18G8.] 



MAW VAKIEGATUD STRATA. 



367 



Fig. 42. Bunter Sandstone, 

 Linlet/, near Bride/north. 



first example is represented in fig-. 42, of a vertical bleached line in 

 the Bunter Sandstone between Linley and Bridgnortli. 



The analysis of the lied Sandstone (No. 1) already given at p. 3(53, 

 showed that it contained 1*3 per cent, of 

 sesquioxide of iron, whilst the bleached 

 seam contained less than one-third of 

 the amount, the general composition of 

 the stones being otherwise similar. 



A rather different form of joint- 

 bleaching, analyses of which (Nos. 11 

 and 12, p. 364) have also been given, 

 is represented in fig. 24, PI. XIII., of 

 chocolate-coloured Cambrian Grits, 

 Bayston Hill, near Shrewsbury, in 

 which the discoloured bands follow the 

 intersecting lines of jointing both ver- 

 tically and horizontally, often isolating 

 the dark ground into oblong brick- 

 shaped forms, entirely surrounded by 

 the bleached bands. In this the same 

 process seems to have taken place as 

 iu the case of the Bunter, there being 

 nearly two-thirds less iron in the light 

 than the dark portions, with no mate- 

 rial difference in the proportion of 

 protoxide to sesquioxide, the insoluble 

 sesquioxide being the condition in 

 which it chiefly occurs throughout 

 the rock. Now on looking at these 

 bleached lines ranging with the permeable joints, they appear at 

 first sight due to some kind of solubility and washing out of the 

 colouring-matter ; but a careful examination shows that there is no 

 essential difference between them and the isolated spherical blotches. 

 In the vertical bleached lines in the Bunter (fig. 42), the discoloured 

 line is here and there interrupted and broken up into isolated 

 blotches. In the Cambrian grits of Shropshire (fig. 24, PI. XIII.) 

 eveiy gradation occurs between the discoloured joint-lines and the 

 completely separate bleached patches; indeed all these examples 

 serve to show how capriciously the discolouring action has been 

 localized — in some cases strictly following the joints, and in others 

 branching out into irregular masses that have no reference to them. 



In comparing these two forms of joint- variegation, viz. the rust- 

 ing of beds charged with protoxide, and the bleaching of those 

 coloured with the red sesquioxide, it seems difficult to assign to in- 

 filtration, which has failed to remove the more soluble carbonate of 

 protoxide of iron in grey beds, the power of removing the compara- 

 tively insoluble sesquioxide of iron adjacent to the joints in red beds. 



In considering the subject of the influence of org\anic matter in 

 the bleaching of red beds, certain cases will be referi^d to in which 

 the acids formed during the decomposition of organic matter appear 

 to have acted as solvents of sesquioxide of iron ; but the great 

 majority of cases, in which bleached spots and joints occur in red 



VOL. XXIV. PAET I. 2d 



