 of the constituents of the latter. In the quartz tourmaline 

 reefs of the Heemskirk district the greater part of the 

 material has been deposited as a replacement of the wall 

 rock, and only a very small portion as the actual filling of 

 the fissure. These two portions of the vein we propose to 

 distinguish by the terms " vein rock " and " vein stone," 

 the former denoting that portion of the vein which has 

 been formed as a replacement of the wall rock, and the 

 latter that portion which has been deposited along the plane 

 .of the fissure, and generally in an open cavity. 



The quartz tourmaline veins consist of tabular deposits 

 composed essentially of quartz and tourmaline, traversing 

 both the granite and the surrounding Silurian strata. 

 There is a central seam or fissure filled usually with tour- 

 maline, or quartz and tourmaline (the vein stone), and on 

 either side of this is a granular rock composed of quartz, 

 quartz and tourmaline, or quartz and white mica (musco- 

 vite or lithia mica) and tourmaline (the vein rock). Both 

 the vein stone and the vein rock may carry tin oxide. The 

 former is sometimes very rich in tin, the ore being often 

 beautifully crystallised ; in the latter the tin oxide occurs 

 in crystalline grains, and is often finely disseminated. 

 Pyrites appears to be an invariable constituent wherever 

 the veins have been explored below water-level. It occurs 

 in bunches in the vein stone, and also disseminated through 

 the vein rock. In some cases the vein rock consists almost 

 entirely of granular quartz and pyrite, the latter having 

 the appearance of replacing the felspar of the granite. 

 Besides those already mentioned, most of the other minerals 



