XXI. On a peculiar Disposition of the Colouring Matters in a Schistose Rock.[1]
By the J. Mac Culloch, M.D. F.L.S. President of the Geological Society, Chemist to the Ordnance, Lecturer on Chemistry at the Royal Military Academy, and Geologist to the Trigonometrical Survey.
[Read 6th December, 1812.]
The rock which this drawing represents is well known in the
country where it occurs by the popular name of Killas; and as the
reasonings to be founded on it will not be affected by the use or
omission of more scientific terms, I shall not wait to determine
under which of these names it ought to be ranked.
It is to be observed at the back of the Gun wharf at Plymouth dock, where it has been cut to a smooth face to make room for the Ordnance department in that yard.
On inspecting the drawing, it will be seen that the fissure of the killas is perpendicular to the horizon. The general colour of the mass is a faint brown red, and a number of dove-coloured stripes of unequal thickness may be seen traversing it in very irregular curved lines, but bearing a sort of parallelism or relation to each other. To say that it resembles strongly a piece of marble paper, will be a comparison as illustrative as it is familiar.
If we pursue the same familiar analogy we may be led to explain the method by which the mass of killas acquired this peculiar disposition of its colouring matters.
It is well known that the operation of marbling, either in oil or water, is produced by partially mixing together two or more
- ↑ Pl. 28. Fig. 2.
