360 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [April 22, 



range of these discoloured areas in relation to the mechanical com- 

 position of the stratum, which is well shown in fig. 14, Plate XII., 

 representing a portion of the Old Red Conglomerate of Porfarshire, 

 composed for the most part of red pebbles held together by a com- 

 minuted sandstone of the same material. In this example the 

 blotches of secondary discoloration range in common through the 

 impervious pebbles and the red matrix. In some instances the 

 chemical nuclei of the light blotches occur in the sandstone ground, 

 whilst the range of bleaching extends into the pebbles that happen 

 to come within its radius ; and in others the segregated nucleus 

 occurs in the very body of the pebble, whilst the circumscribing 

 area of discoloration spreads itself in common partly through the 

 pebble and partly through the surrounding sandstone. 



A more singular case than this occurs in the Carboniferous lime- 

 stone at Trevor, near Llangollen, where an interstratified layer of 

 red gravel (fig. 7, Plate XI.), probably derived from the Old Eed beds, 

 is permeated by bands of greenish discoloration, which run through 

 the loose mass, affecting the one half of individual pebbles that in- 

 tersect its line of boundary, leaving the other half unchanged. This 

 partial change of colour cannot be the effect of mere infiltration, 

 because the pebbles lie loosely without the slightest cohesion, so that 

 any water passing through one part of the bed must have pervaded 

 the whole mass. 



The more common forms of variegation of red beds are so fafniliar 

 to geological observers that any further general description is un- 

 necessary, and it now remains to consider the chemical composition 

 of the several coloured areas. 



The following analyses have been made with special reference 



(1) to the amount of the colouring oxide and of the metallic base, 



(2) to its state of combination in the several coloured areas, and 



(3) to general composition in relation to variation of colour. 



The first example (Analyses, Nos. 6, 7, 8) is a nucleated form of 

 variegation of Permian Sandstone near Coalport, Shropshire (fig. 9, 

 Plate XI.), viz. a red ground mottled with light blotches concen- 

 trically surrounding much darker nuclei. 



No. 



6. The red ground contained Protoxide of iron 0'538 "I Metallic iron 



„ „ „ Sesquioxide of iron 1'520 J 1470 percent. 



7. The dark central nucleus Protoxide of iron 0-646 1 Metallic iron 



Sesquioxide of iron 12-260 J 9-000 per cent. 



8. Light zone, siu'rounding nucleus Protoxide of iron 0"010 1 MetaUic iron 



,, „ „ Sesquioxide of iron 0-700j0'491 per cent. 



A somewhat similar disposition of colour occurs in the Permian 

 Sandstone penetrated in sinking the new coal-pits at Kemberton, 

 near Madeley, Shropshire (fig. 8, Plate XL), in which not only the 

 spherical nucleated blotches occur, but also discoloured bands with 

 a central dark line of sesquioxide of iron intersecting ohliquely the 

 heels of stratification — a fact which suggests that the variegation in 

 this case could not have been induced by anything interbedded with 

 the deposition of the sandstone. 



