1868.] MAW VARIEGATED STKATA. 369 



Tlio green bands (Analysis No. 84) contained : — 



T f Soluble in liydrochloric acid 2-232 1 m <. i .1 —i-* ^ 



^^■""1 Insoluble... ^.r,02|Total2-/34 per cent. 



Present as Sesquioxide of iron 0370 per cent. 



Protoxide of iron 2-532 ,, 



Oxides of iron insoluble, "1 ^.^40 

 weighed as protoxide J " 



Combined water 2*223 , , 



The rock therefore contained originally about 3 per cent, of iron, 

 partly as sesquioxide and partly as protoxide and silicates ; and the 

 banding appears to be due to the segregation of the sesquioxide into 

 the concentric purple layers, the green colour of the intervening 

 courses resulting from the eccposure of the green silicates and the 

 protoxide, by the removal of nearly the whole of the obscuring 

 sesquioxide. 



Many jointed rocks exhibit this secondary concentric banding 

 ivifhin each separate 7nass, bounded by lines of joint — some merely 

 ^vith respect to colour, by the rearrangement of the oxides of iron ; 

 but in others (for example, jointed granites and some Trap rocks, 

 which exfoliate in concentric layers tuiihin each portion bounded by 

 joints) there appears to have been a partial rearrangement of the 

 mass of their constituents, and a corresponding modification of me- 

 chanical structure. 



These curious phenomena seem tcf be more related to the isolation 

 of the masses by the joints than to the mere fact of the jointing; 

 for in Avaterworn stones the direction of the lines of secondary 

 banding is often determined by the contour assumed after separation 

 from the parent rock. In the case of flints banded with yellow and 

 grey concentric layers of iron, in different states of combination, the 

 bands range with the waterworn outline and not the original con- 

 tour of the flint. 



6. On the Variegation of the Keuper Marls. — The arrangement of 

 the red and grey colours in the Xeuper Marls is almost as capricious 

 and anomalous as in some of the examples already referred to ; and any 

 satisfactory explanation is rendered difiicult, on the one hand, by the 

 disposition of colour being apparently related to stratification, and, 

 on the other, by its being evidently the result of secondary causes. 

 Fig. 13 (PI. XII.) represents a portion of the section at Worcester 

 station ; its general aspect suggests that the alternation of the red 

 and grey bands is simply the result of interstratification ; but the 

 interlacing outline at their junction shows that the grey beds are 

 merely an altered condition of the red — isolated patches of grey 

 breaking irregularly into the general red ground. There are also 

 continuous grey beds, some of which are harder than, and of diff'erent 

 mechanical composition from the red ; and there are isolated patches 

 of grey, which in general have a horizontal range, coincident with the 

 stratification ; but the recurrence of the individual blotches on their 

 horizontal range is evidently determined by secondary causes. A 

 portion of the beds included in the section are intersected by ver- 

 tically disposed zigzag lines of hard stony matter, possibly repre- 



2d2 



