_Lewisian Gneiss._ Ever since the researches of Murchison and Nicol in the north-west of Scotland, it has been known that two distinct systems of rock underlie the quartzites to which I have just alluded. Murchison regarded the upper of these as of Cambrian age, while he assigned the unconformable quartzites and limestones above it to the Lower Silurian period. But the recent discovery of the _Olenellus_ zone intercalated conformably between the quartzites and the overlying limestones may be regarded as proving that all the rocks which underlie the quartzites and are separated from them by a strong unconformability must be pre-Cambrian. It is thus established beyond any reasonable doubt that two great pre-Cambrian systems of rock exist in the north-west of Scotland. These two systems differ so entirely from each other that their respective areas can be defined with minute accuracy. The uppermost consists chiefly of dull reddish sandstones with conglomerates, and especially towards their base in Rosshire, some bands of dark grey shale, the whole having a thickness of at least 8,000 or 10,000 feet, though as both the base and the top of the series are marked by strong unconformabilities, the whole original thickness of deposits is nowhere seen. As these rocks are well developed around Loch Torridon, they were named by Nicol the Torridon Sandstone--a designation which has more recently been shortened into "Torridonian." The lower system is mainly composed of various foliated rocks which may be embraced under the general term "gneiss." These masses present the usual characters of the so-called "fundamental complex", "Urgebirge," or "Archæan Series" of other countries. The contrast between the thoroughly crystalline, gnarled, ancient-looking gneisses below, and the overlying, nearly horizontal Torridonian conglomerates, sandstones, and shales, which are largely made out of their debris, is so striking that every observer feels persuaded that in any logical system of classification they can not be both placed in the same division of the geological record. They are certainly both pre-Cambrian, but they must belong to widely separated eras, and must have been produced by entirely different processes. If it is proposed to regard the gneisses as "Archæan," we must refuse to include the Torridonian strata in the same section of pre-Cambrian time. But so much uncertainty exists as to the application of this term Archæan, examples are so multiplying wherein what was supposed to be the oldest and truly Archæan rock is found to be intrusive in rocks that were taken to be of much younger date, and there are such slender grounds for correlating the so-called Archæan rocks of one country with those of another, that I prefer for the present, at least, not to use the term at all. Let me very briefly state some of the main characteristics of the two sharply contrasted rock-systems of the north-west of Scotland.