rOURMALlNE-BEARlNG ROCKS, MT. HEEMSKIRK. 147 



rounding rock, and this causes them to stand out on the 

 weathered surface, giving the rock a very remarkable 

 appearance. From the same cause the subsoil in some 

 localities is largely composed of these nodules. 



Under the microscope the ground mass of the tourmaline 

 aplites consists essentially of orthoclase, plagioclase, and 

 quartz, with small quantities of both biotite and muscovite. 

 The plagioclase is oligoclase-andesine, or albite. Measure- 

 ments of extinction angles on sections cut approximately 

 perpendicular to the twining lamellae give results ranging 

 from 90 to 16°. Some of the quartz appears to have pre- 

 ceded the felspar in order of crystallisation. Some of the 

 grains have defined crystal edges, and they often protrude 

 into, or are included in, the felspar crystals. Like the 

 tourmaline granites, the quartz contains no microscopic 

 rods of tourmaline. One section was prepared, showing the 

 junction between a quartz tourmaline nodule and the sur- 

 rounding rock. As the centre of the slide is approached, 

 grains and hypidiomorphic crystals of tourmaline make 

 their appearance ; they are, for the most part, yellow-brown 

 in colour, and show faint pleochroism. As the centre of 

 the nodule is approached, the felspar — which is almost 

 entirely plagioclase — dwindles, and the rock appears to be 

 entirely constituted of quartz and tourmaline. The quartz 

 occurs sometimes in the form of hypidiomorphic crystals 

 surrounded by tourmaline, which is almost entirely without 

 crystal boundaries ; the tourmaline is blue in colour, and 

 moderately pleochroic. 



Other slides of nodules show an intensely pleochroic 

 tourmaline of blue colour; the crystals are sometimes of 

 tabular habit, and enclose grains of quartz in such a manner 

 as to give rise to a poecilitic structure. A small amount of 

 much-altered plagioclase felspar is also present ; it has pre- 

 ceded the tourmaline in the order of crystallisation. Eods 

 of tourmaline are absent from the quartz of these nodules. 

 Although not observed in the slides, a small quantity of 

 tin oxide appears to be always present in the quartz tour- 

 maline nodules. A small prospect was always obtained by 

 crushing and vanning the stone, while a bulk assay taken 

 from the nodules occurring at the Federation Mine yielded 

 • 2 per cent of metallic tin. 



The tourmaline aplites may be said to differ from the 

 tourmaline granites in the following particulars : — 



(1.) They are finer grained. 



(2.) They contain quartz tourmaline nodules in greater 

 abundance, and in larger and more perfectly 

 developed spheres. 



