150 MR. G, BAKROW ON THE OCCURRENCE OF [May 1 898, 



the fact that, even in the slate, the white spots are still big 

 enough to recognize with a hand-lens. Microscopic examination 

 proves that they are present in great number, but the crystals are 

 extremely small and ill defined, tending to occur in groups. In 

 fact they are almost identical with the ottrelite described by Mr. W. 

 M. Hutchings^ as occurring in a slate near Tintagel. As the con- 

 taining rock becomes more crystalline, both the chloritoids and the 

 spots tend to become larger, till in the schist in Friar Glen Burn 

 we see numerous dark-green glistening crystals, often as large as a 

 pin's head, and white spots as big as peas. In some of the speci- 

 mens of this type the microscopic sections show a perfect gradation 

 from the largest to the smallest crystals, and clearly prove that the 

 mineral in the slate is the same as that more commonly seen in 

 this schist. Without this graduated series one could not have been 

 sure of this identity. 



Description of the Rock. 



The chloritoid-rock is in all cases a typical ' felt ' of white mica 

 and chlorite, associated with a considerable amount of quartz, but 

 very little felspar. The larger chloritoid-crystals are either irre- 

 gularly-shaped plates or flakes, giving a mere suggestion of prismatic 

 outline, or lath-shaped, with jagged ends — obviously the plate seen 

 edgewise. Binary twinning is almost always shown by the latter 

 under the microscope (see figure, p. 154) ; polysynthetic twinning 

 is distinctly rare. When the crystals fall below a certain size they 

 never show any twinning. 



The spots are numerous, and so full of inclusions of the minerals 

 of the rock-matrix that at times their boandaries can be made out 

 only under crossed nicols, when they form areas of very low double 

 refraction. Very small chloritoid-crystals (one might almost say 

 ' specks,' they are so small) occur sometimes in this spot-material, but 

 they are not common. The real nature of this material we have 

 been unable to determine ; the optical properties are ill defined, 

 probably because it seems very liable to decompose. A considerable 

 amount of it was obtained after treatment with hydrochloric acid, and 

 proved to be composed chiefly of silica, iron, and alumina. The field- 

 evidence leaves little doubt that the spot-material represents a stage 

 in the formation of staurolite, but this cannot be actually proved. 



One other feature of the chloritoid-rock may be worth noting 

 here. In common with most of the finer sediments of the High- 

 lands, originally rich in chlorite, this band contains a great number 

 of schorl-crystals. As in the case of the chloritoid and the spots, 

 the crystals increase in size as the rock containing them becomes 

 more coarsely crystalline. 



Optical Properties and Hardness of the Mineral. 



The binary twins show strong pleochroism ; when the shorter 

 diagonal of the prism is at right angles to the trace of the twin-face 



1 See G^eol. Mag. 1889, p. 214. 



