872 ME. J. B. SCEIVEXOE OX THE [Dec. I 9 I 4» 



An alternative theoiy that suggests itself regarding the schorl- 

 rock, is that the tourmaline may be in part the result of segregation 

 from the vein -rock on the cool margin of the vein, but this could 

 not be applied to some of the very small veins, where the reaction- 

 border, full of tourmaline, is larger than the vein itself, in which 

 tourmaline is not very common. Moreover, if the abundant tour- 

 maline of the reaction -border were even in part a result of segre- 

 gation, we should expect to get abundant topaz also (as in the clear 

 ease of segregation in Quarry M), but no case of a reaction -border 

 with abundant topaz has been found. The explanation of the 

 abundant tourmaline must be that the media contained more boron 

 than in the case of the vein in Quarry M. 



(6) The Topaz-Aplite. 



The quartz-topaz veins do not contain any felspar, but in the 

 topaz -aplite felspar is abundant. Generally it is almost completely 

 kaolinized, but rock was found fresh enough for cutting sections. 

 The following minerals occur : — 



Quartz : this is common. 



Orthoclase : highly kaolinized. 



Plagioclase : not so much altered as the orthoclase, and showing- ex- 

 tinction-angles as high as 20°. 



Topaz : where the rock is too soft for sections to be cut, the topaz has 

 been almost entirely altered to a micaceous mineral. Consequently, 

 it is difficult to say how the amount compares with the amount 

 present in the quartz-topaz rock. The sections cut show less than 

 i> found generally in the other rock, and probably the amount 

 throughout was less. 



Pale-brown mica : not abundant. 



Muscovite : not uncommon. 



Cassiterite : not abundant, in irregular grains. 



Tourmaline : brown and blue ; not abundant ; some enclosed in fresb 

 felspar. 



In one of the drives near Quarry H a modification of this aplite 

 occurs, with very abundant dark zinnwaldite, rich in iron, and 

 apparently uniaxial like the mica in Quarry M. The bulk of the 

 rock is formed of potash-felspar, but there is a little topaz and a 

 little cassiterite. 



In Quarry H, again, a nest of lithia-mica was found in a rock 

 believed to be part of the aplite-intrusions. In this mica the axial 

 figure opens slightly. 



Xumerous sections occur where the topaz-aplite may be seen 

 cutting the quartz-topaz veins. Xo evidence was noted of reaction 

 between the two rocks or the media carried by them. Where the 

 aplite cuts the granite, there is no marked alteration of the latter 

 as in the case of the quartz-topaz veins. 



Two occurrences of pegmatite have been noted in connexion 

 with the topaz-aplite. One is in the underground workings near 

 Quarry B. A section of topaz-aplite is exposed showing the junction 

 with the porphyritic granite, and from the aplite a thin vein of 

 pegmatite with a border of schorl-rock protrudes into the granite. 



