of the Scottish Highlands. 173 



The quartz-schists and mica-schists of this zone represent between 

 their two extremes an infinite variation of their two constituent 

 minerals — mica and quartz. The quartz-schists seldom, however, 

 lose so much of their mica as to approach the typical quartzites of 

 the lower zone, while the mica-schists, on the other hand, rarely 

 become so argillaceous as to approach the finely foliated mica-schist 

 of the upper argillaceous series. Both the micaceous and quartzose 

 varieties are often highly charged with garnets, as in and around 

 both Loch and Strath Tay. 



The grits of this series, like the other members just described, 

 show a more distinct tendency to foliation than those belonging to 

 the lower series, the finer matrix in which the pebbles of quartz are 

 imbedded becoming more or less foliated, and wrapping each in- 

 dividual pebble in a series of plates of mica, giving the rock 

 somewhat of a gneissose aspect. The larger pebbles of quartz show 

 evidence of having been drawn out along the lines of shear, and look 

 more like veins of segregation than original quartz pebbles. 



VI. The Looh Tay Limestone Zone. 



The next zone which falls to be mentioned is that of the Loch 

 Tay limestone, which, as we have already noticed, is found inter- 

 calated in our middle arenaceous group, the quartz and mica-schists 

 of the latter being found both to underlie and overlie the band of 

 limestone. The rocks of this zone are not always pure limestone, 

 but often pass into a calcareous mica-schist, the whole pointing, as 

 in the lower argillaceous zone, to the land-surface having readied 

 a maximum of depression below the level of the sea. Below this 

 limestone band and upon this horizon occur massive beds of sheared 

 basic rocks, which follow the limestone in a remarkable manner ; 

 wherever it is found, in fact, they may be said to be coextensive 

 with the limestone from the east to the west coast across the whole 

 of the Southern Highlands. They are evidently basic igneous rocks 

 which have been intruded into the schists and limestones at a period 

 prior to their metamorphism, and which have subsequently under- 

 gone the same shearing process as the clastic rocks, being now 

 structurally identical with the schists. 



The Loch Tay limestone has been traced by the officers of the 

 Geological Survey from shore to shore across the Southern High- 

 lands, through the shires of Banff, Perth, and Argyll. In Perthshire, 

 where I have principally been enabled to trace its outcrop, it is found 

 to extend from Crianlarich in the east, down the north side of Glen 

 Dochert to Killin, and thence north-eastwards by Loch Tay to 

 Fernan, where it is abruptly cut off by a fault. It does not seem to 

 appear again in Strath Tay, but after passing Strath Ardle it appears 

 again at Ashintully, Kirkmichael, and Mount Blair. 



VII. The Upper Argillaceous Zone. 



The upper argillaceous zone, which I have given as succeeding 

 the higher members of the middle arenaceous group, is characterized 

 by a series of highly altered argillaceous rocks, the principal 



