352 PEOCEEDrS-GS OF THE GEOLOGICAL SOCIEtT. [April 22, 



largely affected their aspect than the recombinations and rearrange- 

 ment of iron. 



In continuation of this subject, treated of in a short paper read 

 before this Society last year *, the following communication records 

 some further observations on those forms of ferruginous variation 

 which appear to have been due to secondary causes, subsequent to 

 original mechanical deposition. 



1. Literature. — It may be convenient in the first place to give a 

 short resume of the previous geological and chemical papers that 

 directly refer to, or bear on the subject. 



Sir Henry James, in a short paper dated May the loth 1843, 

 published in the * London, Edinburgh, and Dublin Philosophical 

 Magazine ' for July 1843, notices that the disposition of the bluish- 

 green discoloration of the Old and New Eed Sandstones is indepen- 

 dent of stratigraphical arrangement, and suggests that, as the light 

 lines and blotches are generally adjacent to joints, the cause of the 

 discharge of colour is due to infiltration. The author, in a note to 

 this paper, also refers to an observation by Mr. Mallet, that ^'if 

 through a fissure in a rock containing peroxide of iron a stream of 

 water should pass containing an earthy sulphate and organic matter, 

 the sulphate will be decomposed, and sulphuretted hydrogen evolved, 

 which might reduce the peroxide of iron to a lower oxide." With 

 reference to this suggestion, I will here only observe that the pre- 

 sence of sulphate of lime in the Keuper marls of Cheshire and Der- 

 byshire seems to have no relation to their variegation, as the bands 

 and crystals of gypsum occur in contact with both the red and the 

 grey portions, and, furthermore, the grey blotches in the marl often 

 occur independently of the presence of joints. 



Sir Henry de la Beche, in the first volume of the ' Memoirs of the 

 Geological Survey,' pubhshed in 1846, p. 254, refers to the alterna- 

 tion at Aust Chff, Gloucestershire, of red marls of the Keuper with 

 blue or greenish bands, and gives analyses of each, indicating a 

 nearly similar composition, excepting that the iron in the blue marls 

 was said 4;o exist wholly in a state of protoxide, and in the red as a 

 mixture of protoxide and sesquioxide, the amount of iron in each 

 being nearly identical ; and he attributes the difference of colour to 

 the reduction of sesquioxide to protoxide by the agency of decaying 

 organic matter in the lighter parts of the marl. At pp. 52, 53, 57, 

 & 267, reference is also made to the particoloured strata of the Old 

 Eed Sandstone, in the grey beds of which carbonaceous matter and 

 protoxide of iron are said to occur, whilst in the red beds the iron 

 was found to be wholly in a state of sesquioxide. 



The next paper bearing on the subject is one by Dr. J. W. Daw- 

 son, in the Quarterly Journal of the Geological Society, vol. v. 

 p. 25, read May 31st, 1848, relating to the red beds of Nova Scotia. 

 It discusses the question whether the sesquioxide of iron colouring 

 them is in its prinuordial condition, or the result of a secondary 

 change from the decomposition of iron pyrites ; which latter view 

 the author supports, and attributes the blotchy discoloration to the 

 * Quart. Journ. Geol. Soc. vol. xxiii. p. 114. 



