18G8.] MAW VARIEGATED STRATA, 371 



matter ; and their general composition was identical, excepting as 

 regards the iron, of which there seems to have been a departure 

 of the greater part of the sesquioxide from the grey portion. The 

 protoxide of iron in the grey is somewhat in excess of that in "the 

 red ; but the difference is scarcely more than might occur in any two 

 portions of the same stratum, and is insufficient to prove its secon- 

 dary derivation from the red sesquioxide. The mottling of the 

 Keuper Marls seems, therefore, in principle to differ in no respect 

 from the blotchy variegation of other red clays and marls ; and the 

 two analyses of the Worcester example are worthy of note because 

 they fail to exhibit the kind of change in the state of combination 

 of tlic iron indicated by the analyses, given by De la Beche, of the 

 Keuper beds at Aust Passage, viz. the reduction of the sesquioxide 

 to protoxide without any departure of the iron. 



7. On the Influence of Organic Matter in inducimj Variegation. — 

 The connexion between the blotching and discoloration of red beds 

 and the presence of fossil carbonaceous and peaty matter has re- 

 peatedly been noticed by geologists, and reference has already been 

 made to the observations of De la Beche on this subject in the 

 Memoirs of the Geological Survey. The generally accepted theory, 

 and that suggested by De la Beche in explanation of the pheno- 

 menon, is, that the discoloration has been brought about by the 

 reduction of the sesquioxide to a lower state of oxidation of less 

 colouring-power by simple chemical reaction with the fossil carbon. 

 The experiments of Bischof, and also those of Kindler, published in 

 Poggendorff's Annalen, establish the fact that sesquioxide of iron, by 

 simple contact with organic matter, is capable of being reduced to a 

 state of protoxide. 



With a view to ascertain how far the reduction of the colour of 

 red beds may be due to this process, I have procured analyses of 

 the bleached and unbleached portions of a number of examples of 

 ferruginous strata in which the variegation seems to be connected 

 with the presence of fossil carbon. 



The mottled beds of the Woolwich and Reading series, especially 

 in their development as the Argile plastique of the Paris Basin, are 

 particularly instructive in explaining the nature of the secondary 

 disposition of ferruginous colouring as apparently dependent on 

 accompanying organic matter. 



Figure 45 represents a section at Yaugirard, Paris, from the 

 Upper Chalk to the Calcaire grossier ; the mottled beds of the Argile 

 plastifjue (fig. 21, Plate XIII.) included in the section are overlain 

 by 15 or 20 feet of grey laminated clays containing beds of impure 

 lignite and pervaded throughout by carbonaceous matter. The 

 gradation of this character downwards seems to be connected with 

 the arrangement of the mottling of the underlying Plastic Clay. 

 These beds include five distinct and well-marked colours, viz. : — 

 Ist. The bright blood-red, which ax)pears to have been the pri- 

 mordial tint ; 

 2nd. A light pinky red ; 

 3rd. A dark brown ; 



