368 PEOCEEDES-GS OF THE GEOLOGICAL SOCIETY. [April 22, 



beds, present features inconsistent with the mere dissolution of the 

 colouring oxide. 



The influence of joints, apart from their functions as channels of 

 infiltration, will also be referred to in consideiing the phenomena 

 of the banding of yellow sandstones (see p. 391). 



There is also another singular form of banding influenced by the 

 presence of joints, viz, in the rock-masses isolated by them. The 

 banded purple and green beds of the Coniston Grits, represented in 

 fig. 30 (PI. XIY.), and in fig. 43, to which my attention was drawn by 



Fig. 43. Banded purple and green Coniston Grits, Ausiwick, liead 

 of Crummuch-Water heck, Clapliam, Yorkshire. 



Professor Harkuess and Dr. Xicholson, present the following cha- 

 racters. The whole mass of the rock is made up of concentric series 

 of narrow purple and green bands, disposed without reference to the 

 stratified structure of the rock, each nest of concentric bands being 

 bounded by the main Knes of jointing; for instance, in a triangular 

 mass the bands concentrically follow its three sides, and in cubical 

 portions have a square disposition, tending to curvilinear forms 

 towards the centres. The finer cracks, which happen to die out and 

 terminate in the middle of a mass, have also curiously influenced the 

 direction of the banding, causing, as in fig. 30 (PI. XIT.), a looping 

 and deflection of the purple and green lines, as though a series of ad- 

 vancing lines had been arrested and held back where intersected by 

 the joints, resulting in a structure resembhng and, I believe, identi- 

 cal in principle with that of folded agates (see fig. 1, pi. xiii. vol. v. 

 Geological Magazine, illustrating Mr. Euskin's paper on '- Banded 

 and Brecciated Concretions "\). The states of combination of the iron 

 in each portion is given in the following analyses, made in the 

 laboratory of the Museum of Practical Geology : — 



Determination of iron in purple and green bands, Coniston Flags, 

 Head of Crummuch-"Water beck, Austwick, nearClapham, Yorkshire. 



The purple bands (Analysis 83) contained : — 



T r Soluble in hvdrocKloric acid 3*104 1 m . i r,r--n ^ 



^^^i Insoluble....* 0.5^1 Total O-6.0 per cent. 



Present as Sesquioxide of iron 1 -347 per cent. 



Protoxide of iron 2-778 ., 



Oiides of iron insoluble, "1 q.-.^o 

 weighed as protoxide J '" " 



Combined water . 1 "990 



