Vol. 60.] OF THE EAST-CENTRAL HIGHLANDS. 425 



Clunie and Glen Callater, continuing a short distance up the latter. 

 Two other exposures occur in the neighbourhood : one in the nose 

 and crest of the hill overlooking the junction of the two streams : 

 the other along the crest of the southern face of the corrie behind 

 Coldrach. It is easily seen that these passage-rocks, before they 

 were folded, could not have been more than a few feet thick. 



3. The Little Limestone. — The typical form of the Little 

 Limestone, as seen in Glen Callater and Glen Clunie, is a remarkable 

 rock, the origin of which becomes clear only after the bed next to it 

 has been examined. The latter shows conclusively that the Little 

 Limestone was originally an admixture of calcite, very finely-divided 

 clastic chlorite, and marcasite, with possibly a small portion of car- 

 bonaceous material. When raised to a high temperature, an unusual 

 combination of elements took place, resulting in the production of a 

 glass-white hornblende (tremolite), in which lime and magnesia are 

 mixed in equal parts, the iron-ore being rejected. The latter being 

 dusted through the rock, which is essentially of massive habit, 

 imparts to it an almost black colour, although the dominant 

 constituent is glass-white. Occasionally yellowish films, mainly 

 composed of epidote and zoisite, occur in it. In some cases there 

 was slightly more lime than was necessary for the formation of the 

 tremolite, and this is now scattered through the rock in small 

 grains of crystallized calcite. The rock often bears a close resem- 

 blance to an epidiorite, and has to be carefully examined in order to 

 place its identity beyond dispute. So long as the Dark Schist, to be 

 described next, is of constant composition, the Little Limestone 

 retains this aspect, and has been recognized as far away as the 

 neighbourhood of Ben Yrackie, near Pitlochry ; but, if the Dark 

 Schist changes in composition, the Little Limestone changes too. 

 The rock is met with in Glen Callater at the first small rapid above 

 the bridge, though another and more interesting outcrop occurs at 

 the sharp bend farther up, a little beyond the quarry, in the flaggy 

 hornfels. The total number of outcrops in this neighbourhood is 

 almost incredible, and shows conclusively the intense and com- 

 plicated folding of the rocks. 



4. The Dark Schist, with the ' twinned-chlorite-rock " 

 and the ' f els par- rock.* — In a type-area, such as that south 

 of Auchallater, where the Dark Schist attains its full development, 

 it is characterized by the presence of an excessive amount of mag- 

 nesian silicates, due to the existence in the original rock of an 

 extraordinary quantity of finely-divided clastic chlorite. This 

 material attained its maximum in the film of rock next the 

 Little Limestone (the Twin-Chlorite Hock), and this is now seen in 

 the form of abundant twinned crystals of chlorite. From this zone 

 upwards, the clastic chloritic material steadily diminishes, on the 

 whole, attaining its minimum close to the Main Limestone, 

 where the most characteristic aluminous silicate is kyanite. or 

 more rarely andalusite. containining no magnesia. As we descend. 



