TOURMALINE-BEARING ROCKS, MT. HEEMSKIRK. 145 



this normal granite has undergone a small amount of meta- 

 somatic change. 



Tourtnaline Granites and Aplites. 



These rocks occur in considerable quantities in many parts 

 of the district, especially in the vicinity of known tin-bearing 

 deposits. Tourmaline granites occur in the form of dykes 

 and masses in the normal granite, and in the latter con- 

 dition often extend over areas of some hundreds of acres. 

 The tourmaline aplites, or fine-grained tourmaline granites, 

 occur as irregular masses and nodules in the tourmaline 

 granites, and also as dykes, both in the tourmaline granites 

 and in the normal granites. Dykes, nodules, and masses of 

 non-tourmaline-bearing aplite are also found in the normal 

 granites, and dykes of the same rock traverse the tourmaline 

 granite. 



The tourmaline granites exhibit a large amount of 

 variation, both in the proportions of the several minerals 

 contained and also in the size of the constituent grains. 

 We have only two somev/hat imperfect slides of these rocks 

 at our disposal, so that our description must be principally 

 confined to macroscopical characters. The minerals which 

 may be seen with the naked eye com^prise the following : — 

 Felspar, quartz, tourmaline, muscovite, and a dull green 

 mica (probably biotite which has been partly altered to 

 chlorite). 



The most striking characteristic of the rock is in connec- 

 tion with the distribution of the tourmaline. This occurs 

 in patches or bundles of radiating needles or prisms, from ^ 

 1 up to 3 inches in diameter, usually associated with quartz. 

 As the granite becomes finer grained, and approaches to 

 the aplites in structure, the tourmaline loses its radiating 

 character, and, together with quartz and a little felspar, 

 form_s more or less well-defined ba.lls or nodules (quartz 

 tourmaline nodules), which occur sometimes distributed at 

 regular intervals through the granites, and sometimes 

 collected together in masses. The groundmass of the rock 

 consists of a medium^ to fine-grained crystalline granular 

 mixture of quartz, felspar, and two micas. Small miaro- 

 litic cavities may be often observed, into which the crystal 

 faces of quartz and felspar protrude. Under the microscope 

 the felspar is seen to consist both of orthoclase and plagio- 

 clase, both much decomposed. The quartz differs from that 

 of the normal granite, in that it contains no microscopic 

 rods of tourmaline, the whole of the latter mineral appear- 

 ing to be concentrated in the quartz tourmaline nodules. 



