part 2] GEOLOGY OF THE MELJ)<»' VAI.I.KVs. 105 



Both in the normal rock and in the coarser veins microperthito 

 occurs, and is sometimes the dominant felspar. 



The plagioclase is almost pure albite, as judged by its extinction- 

 angles. It occurs chiefly in rather broad laths, more usually with 

 somewhat irregular ends. The size of the larger laths may be 

 judged from the four following examples, selected from two typical 

 sections: — (a) 075 mm. x 0-125 mm., (b) 1*00 mm. x 0*35 mm., 

 (r) 0-50 mm. X 0;20 mm., (d ) 02o mm. x 025 mm. The crystals 

 are repeatedly twinned on the albite plan, sometimes combined with 

 the pericline. The fact that some laths are broken, some bent, and 

 some optically irregular from strain has already been mentioned. 



All the felspars are relatively fresh, only slight clouding by 

 decomposition being present (except in rare instances) ; this 

 statement must, however, be qualified in that in some of the coarser 

 veins a considerable alteration of the felspars has taken place. 



Quartz in the normal rock is wholly in irregular grains, of 

 much the same range of dimensions as the orthoclase. In one slide, 

 from a vein of aplite penetrating an igneous inclusion (Q. Ill, 76), 

 the quartz shows a tendency to rhombic outlines. 



Cavities, either empty or with fluid and bubble and at times a 

 cubic crystal, are (on the average) distinctly less prominent in size 

 and number than in the ordinary granite of Dartmoor. In the 

 coarser veins the cavities are larger and more numerous than in 

 the normal rock. 



Inclusions in the quartz are : mica, tourmaline, and plagioclase. 

 Some crystals, notably in section Q. I, 70, appear almost like 

 micropegmatite ; fairly high magnifications show many small grains- 

 and sub-prismatic forms of a mineral which is probably felspar. 



Mica in the normal rock is a subordinate mineral, present 

 chiefly as small flakes of 0*25 to 0*075 mm. and less. Larger 

 forms are sparsely scattered : three such from two typical sections 

 are approximately rectangular, and measure 1*00 x 0*38, 0*58 x 0*50, 

 and 023 x 0*38 mm. respectively. On some joint-faces, and in the 

 coarser veins, mica is freely developed in places, and some flakes 

 are as much as 16 mm. long. In hand-specimens it ranges from 

 colourless to a very pale pink-brown, in sections it is usually 

 colourless, but sometimes faintly tinted with brown ; it is some- 

 times zoned, the centre being tinted and the margin clear. Where- 

 ever conspicuously developed it is associated with tourmaline or 

 apatite, or with both. All Hakes large enough for testing have 

 given a strong lithium (lame, and have readily fused to a pale-grey 

 globule in the Bunsen burner; the species is undoubtedly lepidolite. 



In some sections a few small ragged inclusions may be haematite; 

 small flakes of fluorspar are more frequent. In the coarse veins 

 (Q. I, 61) the fluorspar is sometimes very markedly developed in 

 and across the cleavage of the mica ; it is difficult to say whether 

 it is an intergrowth or a replacement, more probably the latter 

 (PI. Til, fig. 15). 



Narrow bands occur in the aplite of the main dyke, which, 



