part 2\ GEOLOGY OF THK M KMH >\ VALLEYS. 113 



Quarry, South Tawtpn, where it was found by Dr. E. H. Young. 

 But the richest in andalusite of any of the known veins was found 

 by me in 1904 in the valley of the Lyd, in aid', at LXXVIII, 

 N.W. 1, long. 4° 4' 4", lat. 30° 39' 54". ■ 



Andalusite and topaz have not as yet been seen associated in 

 the same Dartmoor granite; the granite or aplite may, however, 

 contain topaz, while its contact-shale develops andalusite. It 

 seems very probable that where the conditions are such that topaz 

 can form, andalusite is thereby excluded. 



True aplites, mica-free, constantly occur as a border to the 

 Dartmoor granite, and are frequently of a bright-red colour — in 

 fact, red shades are never absent from the border for any consider- 

 able length. Of these, one from this localitv may be described, 

 LXXVIII, X.E. 4, long. 4° 1' 44", lat. 50° 41' 17, near Black 

 Tor. A pink rock, with small nests of tourmaline. The felspars 

 are orthoclase and a little albite, both with inclusions which give 

 the red colour to the rock. Mica is absent. The quartz is in some 

 grains fairly free from cavities; in others cavities are frequent, 

 mostly small, irregular, sometimes empty, at others with fluid and 

 bubble, and an occasional cubic crystal. The tourmaline is olive- 

 brown with irregular tints of indigo, and halos of the latter 

 colour. A fine-grained rock of granitic texture and without por- 

 phyritic crystals. 



Another genuine aplite forms veins in the granite of the 

 ridge between Yes Tor and High Wilhays, LXXVIII, X.E. 3, 

 long. 4° 0' 39", lat. 50° 41' 20". This is a grey rock, texture 

 microgranitic. Albite is in greater quantity than orthoclase, and 

 the albite-crystals are more consistently broken than in any speci- 

 men of the Meldon aplite. The quartz contains relatively large 

 cavities, many over "02.3 mm. long, some are empty, but fluid, 

 bubble, and cubic crystal are common. There is a fair quantity 

 of rather dark tourmaline, zoned olive-brown within and indigo 

 without, and showing indigo halos around inclusions. 



Fine-grained granite also traverses the normal granite : thus at 

 Black Tor, LXXVIII, X.E. 5, long. 4° 1' 46", lat, 50° 41' l\\" , 

 a grey rock as veins some 10 inches wide, following the vertical 

 jointing of the Tor. This is a microgranite. The quartz usually 

 presents rounded outlines, to which the felspar, which is almost 

 wholly orthoclase, conforms. An occasional small crystal of mica 

 occurs in the quartz: cavities are comparatively scarce, rather 

 regular in form, and more usually empty, but fluid, bubble, and 

 eul lie crystal do occur. The mica shows pleochroism from straw- 

 colour to brown-black. Tourmaline, blue and brown, in places 

 irregularly disposed, elsewhere in alternating lamellae. There are a 

 few grains of sphene. 



The general mass of the granite is of the normal Dartmoor type, 

 with large porphyrinic felspars. It differs from some of the 

 Dartmoor granites, and agrees with others, in that nests of schorl. 

 lip to 2 or perhaps 3 Inches in diameter, are rather freely developed. 

 Detailed consideration of this rock is here purposely excluded. 



