79 



ON THE DISTINCTIVE COLOURS OF 



THE CARBONIFEROUS AND PERMIAN OR TRIASSIC 



(POIKILITIC) ROCKS OF NORTH CUMBERLAND. 



By T. V. HOLMES, F.G.S. 



(Read at the Carlisle Annual Meeting.) 



My object in this paper is not to discuss the chemical differences 

 in the colouring-matters tinting the Carboniferous and Permian 

 rocks, but to show the importance of these variations of colour as 

 guides to the geological classification of the rocks of North Cum- 

 berland. For the sections in these formations are often so few 

 and far-between that, but for the reliance to be placed on the 

 colour-test, the mapping of their boundary lines would be occasion- 

 ally almost impossible. On the other hand, the remarks I have to 

 make will also demonstrate the fact that colour is an attribute 

 that may lead the geologist very far astray, if unchecked by evidence 

 of a more fundamental character. In short, the importance of 

 colour, in any case in which it affords the sole available evidence, 

 depends upon the extent to which it has been found a true guide 

 where there has been additional evidence sufficient to settle the 

 question without its aid. And the colour-test that might be 

 sufficient in itself where there is much rock exposed to view, 

 might be almost worthless where but two or three square feet are 

 seen. 



The usual, and most characteristic, colours of Carboniferous 

 sandstones are various shades of brown or buff, varying from a 

 deep brown to an almost pure white. The shales interbedded 



