152 TOURMALINE-hEARING ROCKS, MT. HEEMSKIRK. 



masses, sometimes fibrous in structure, and then showing 

 tendency to radiant arrangement. We believe that this 

 tourmaline is a replacement mineral after felspar and 

 biotite, no trace of either of which minerals can be seen 

 in the slides. Contrary to what was observed in the case 

 of the replacing quartz, however, the felspar crystals have 

 not determined any special orientation in the case of the 

 tourmaline. In the slide in which replacing quartz is 

 absent the original quartz grains do not seem to have 

 entirely escaped change. They appear to be corroded 

 around the edges, and to some extent replaced by tourma- 

 line ; this appearance is nearly absent from the slides, 

 which contain secondary quartz. 



One slide was prepared of greisscn, occurring as a re- 

 placement of a granite or aplite, containing quartz tourma- 

 line nodules. The rock consists of quartz, tourmaline, and 

 a fibrous mica ; the quartz ajDpears for the most part in 

 the form typical of the quartz of tourmaline granite, but 

 there is in addition a small number of minute grains of 

 quartz surrounded by a confused mesh of mica ; the mica 

 is in fibrous crystals, sometimes arranged with radial 

 structure; it shows no pleochrism, but there is a con- 

 siderable amount of absorption of light as the slide is 

 rotated. It polarises in very high colours, and appears to 

 have the optical properties of maiscovite. Tourmaline 

 occurs in small quantities as hypidiomorphic pleochroic 

 crystals, but it is not present in the form of rods in the 

 quartz grains. The mica would appear to be for the most 

 part a replacement after felspar, but the occurrence of 

 small nests and meshes of mica in the larger grains of 

 quartz shows that the latter mineral hap also suffered 

 replacement. In the slide are a few very minute crystals 

 of a highly refractory substance, v^hich may be zircon ; 

 the material of the slide surrounding these specks is 

 frequently coloured brown. One slide was prepared of 

 quartz tourmaline rock, replacing slate which appears 

 originally to have possessed a finely laminated structure. 

 The laminae have been for the most part replaced by grains 

 of pleochroic tourmaline arranged linearly, and are 

 separated from each other by minute grains of clear 

 quartz. 



The vein stone was not examined microscopically. It 

 often consists almost wholly of tourmaline, but is some- 

 times very rich in tin oxide. The tourmaline often 

 occurs in large bunches of radiating prisms or needles 

 going up to 6 or 8 inches in length. Occasionally, however, 

 a good deal of quartz is present, and this is especially the 



