1868.] MAW VARIEGATED STRATA. 379 



there tinged with red, and somewhat resembling in physical character 

 the white Tertiary clays ; but instead of burning of a light cream- 

 colour in the kiln, the presence of much iron was indicated by the 

 fire changing it to a dark yellowish-brown colour. 



An analysis by Dr. Voelcker (No. 38) indicated that it contained 

 nearly 1| per cent, of iron occurring as 



Basic sulphate of sesquioxide 1 '68 per cent. 



Bisulphide of iron 0*094 ,, 



Patches of pinky red in portions of the clay indicated the former 

 presence of the iron in a state of anhydrous sesquioxide, which also 

 occurs largely in the middle beds of the "VVealden. 



The partial decomposition of the pyrites, of which some still re- 

 mains, seems to have brought about the discoloration. It appears 

 that the whole of the sulphur of the decomposed pyrites has become 

 oxidized, and has entered into combination with the ferric oxide, 

 l^roducing the neutral grey basic sulphate ; and the original light-red 

 colour of the bed has thus became partly obliterated. This is the only 

 case of discoloration distinctly traceable to the decomposition of 

 bisuljihide of iron that has come under my notice ; and few red beds 

 contain a sufficient proportion of pyrites to convert by its decom- 

 position the whole of the colouring sesquioxide into the almost co- 

 lourless basic sulphate ; but the case, though exceptional, should be 

 recorded among the phenomena of variegation. 



9. Variegated Cambrian Slates. — The variegation of Welsh slates, 

 of which examples are given in figs. 29, 31, and 32 (Plate XIY.), and 

 which seem to be the result of a combination of sedimentary and 

 secondary causes, is of two kinds : — first, that consisting of well-de- 

 fined blotches and bands disposed with more or less continuity, and 

 always in harmony with the stratification ; and, secondly, the conver- 

 sion of the blue and purple slates to green, in contact with Trap dykes, 

 and in large fields of colour interlacing irregularly with the normal 

 colours, and disposed without regard to the stratification they 

 vertically intersect. 



These two forms of variegation appear to be due to independent 

 causes. 



The first case of stratified variegation (illustrated in fig. 32, 

 Plate XIV.), when most fully developed, occurs in the form of inter- 

 rupted green bands of two shades of colour, viz. dark olive-green 

 layers of mechanical constitution difi'erent from that of the slate, and 

 adjacent to them a zone of light-green discoloured slate, sometimes 

 occurring on both the upper and under side, symmetrically enclosing 

 the dark layer as a central nucleus, but frequently occurring only 

 on the under side. 



Having but recently described these beds in the Geological 

 Magazine *, I need now only state that whatever may be the precise 

 nature of the dark olive-green bands, it is evident they are of me- 

 chanical origin, and not the result of secondary segregation. Though 

 often continuous for many yards, they are generally interrupted at 

 * Geological Magazine, March 1868, vol. v. p. 123. 



