112 MR. R. II. WORTH ON THE [vol. IxXY, 



VII. Granite-Veins. 



The granite-veins have not been thoroughly surveyed. They 

 all lie between the aplite and the main mass of the granite, or else 

 within the latter. 



On the lledaven, between LXXVI, S.E. 32 and 33, is a dyke 

 some 230 feet wide, which strikes with the shales. This rock 

 is a fine-grained granite, without porphyritic constituents. The 

 quartz shows a rough cleavage, which bears a constant relation to 

 the direction of extinction. It is almost entirely in rounded 

 forms, and the felspar has grown around it. But the mica, which 

 is brown and highly pleochroic, has asserted its outlines as against 

 both quartz and felspar, and there are some inclusions of mica in 

 the quartz. Cavities in the quartz are rather rare, and are mostly 

 empty ; but fluid, bubble, and cubic crystal are to be found. 

 Orthoclase and microperthite are the commoner felspars, and are 

 somewhat clouded ; albite is also present, and is comparatively 

 clear. One grain of tourmaline occurs in the slide. Another 

 slide (32 a) of a junction with the shale shows prisms of brown 

 tourmaline with, in places, a \ery narrow outer border of blue, and 

 with blue halos around inclusions. 



There is probably another granite-dyke immediately north of 

 the point at which the 1250-foot contour crosses the Kedaven. 



A narrow vein occurs at LXXVI, S.E. 46, bed of the western 

 tributary of the lledaven. This is very fine-grained, of typical 

 granitic structure. Much of the felspar is badly clouded and 

 stained, but obviously both orthoclase and plagioclase are present. 

 There is some original white mica and much secondary mica in the 

 felspars. 



A vein 1 foot wide lies about 200 feet below the Island of 

 liocks on the West Okement, at LXXVI, S.E. 4. The quartz 

 somewhat resembles that in the aplite from LXXVI, S.E. 9. It is 

 zoned with a dusty-brown pigment. Lines of cavities, mainly 

 empty, but some with fluid and bubble, cut across the crystals, 

 frequently parallel to a prism face, and thus coincident with the 

 zoning, but elsewhere cutting across the zones. The quartz is 

 also traversed by planes upon which a bladed mineral, apparently 

 mica, has developed. The felspar is largely microperthite, invaded 

 by mica along the cleavages ; but there is a little much-clouded 

 plagioclase. A little tourmaline occurs, with pleochroism, warm 

 buff to cinnamon-brown, no halos seen. Grey-green mica is noted, 

 only slightly pleochroic, but mingled with mica showing pleochroism 

 from straw-colour to rich red-brown ; this sometimes encloses 

 tourmaline, and in one instance encloses andalusite in good crystal 

 form. Andalusite and this mica are constantly associated, but 

 there is no indication of the latter being derived from the former. 

 The andalusite is at places in good crystal form, and its pink pleo- 

 chroism is well marked ; thin films of mica sometimes invade the 

 cleavages. In the slide there is no andalusite in the shale. 



Andalusite in vein-granite is not uncommon on this side of 

 Dartmoor, it is developed in as large forms as anywhere in Ford 



