slate. Although dark blue or grey is the predominant colour, it occasionally varies through different shades to pale grey and greenish grey, and its texture is equally liable to variations. It is a pure carbonat of lime with the exception of the colouring ingredient iron, containing no notable proportion of other earths, except where it is intermixed with the siliceous or argillaceous laminæ. The same is true of the white varieties, except that where they are much mixed with steatite and serpentine they yield magnesia upon being analyzed.
The dip of the beds is invariably to the south, but the quantity of that angle is not sufficiently constant to render its measurement an object of interest. It seems to vary from five to fifty and even to sixty degrees. In one place and one only I observed a considerable contortion of the beds, and in many others there are fractures and dislocations to be seen. Yet with such partial irregularities we may still safely consider the general parallelism and stratification as regular, and the dip as a medium constant quantity of perhaps twenty degrees. A few porphyry veins are found to traverse these beds, an appearance too common to call for any particular notice. Nor is it possible for want of marks to refer to the places where they occur. Contortions similar to those here described are not infrequent in Scotland, and they have been often supposed to depend on the vicinity of trap or granite. They are however to be seen in many places where neither of these rocks can be found, and I have observed in the island of Sky[1] a series of stratified rocks of which the evenness and parallel horizontality is perfect, although they are traversed by trap veins of enormous magnitude and great frequency. No general conclusions therefore respecting
- ↑ See the first paper in this volume.
