neae new galloway. 577 



§ 5. Chiastolite-mica-Schists. 



Amongst the grits and flags marked C on the map are some 

 unaltered shales, and others which may be called chiastolite-mica- 

 schists. These are very black (fig. 9, PI. XXIII.), and cleave along 

 surfaces with the lustre of highly polished black lead. The cleaved 

 surface is often rough with garnets of very varying sizes, or shows 

 clusters of chiastolite needles, from ^ to ^ inch in diameter and 1 or 



2 inches long. A cross-fracture has a more brilliant lustre, as the 

 black mica, which forms a great part of the rock, is more conspicuous. 

 The chiastolite is water-clear, and often shows the pink and green 

 dichroism as well as the characteristic black figure. Parts of the 

 crystals are sometimes replaced by white mica, and sometimes by 

 quartz. The chiastolite usually includes garnets, and often, with 

 them, little masses of fine-grained quartz. Very rarely it includes 

 mica, and this is in small flakes. 



The garnets are sometimes ^ inch in diameter, but usually 

 smaller. In sections a dark ring is often seen round the centre, 

 sometimes due simply to diff'erence of colour, sometimes to inclusions 

 of black grains. Often on each side of the garnet is a small tri- 

 angle of quartz, making with it a clear eye. 



The main mass of the rock consists of quartz and black mica, 

 with much colouring matter, which renders much of a slide opaque. 



There are quartz-garnet lenticles, like those described in the 

 gneissic rocks ; and round their margins are large masses of chia- 

 stolite with few or no crystal faces. 



§ 6. Aplite Veins. 



On Knocknairling Hill and at other parts of the margin both 

 granite and sedimentaries are veined by a granite, which would 

 answer to the description of aplite given by Prof. Eosenbusch, if it 

 were not that he says that muscovite is present only in small quan- 

 tities. The rock is usually very coarse, but here and there becomes 

 fine-grained. It consists of felspar (often in large masses), quartz, 

 and a considerable quantity of white mica. Tourmaline and gar- 

 nets are accessory minerals. 



The felspar is mostly microcline. In some veins it forms a micro- 

 pegmatite with the quartz ; in others it forms the bulk of the rock. 

 In one vein, to the west of the granite, there are masses of it 2 or 3 

 feet across, with continuous cleavage-planes some inches long. The 

 felspars often include garnets. 



The quartz is traversed by many lines of small inclusions. That 

 associated with the large felspar masses just mentioned can be seen, 

 with a hand lens, to consist of small, flattened, hexagonal prisms, 

 with pyramids at one end, built up in parallel rows. 



The white mica is often in hexagonal plates, as much as ^ inch 

 in diameter. Along the edge of one of the veins on Knocknairling 

 Hill there are triangular flakes arranged in a plumose manner. 

 The mica feathers point inwards from the edge of the dyke, and are 



3 or 4 four inches long. 



