8. AlIpoH — On the Pitchstones of Arran. 5 



Tlie total abseflce of dichroism is sufficient to distinguisli it from 

 hornblende. 



The magnetite occurs, as usual, in opaque black grains, and is 

 found in the augite and felspar crystals, and also in the base. 



Drumadoon Point, on the west coast of the island, is a bold, rocky 

 headland of quartz porphyry, exhibiting on its seaward face a 

 magnificent group of lofty columns, which rest perj^endicularly on 

 beds of Carboniferous sandstone. On the shore, at the base of the 

 cliff, there is an intrusive mass of quite similar rock, together with 

 numerous dykes of basalt ; the latter cut through the sandstones and 

 porphyrites, and extend upwards through the great vertical columns. 

 Northwards, towards Tormore, the dykes become still more nume- 

 rous, and at one point, the side of an immense dyke of quartz 

 porphyry forms a cliff rising abruptly from the water's edge. Im- 

 mediately to the north, the shore is completely seamed by great 

 dykes of basalt, porphyrite, felsite, and pitchstone. One large dyke, 

 sixteen yards wide, is composed of basalt on one side and pitchstone 

 on the other ; in other cases, the sides are formed of basalt and the 

 centre of pitchstone ; in many places the latter has been altered to 

 hornstone (so-called) for some little distance from the line of contact 

 with the sandstones. One of these dykes just north of King's Cove 

 consists of a greenish black porphyritic pitchstone, containing nume- 

 rous small crystals of glassy felspar and quai-tz; the fracture is 

 sub-conchoidal, lustre vitreous. Some parts of the rock exhibit 

 numerous parallel stripes of a lighter colour, which joass as bands 

 through the mass of the rock, their direction being parallel with the 

 sides of the dyke. Under a one-inch objective, the texture is seen 

 to differ considerably from the specimens described above. The 

 base is a pale brown glass irregularly mottled with clear colourless 

 spaces, and crowded in every part with slender pale green belonites 

 and minute granules ; they are not arranged in groups, as in the 

 other varieties, but are evenly distributed throughout the mass, and 

 for the most part lie with their long axes in a direction more or less 

 parallel with the light-coloured bands. Porphyritically imbedded 

 in this base are a number of crystals of felspar and quartz, a few of 

 augite, and a few grains of magnetite. 



To the sides of some of these crystals the belonites have attached 

 themselves by one end in great numbers, projecting at right angles 

 from the surface, or crossing each other in various directions. As 

 the microlites of the base usually lie with their long axes more or 

 less parallel, the general appearance is that of a stream of particles 

 flowing in one direction ; but when a crystal of felspar or quartz 

 stands in the way, the stream is divided, the belonites appear to be 

 diverted to either side, arrange themselves with their axes parallel 

 to the sides of the obstacle, then sweep round it, and are again xmited 

 in a direct course on the oj)posite side. This remarkable structure 

 is well represented in Fig 13, which shows a group of compound 

 felspar crystals, augite, and black grains of magnetite, surrounded 

 by streams of belonites. The small group of augite crystals partly 

 penetrates the felspar, and the sides of the latter are thickly studded 



