The problems in tectonic geology presented by the complicated structures of the northwest of Scotland have been ably worked out by the officers of the Geological Survey, to whose report in the _Quarterly Journal of the Geological Society_ for 1888, I would refer for full details. It has been shown that, besides stupendous dislocations and horizontal displacements, the rocks have been cut into innumerable slices which have been driven over each other from the eastward, while at the same time there has been such a general shearing of the whole region that for many hundreds of square miles the original rock-structures have been entirely effaced, and have been replaced by new divisional planes, which, when they approach the underlying Cambrian strata, are roughly parallel with the bedding planes of these strata. In this region, therefore, we have striking proofs of a stupendous post-Cambrian regional metamorphism. But there is still much uncertainty regarding the geological age of the rocks which have been affected by it. There can be no doubt that large masses of the old gneiss, torn up from below, have been thrust bodily westward for many miles, and are now seen with their dykes and pegmatites resting on the Durness limestones and quartzites. It is equally certain that in other districts huge slices of the Torridon sandstones have been similarly treated. But where all trace of original structure has disappeared, we have, as yet, no means of definitely determining from what formation the present eastern schists have been produced. The ordinary gneissose and quartzose flagstones do not appear to me to be such rocks as could ever be manufactured by any chemical or mechanical process out of the average type of Lewisian gneiss. I have long held the belief that they were originally sediments, but whether they represent altered Torridon Sandstone, or some clastic formations which may have followed the Durness limestones, but which have been everywhere and entirely metamorphosed, remains for future discovery. For my present purpose, it is sufficient to observe that, in the meantime, as we can not be sure of the origin of most of the rocks, which, between the West Coast and the line of the Great Glen, have been subjected to a gigantic post-Cambrian regional metamorphism, it seems safest to exclude them from an enumeration of the pre-Cambrian rocks of Britain.