GRANITE VEINS. 219 



is that the concretion contains less silica, more iron, more lime, more 

 soda, and less potash. There are no inclusions or concretions in the 

 Cheese-ring granite near Liskeard. In several localities, as at Foggen 

 Tor, near the Prince's Town prison on Dartmoor, concretionary patches 

 occur in which the outlines are not sharply defined. 



Similarly the granite of Westmoreland, at Shap, frequently ex- 

 hibits rounded patches of a dark colour, in which Mr. Phillips has 

 detected crystals of felspar, partly contained in the concretion and 

 partly in the granite, 1 as may be seen in the St. Pancras Railway 

 Station. These concretions vary in size from a pea to a water-melon, 

 and, like all the others, owe their dark colour to the abundance of 

 mica. They have a ground mass of quartz, felspar, and mica, and con- 

 tain accidental minerals such as magnetite, titanite, apatite, and horn- 

 blende. Occasionally, however, the patches, owing to the absence of 

 black mica, are lighter in colour than the surrounding rock. 



In the granite of Aberdeen, the pebble-like concretions are almost 

 unknown. This granite consists of orthoclase, a large proportion of 

 oligoclase, quartz, white and black mica, minute garnets, and occasional 

 crystals of apatite and sphene. The quartz is frequently traversed by 

 hair-like crystals of rutile. Fragments of schistose rocks sometimes 

 occur. Mr. Phillips describes one lenticular mass weighing over a 

 hundredweight found in the Dyce quarry, north-west of Aberdeen. It 

 was covered with a layer of mica so as readily to separate from the 

 rock. On being broken the mass consisted of granite half as fine 

 again in texture as the surrounding rock, and only differed in contain- 

 ing more oligoclase. Near Peterhead, ovoid concretions from the size 

 of a nut to that of an apple are found. They are nearly always dark, 

 fine in texture, and sharply defined. On analysis they contain less 

 silica and less potash, but more iron, lime, and alumina than the 

 surrounding rock. The granite of Ballachulish contains both concre- 

 tions and fragments .of foliated rocks. Many of the Irish granites 

 abound in dark-coloured concretions. 2 



Granite Veins. The size of granite veins is very variable. They 

 may be fine and interlace like a network. Such, are well known in 

 Glen Tilt. At Wicca Pool they often enclose pieces of mica slate. 

 They are numerous in Cornwall. On the Continent they are well seen 

 on the south side of the Brocken in the Harz. Sometimes in such 

 veins the mica disappears, and further on occasionally the felspar also 

 disappears, so as to leave at last nothing but veins of quartz at the 

 termination. These changes are well seen in Arran, in the Valley of 

 the Drummond, and at St. Marie in the Pyrenees. Granite veins 

 three miles long have been described as penetrating the granite near 

 Rossau in Saxony. The texture is often coarse in the middle of a 

 large vein, but it almost invariably becomes fine-grained or felsitic 

 towards the border, showing that the granite was consolidated before 

 the vein penetrated into it. Occasionally, however, the vein may 

 have a coarse texture, while the granite which it penetrates is fine. 



1 Quart. Jour. Geol. Soc., vol. xxxviii. p. 217. 

 8 J. A. Phillips, Q. J. G. S., vol. xxxvi. p. i. 



