Transactions of the Geological Society, 1st series, vol. 4/On certain colours in Killas

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 coloured fluids of considerable density or tenacity: should the layers of the several fluids have been straight, the curved and wavy appearance is given by producing short and partial disturbances in different parts of the compound. There can be very little question that this rock must have been coloured by a similar operation while in a semifluid state, for on no other hypothesis can the peculiar distribution of the two coloured substances through the whole mass be explained. The continuity of the lines of colour precludes all possibility of a succession of deposited layers, otherwise than in those very lines, and affords at the same time a proof, if any were wanting, that the fissile property of this killas has not been the result of stratification. The whole must in fact be considered as formed either of one deposit, of a semifluid red mud, coloured afterwards by a mixture of blue mud, or of successive layers of red and blue mud. In this state the application of external disturbing force has produced the peculiar contortion here exhibited. It is evident that the theory of softening used to explain the contortion of rocks, is in this case insufficient: a species of fluidity is requisite, otherwise the elongation and narrowing of the blue lines, could not have taken place.

Having established the necessity of consolidation from a fluid state, it remains to ascertain by what powers both the fluidity and the consolidation were effected. There is no difficulty in supposing that the requisite state could exist in a mere mixture of clay and water at the ordinary temperature: but when we consider the large proportion of water requisite to maintain that state in a given quantity of clay, it is difficult to conceive how the disposition of its parts could have been preserved during the great contraction which it must have undergone in the act of consolidation.

  1. Pl. 28. Fig. 2.