The next granite to the north must be sought in the Orkney and Shetland islands, where it is very scatter ed, and seldom of any great extent. In Orkney in particular, the appearances are extremely minute, be ing confined to a small spot near Stromness, and an other at no great distance from it in one of the neigh bouring islands. In the Shetland islands, Foula pre sents an equally insignificant patch on its eastern shore; but on the main islands the appearances are more extensive. Here it chiefly occupies the western side of the Mainland, reaching in an interrupted man ner, or in two distinct regions, from the north to the south of its widest portion. Ronas Hill, the princi pal eminence, is formed of granite. The other ap pearances are so scattered, as to be nearly undefinable in words; but we are less anxious to detail the exact geology of the Shetland islands, as we can refer to a very accurate and detailed paper drawn up by Dr. Hibbert in the Edinburgh Philosophical Journal, and accompanied by a map, as well as to his more exten sive and complete work on those islands.
In the western Highlands, on the mainland, the appearances of granite are so scanty and scattered, that we are unable to point out the unmarked and un known places where. they exist, and shall here there fore name only one mass in Kintail, near Loch Duich, because we can refer to a mark for it. In the islands of the same coast, the only mass of this rock is that which forms the Ross, or western promontory of Mull, already noticed for its commercial facilities; a mass which extends also to a little island at some distance from the shore. The granite which was formerly no ticed as found in Harris, is only a vein. On this shore, however, we ought also to indicate a small tract which lies in the neighbourhood of Strontian.
This brings us to Fort William, whence we may re turn eastward through Perthshire. The granite of Fort William forms the base of Ben Nevis; and, after some interruption, it reappears in the moor of Ran noch, and again, in Cruachan and the surrounding mountains, which forms another of its most consider able appearances in Scotland. From the moor of Rannoch, it appears at various places, till, in a certain sense, it may be conceived to join with the great mass of Aberdeenshire, though the exact points of appear ance, and the limits, are not definable without a geo logical map.
Excepting a small portion near Comrie, Arran pre sents the only remaining granite to the northward, forming its well known group of mountains; but as we have already referred to Dr. Ilibbert's accurate
work for the Shetland islands, so we shall here refer for all the nicer details of Arran to that of Mr. Headrick, a similar pattern of minute accuracy; as for the corrector details of the islands in general, we must refer to Dr. MacCulloch's general work on these, comprising matters far too extensive for our present narrow space.
The remaining granite of Scotland must be sought in Galloway, in Criffel, and in the mountains which range from the Fleet along the ridge of Cairns-muir.
The next rock in geological order is gneiss, and this forms the most extensive of the rocks in the northern division of Scotland. To define it by words would be a hopeless task. In Aberdeenshire, it occupies a large portion of that space which is not granite; being, in a superficial view, irregularly intermixed with it. On the north of Scotland, if we commence near Reay in Sutherland, it may be considered to predominate on a line nearly due south nearly as far as Killicrankic; while, westward, it occupies the whole country, with the exception of the granite already mentioned, of some porphyry, and of some considerable tracts of quartz rock and red sandstone. The future remarks on these will give a better positive idea of the extent of the gneiss, in the way of exception, than if we had here attempted to define its most irregular boundaries.
It is not found, as far as we yet know, to the south of the Highlands, or southward of the Clyde and Tay; but it forms a considerable portion of Shetland. The island of Yell is nearly all composed of it, and it occu pies also a conspicuous part of the Mainland. In Foula and Orkney it forms little patches accompany ing the granite. In the Western islands, almost the whole of the long island consists of gneiss, as do Coll, Tirey, and Iona; as well as Rona, and a certain por tion of the south-east part of Sky, and a small part of Mull, connected with the granite of that island.
The characters of this rock present endless variety in Scotland, but it is nowhere used for building. A few of the most fissile kinds are indeed sometimes em ployed for roofing, in the windy districts, being well calculated by their weight, for this office. We already remarked, in the account of the Physical Geography, that the general bearing of the stratification was north eastward; and shall only further add, that though the prevailing dips are southerly, they arc often reversed, while the angle of inclination is also very irregular.