Page:A Treatise on Geology, volume 1.djvu/133

 CHAP. VI.

In the British islands we have but few opportunities of beholding a complete section of the gneiss and mica schist system; the Scottish Highlands, in fact, must alone be appealed to and from these, perhaps, the most satisfactory result which can be gathered, is that which is derived from a general view rather than from any one district, like Braemar, Loch Sunart, or Loch Tay.



In the accompanying diagram the two great formations of which the system consists, viz. gneiss and mica schist, are placed in their order of succession above the granite. The gneiss is generally the lowest, thickest, and most extensive: it includes primary limestone (Iona, Assynt) quartz rock (Assynt, Loch Eribol), hornblende schist (Glen Tilt). Estimates of its thickness must be wholly conjectural; but we may believe it to exceed many thousand yards.

Passing to the south-east from the granite of Strontian King's House, or Cairn Gorum, we traverse the gneiss and reach the mica schist, near the base of which (Schihallion, Ben y gloe, Balachulish) quartz rock usually occurs. In different parts of the mica schist, primary limestone occurs in stratified masses, of limited extent and, sometimes, lenticular shape (Balachulish, Killin, Loch Earn, Inverary); and it seems probable these might be employed to subdivide the great mass of mica schist, were it likely to be of any use or interest where no organic remains and few mineral variations are to be recorded. The mica schist rocks are some thousand yards in thickness.

The upper parts of the mica schist (Loch Earn, Loch Lomond) become chloritic, and might, perhaps, deserve