108 MR. R. H. WORTH ON THE [vol. lxXV y 



Pleochroic halos around inclusions are somewhat difficult to 

 detect in the emerald-green mineral, but are well marked in the 

 olive variety. The halo is accompanied by an outer corona, and 

 both are in blue shades whatever the colour of the prystal. Measure- 

 ments are very consistent, the radius of the halo being '02 mm. 

 and of the corona "032 mm. An inclusion in the tourmaline of 

 LXXVI, S.E. 27, Iledaven, almost parallels the inclusion in the 

 mica of LXXVI, S.E. 38; it is globular, '041 mm. in diameter, and 

 surrounded with a halo 022 mm. thick — there is no corona. 



It appears certain that tourmaline crystallized simultaneously 

 with albite, with topaz, and with apatite. Only in one slide, 

 LXXVI, S.E. 22 (South Down), are there a few small radiate 

 aggregates of needles of this mineral. 



Topaz. — First recorded at Meldon by Sir Jethro Teall (2), 

 since spoken of by the Geological Survey as probably more frequent 

 than in any other granite in the West of England. This state- 

 ment requires some modification : if for ' granite ' we substi- 

 tute ' granite occurring in quantity,' then it would perhaps be 

 near the truth ; but veins in Dartmoor granite and its bordering 

 rocks sometimes show more and better developed topaz than the 

 Meldon aplite, whether main or minor dykes. A vein or dyke in 

 the sedimentaries in the valley of the Lyd, near Great Nodden, 

 may be cited ; but, as to this, I have been anticipated by Mr. F. 

 P. Mennell. 1 A more striking example would be LXXVIII, 

 N.W. 4, long. 4° 4' -I", lat. 50° 27' 4", in situ, bed of the Avon 

 above Shipley Bridge ; this is a vein in the granite, and topaz is 

 not only present in quantity but in exceptionally good outline. 



In every slide of the Meldon aplite there is some topaz, and in 

 but few is it comparatively scarce : Q. I, 29 is most marked in 

 this respect. Often merely granular, it not seldom occurs in 

 approximately rectangular forms, and occasionally shows good basal 

 cleavage. In the specimens from Sourton Tors it is as well 

 developed as anywhere, the larger forms measuring about - 63 mm. 

 by - 37 mm. : these are distinctly rare. 



Topaz is associated with tourmaline and with apatite in such a 

 manner as to indicate that it was one of the earlier minerals to 

 crystallize ; there is no ground for the suggestion that its presence 

 is due to absorption of some part of the neighbouring sedimentary 

 rocks. The nature of the sedimentaries, whether calcareous or alu- 

 minous, exercises no influence, and further the extremely-low iron 

 content of the aplite shows that no such absorption can have taken 

 place. It will also be remembered that, as in one instance cited 

 above, topaz may be freely developed in veins penetrating the granite 

 itself. A more suggestive fact is that on Dartmoor richness in 

 topaz is associated with poverty in mica. 



Only in one slide, LXXVI, S.E. 8, south of Forest Mine, is the 

 topaz in any way altered, and there pale mica is marginally 

 developed — this is from one of the minor dykes. 



1 Q. J.G.S. vol. lxxi (1915-17) p. 635. 



