In former memoirs upon the crystalline rocks of the north of Scotland, read before this Society, it was shown by one of us, that in the county of Sutherland, in addition to the existence of a fundamental gneiss and an unconformably superposed Cambrian sandstone, there is a conformable ascending series, from certain Lower Silurian quartz-rocks and limestones, up into a group of micaceous and gneissose schists. It was also pointed out, in a general sketch-map of the Highlands, that the order thus observable in the north-western regions of Scotland probably extended southwards across the mountainous tracts to the south of the Caledonian Canal. But this application of the classification of the rocks of the north-west was based on general observations only of earlier years; and a more exact survey was called for before it could be held as proved that the great mass of the Scottish Highlands displayed the same order of succession as had been demonstrated to exist in the north-western tracts. It was also necessary to trace the development of the Sutherland-shire series to the south-west, through Boss-shire, so as to complete the base-line from which the rest of the Highlands should be worked out in detail.