1868.] MAW VARIEGATED STRATA. 359 



with fragments of other rocks of various colours, which have not 

 been so changed, and also with derivative fragments of dark-red marl 

 containing more sexquioxide of iron than the general matrix. 



4. On the Bleaching of lied Beds due to Abstraction of the Colour- 

 ing Oxide. — Whatever may have been the primordial condition of 

 the sesquioxide of iron pervading red beds, the occurrence of pale 

 blotching seems to have almost invariably supervened on a uniformly 

 red colour ; in short, the pale portions have been produced on a red 

 matrix, and not the red colour partially introduced on a lighter 

 ground. This is a point at once rendered obvious by the relative 

 disposition of the light and dark colours. 



The several forms presented by the blotching of red beds have been 

 fully described with reference to the red beds of Devon in the 

 Memoir by Mr. Pengelly already noticed ; and the accompanying il- 

 lustrations (figs. 4-11, Plate XI.; 12-1 6, Plate XII.; 27,PlateXIV.; 

 and figs. 41, 42, 44, 45, 49, infra) represent the principal characte- 

 ristics presented by the phenomena in various formations. Of all the 

 forms that variegation takes, the occurrence of isolated blotches en- 

 vironed on all sides by the primordial colour seems to be most ob- 

 viously independent of mechanical arrangement. In the red beds 

 of the Bunter, Keuper, Permian, and Carboniferous, such blotches 

 are of common occurrence, and are more often than otherwise inde- 

 pendent of any apparent predisposing cause. 



In some instances, especially in the Keuper beds, the blotches seem 

 to have a tendency to range with the stratification ; and the lines of 

 unconnected light patches (fig. 13, Plate XII.) merge by insensible 

 gradations into regular stratified beds of alternating colour. In other 

 cases, as in the Permian and Gres bigarre (figs. 8 & 9, Plate XI., 

 and fig, 12, Plate XII.), the blotching is altogether independent of 

 the lie of the beds ; and fields of light colour, obviously of secondary 

 origin, vertically intersect alternating strata of difi'erent physical 

 character and composition ; and in contrast with this apparently ad- 

 ventitious disposition must be noticed the occurrence of light bands 

 and blotches, the localization of which is evidently connected with 

 a predisposing cause, such as the existence of vertical and horizontal 

 joints, and of mechanical nuclei, as pebbles of various rocks, fragments 

 of fossils, &c. 



Perhaps the most perplexing point in connexion with the pheno- 

 mena is, on the one hand, the apparently adventitious occurrence of 

 the discoloured areas, and, on the other, their occasional connexion 

 with predisposing causes — and yet, at the same time, the evident 

 identity of the two extremes, which are connected by every variety 

 of intermediate grade : for instance, variegation apparently deter- 

 mined by a line of joint will branch out and spread itself irregularly 

 into an unjointed mass, and this again leads up to a group of isolated 

 blotches ; again, blotches formed concentrically round a mechani- 

 cal nucleus, may be closely associated with blotches having no such 

 centre, and blotches both with and without a nucleus of segregation 

 may occur on the same piece of stone. 



Another point to be noticed is the entire independence of the 



