THE CRYSTALLINE SCHISTS OF THE HIGHLANDS. 



BY PETER MACNAIR. 



THE area occupied by the metamorphic rocks about to be described may 

 be defined as that lying immediately N. of the great boundary fault 

 which, crossing Scotland from shore to shore, with a N.E. and S.W. trend, 

 divides the Highlands from the Lowlands geographically, and the Crystalline 

 Schists from the Old Red Sandstone geologically. The position of this great 

 line of demarcation has been more or less accurately fixed. It can be traced 

 through Arran and Bute, thence from near Toward Castle to Innellau, and 

 across the eastern point of Rosneath peninsula, and by Helensburgh across 

 Loch Lomond northeastwards by way of Balmaha, Aberfoyle, Callander, 

 and Comrie. 



The valley of the Clyde proper after traversing the Silurian rocks of the 

 Southern Uplands and the later palaeozoic rocks of the midland valley 

 eventually abuts against the great wall of crystalline schists forming the 

 frontier Highlands near Helensburgh, where the Gareloch runs northwest- 

 wards into them. Further on, Loch Long, Loch Goil, the Holy Loch, Loch 

 Striven, Loch Fyne, and other sea-lochs trench deeply into the crystalline 

 schists, exposing along their shores numerous sections of great value in 

 unravelling the structure of this region. 



Above Helensburgh the Clyde receives at Dumbarton the waters of Loch 

 Lomond, which drains part of the Highland area, and is the most easterly 

 drainage system falling into the Clyde valley from the crystalline schists 

 of the Highlands. It will, however, be well here to include the ground 

 further east as far as Loch Tay, as the rocks there still occupy their normal 

 positions, whereas along the Firth of Clyde, owing to a succession of earth- 

 movements, the original sequence has been obscured and their physical 

 structure complicated. 



As it is impossible to speak with certainty as to the physical structure, 

 succession, or age, of the rocks forming the southern or marginal Highlands, 

 it may be well to briefly sketch the history of opinion concerning them. 

 In 18i9 Macculloch l pointed out for the first time the opposite directions of 

 the dips in the schists of Cantyre, where the beds of micaceous schist on the 

 eastern shore have an eastern dip, while those on the opposite side dip west- 

 wards. Murchison 2 also at a later period refers to an anticlinal fold at 

 Loch Eck. In 1860 Jamieson 3 described in more detail the anticlinal axis 

 observed by Macculloch and Murchison, correlating the argillaceous series of 

 Bute with that of Loch Fyne, which he states arches over into an anticlinal 

 axis, the more arenaceous rocks of Cowal forming the core of the anticline at 



i "Western Islands of Scotland," Macculloch, 1819, vol. ii., p. 288. 

 ' 2 Quart. Journ. Geol. Soc., vol. vii., p. 169. 



3 " The Geological Structure of the Southwest Highlands," Quart. Journ. Geil. Soc., 

 1860, vol. xvii., p. 133. 



