OF THE HIGHLANDS. 417 



phyllites have been interstratified, and of the Arenaceous zone which succeeds 

 them to the north, generally reveals the planes of foliation and shear to be 

 roughly coincident with the bedding, though there are numerous exceptions. 

 In many places, such as in the Dunoon section, the foliation-planes in the 

 phyllites have been themselves folded, and that oftener than once. This 

 structure is only explicable on the supposition that the argillaceous beds 

 must have been driven en masse between the more massive arenaceous beds 

 with which they were interstratified, the latter having simply a rough foliation 

 developed along their original bedding-planes, while the argillaceous beds 

 were twisted into innumerable folds with a much more perfect foliation. 



Immediately N.W. of this Lower Argillaceous zone another broad band of 

 rocks, with a more or less developed foliation occurs. Because of the 

 predominance in it of coarsely fragmental materials it is here termed the 

 Lower Arenaceous zone, varying from exceedingly fine-grained light and dark 

 greywackes to massive grits with pebbles often an inch in diameter. These 

 pebbles generally consist of quartz or felspar set in a finely comminuted 

 felspathic matrix. 



Associated with these rocks occurs a group known as "the green beds" 

 because of their marked greenish colour. They generally show a more or 

 less well-developed schistose structure and are characterised by the presence 

 of granular epidote, chlorite, and dark green or brown biotite. That they 

 are of fragmental origin seems undoubted, but the conditions under which 

 they were originally deposited are uncertain. When first mapped by the 

 officers of the Survey they were supposed to be of pyroclastic origin, the 

 green colouring matter being looked upon as similar to the fine dust of 

 volcanic tuff, but the complete absence of interbedded lava-flows, or of any 

 pebbles referable to igneous rock, throws doubt upon such an origin. Like 

 the phyllites they immediately succeed they exhibit a gradually increasing 

 metamorphism from east to west, the grits and greywackes becoming more 

 highly schistose. 



Geographically this Lower Arenaceous zone may be traced from St. 

 Fillans, in Perthshire, through Ben Vorlich, Ben Ledi, and Ben Venue, 

 giving rise to the characteristic scenery of the Trossachs ; thence along the 

 shores of Loch Lomond below Rowardennan and Inverbeg into Cowal, where 

 it may be seen at the head of the Gareloch, and along Loch Eck, where 

 it gives rise to a peculiarly rugged type of scenery. 



Above the group just described occurs one of the best marked zones of all 

 the schists seen in the southern Highlands, that known as the Loch Tay 

 limestone, the typical rock of which is a crystalline, black, grey, or mottled 

 limestone, generally showing distinct crystals of calcite. With it are associated 

 beds of calcareous mica-schist which often appear to split the limestone up into 

 several bands. It is well exposed along the shores of Loch Tay, and can be 

 traced westwards along Glen Dochart to near Crianlarich, and thence into 

 Argyllshire at Ballimore, near Strachur, in Glendaruel, and near Kilfinan. 

 Upon this horizon occur massive beds of sheared basic rocks which accompany 

 the limestone in a remarkable manner from E. to W. across the whole of the 

 southern Highlands. 



Above this zone comes a band of mica-schists and quartz-schists character- 

 ised by the abundance of their garnets, sometimes in such quantities as to 

 form the larger bulk of the rock. This group which has been named the 

 garnetiferous schist-zone, forms a tolerably well-marked horizon immediately 

 above the Loch Tay limestone, and can be traced along the mountains N. 

 of Loch Tay into Glen Dochart, and thence into Argyllshire, where it 

 accompanies the limestone which it apparently overlies. 



A well-marked group of phyllites, schistose-quartzites, and graphite- 

 schists, the Upper Argillaceous zone succeed this. Like those of the Lower 

 Argillaceous zone, the rocks of this zone seem to have been driven between 



2D 



