XXI. On a peculiar Disposition of the Colouring Matters in a 

 Schistose Rock. 



By 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.] 



X HE 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 dis- 

 position 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. 



3e 2 



