THE FRESH-WATER LOCHS OF SCOTLAND. 131 



The Blair Atholl limestone has an important development in the 

 neighbourhood of Blair Atholl, and up the valley of the Tilt towards 

 the limit of the basin. Sharing in all the folds of the associated 

 phyllites and black schists (group 9), its outcrop is irregular and 

 involved. Where these zones appear, in the Tilt, in the Tay, and 

 Strath Tummel, they generally give rise to softer outlines than the 

 quartzite which apparently overlies them. 



The Perthshire quartzite (group 11) is, perhaps, the most striking 

 geological sub-division in the metamorphic series of the Eastern 

 Highlands, from its greater durability and the lofty mountains to which 

 it has given rise. Along its northern margin the rock is more or less 

 coarse-grained, due to the presence of pebbles of quartz and felspar, 

 but this band is repeatedly brought to the surface by means of folding. 

 An interesting feature of this group is the presence of a conglomerate 

 or boulder bed with rounded blocks of granite, foreign to the area, the 

 matrix of which seems to vary with the rock in contact with it. Some- 

 times appearing as lenticular or boat-shaped masses surrounded by 

 black schists, phyllite, or limestone, and again as narrow belts traceable 

 for several miles, the quartzite is always one of the dominant features 

 of the landscape, occasionally forming lofty peaks, as in Ben-y-Ghlo 

 and Schichallion. 



In addition to the sub-divisions of the metamorphic rocks of the 

 Eastern Highlands which have just been described, there is a group of 

 crystalline schists termed the " Moine series " by the Geological 

 Survey, which have a wide distribution in the north-west part of the 

 Tay basin. Their lithological characters are remarkably persistent over 

 wide areas. Consisting mainly of quartzose granulitic schists or fine- 

 grained gneisses with bands of mica-schist, they represent without 

 doubt a highly altered series of sediments, the original clastic grains 

 of which have been destroyed. They form nearly the whole of the area 

 north of Loch Rannoch, up Glen Garry, and northward of Glen Tilt. 



Reference has already been made to the intrusive sheets of basic 

 igneous rock which appear in association with the Green Beds and Loch 

 Tay limestone, but others occur in connection with the zones of calc- 

 sericite schist and black schist. Perhaps the most remarkable example 

 of the latter is the mass of epidiorite and hornblende-schist on Ben 

 Vrackie north of Pitlochry, where the altered sediments have been 

 deflected and bent round the laccolitic intrusion. 



The acid igneous rocks which were injected into the sedimentary 

 series, before the folding and development of schistosity in the latter, 

 are best represented by the foliated granite of Ben Vuroch, north-east 

 of Ben Vrackie. On the north-west slope of that mountain, the 

 sediments, which still retain their original bedding, have undergone 

 contact alteration, the calcareous shales having been converted into 

 calc-silicate hornfels. 



