106 ME. K. H. WOETH OX THE [vol. lxXV, 



while still fine-grained, are marked out from their surroundings by 

 a. generally dispersed tint of lilac or rosy-lilac; a compact substance 

 of similar colour is also present in some of the coarser veins. 

 Where unmixed, as in the coarse veins, the material is readily 

 scratched by a knife. Four sections (Q. I, 4 & 10-12) indicate 

 that this substance consists of a felted mass of mica-scales with 

 some larger forms of the same mineral, and with topaz and tour- 

 maline in small grains. Three other sections (Q. I, 22, 34, 39) 

 show a similar felted structure ; but the double refraction is dis- 

 tinctly low — too low for mica, unless by chance the flakes have 

 approximately parallel arrangement and are viewed normal to the 

 base, which seems very probable. The larger forms of mica are 

 still present in these three sections. The first four sections suggest 

 that the colour is due to lilac-tinted lepidolite. The occurrence of 

 lepidolite in the altered shales has already been mentioned. 



In some of the minor dykes the mica is a little more deeply 

 coloured, and pleochroic halos can be seen around small inclusions. 

 LXXYI, S.E. 38, at the foot of Homerton Hill, and in situ in 

 the dyke above, yields good examples. An inclusion of "004 mm. 

 diameter occupies the centre of a halo of '067 mm. diameter, the 

 halo being thus a shell *032 mm. thick around the inclusion. There 

 is a curious example in which the inclusion is a globe of "047 mm. 

 diameter, the total diameter of the halo is 091 mm. : the halo is 

 here a shell of *022 mm. thickness around the inclusion. Although 

 inner halo and outer corona can nowhere be discriminated in the 

 mica, this looks much as though the first instance was one of 

 inseparable halo and corona, and the second halo without corona. 



Tourmaline is to be found in all specimens of the Meldon 

 aplite, but in varying quantity. It occurs in the body of the rock, 

 and also coating some joint-faces : in the first case it is an original 

 constituent ; in the second it appears to belong to the final stage 

 of consolidation of the rock, and not to subsequent action. In no 

 case is the green tourmaline associated with any such alteration 

 of the adjacent granite as accompanies ordinary schorl-veins. 



In many specimens the presence of tourmaline would not be 

 suspected without microscopic examination. In fact, the aplite 

 might be divided into two varieties so far as this mineral is con- 

 cerned : the one in which there is very little, the other in which 

 it is present in such quantity as to colour the rock distinctly ; 

 but intermediate conditions are very rare. The two varieties fre- 

 quently make an abrupt junction, which is never either an open 

 joint-face or a plane of weakness. Similar junctions occur between 

 the very fine-grained chilled rock of azure tinge, which borders the 

 larger inclusions, and the normal rock. I have described corre- 

 sponding conditions in the Cann Quarry el van. 1 



The typical tourmaline is of a beautiful emerald-green, but 

 in the quarry on the southern bank of the Eedaven there are 

 masses of rock in which the mineral is light olive-green, and this 



1 Trans. Devon. Assoc, vol. xxxiv (1902) p. 505. 



