380 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



intervals, and there exist all gradations between the continuous 

 bands and isolated blotches. 



These spherical bleached blotches generally contain a small nucleus 

 of matter identical with that composing the dark-green layers, though 

 not always visible, from the line of cleavage failing to intersect it. 



The questions suggested by these phenomena refer to the time at 

 which the bleaching took place. "Was it before, or concurrent with, the 

 slaty cleavage ? and was the alteration of colour due to a change in 

 the condition, or in the amount, of the colouring-matter? The first 

 question is easily answered ; for, as has been shown by Mr. Sorby, 

 the form of the blotches has been notably affected by the cleavage 

 attenuating them on the transverse section, whilst on the cleavage 

 section they exhibit no distortion. I find also that the bands and 

 blotches have partaken of all the movements affecting the slate, and 

 are frequently broken by faults and dislocations. It is evident, 

 therefore, that the bleaching was antecedent to, and independent of, 

 the cleavage. 



The composition of the discoloured spheres and bands adjacent to 

 the green layers points to a conclusion respecting their character dif- 

 ferent from that arrived at by Mr. Sorby, that " they have been con- 

 cretions of a peculiar kind, formed round bodies lying in the plane of 

 bedding" ('' On the origin of Slaty Cleavage,'* Edinburgh Xew Phi- 

 losophical Journal, July 1853) ; for analyses of the purple and dis- 

 coloured portions of the slate, which have recently appeared in the 

 ' Geological Magazine ' (vol. v. p. 123, March 1868), exhibit no ma- 

 terial difference in general composition, and no aggregation of matter 

 in the light parts that is not also found in the body of the slate ; in 

 short, they are identical, excepting that there has been a departure 

 of about two- thirds of the sesquioxide of iron out of the bleached 

 portions. 



The blue slate, Glyn quarries, Llanberis (xinalysis No. 45), con- 

 tained 



Sesquioxide of iron 5-68 



Protoxide of iron 0-46 



and the bleached bands underlying the dark-green layers (Analysis 



No. 44) contained 



Sesquioxide of iron 1-59 



Protoxide of iron 0'22 



and appear to be precisely analogous to the discoloured spheres and 

 bands in red beds. In these it has already been observed that matter 

 of various kinds, as pebbles, fragments of stones, and fossils, has 

 evidently induced and localized the motion of the colouring sesqui- 

 oxide ; so in the case of the banded slates, the layers of interstratified 

 green matter appear to have similarly incited the departure of the 

 sesquioxide of iron from the adjacent slate, without inducing any 

 change in its state of combination. In the discoloured spheres of the 

 red beds the iron withdrawn therefrom was frequently found to be 

 segregated into small central nuclei ; and it apx^ears probable that the 



