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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



to his paper, and will here only state the grounds upon which they 

 seem open to question. 



In the first place, the condition in which the iron occurs in bright- 

 red clays and sandstones is principally that of the anhydrous, or 

 slightly hydrotis sesquioxide, whilst the sesquioxide ultimately de- 

 rivable from the decomposition of pyrites is the yellow hydrous 

 form. This, however, is not a fatal objection, as the experiments of 

 Mr. Edward Davies, P.C.S., of Liverpool, recorded at p. 69 of the 

 fourth volume of the new series of the Chemical Society's Journal, 

 prove the possibility of the hydrous ferric oxide being reduced to a 

 brick-red subhydrate, containing only from 4 to 5 per cent, of 

 water, by prolonged heat in the presence of water, at temperatures 

 considerably below the boiling-point. 



The occurrence in common of bisulphide of iron both in red beds 

 and in those containing carbonate of iron supplies an argument 

 against the derivation of the red sesquioxide from its decomposition * 

 indeed the decomposition of pyrites seems occasionally to have caused 

 the obliteration of a preeooisting red colour, instead of having been the 

 agent in its production ; but this point will be considered further on, 

 in connexion with the causes of variegation. 



Dr. Dawson further suggests that sulphate of lime in red beds 

 may be an incidental product of the oxidation of pyrites in con- 

 tact with calcareous matter; but the association of red marls 

 and gypsum is by no means general. On the one hand, many red 

 marls containing calcareous matter are entirely devoid, or con^ 

 tain but a mere trace, of sulphate of lime*; and on the other, gyp- 

 sum and selenite as frequently occur in grey beds containing car- 

 bonate of iron, as, for instance, in the Lower Purbeeks, Oxford 

 Clay, Kimmeridge Clay, the French Tertiaries, and the grey marls 

 of the Muschelkalk ; and the occurrence of sulphuric acid seems to 

 have no regular relation to the presence of either protoxide or ses- 

 quioxide of iron in strata. 



The physical evidence in favour of the primordial occurrence of 

 the anhydrous sesquioxide seems also as strong as the chemical. In 

 the first place, there seems no 'prima facie reason for the greater 

 probability of the detrital accumulation of the bisulphide than of 

 the red sesquioxide ; if the bisulphide were specially characteristic 

 of the earher rocks, and the red sesquioxide of the more recent de- 

 posits, the probability of the derivation of the latter suggested by 

 Dr.^Dawson might be implied ; but, as a matter of fact, dissemi- 

 nated bisulphide of iron is much more abundant in the Oolites, 

 London Clay, &c. than in the earher rocks of Cambrian and Silurian 

 age ; and, furthermore, the earhest red beds (for example the Old 

 lied Sandstone), containing red pebbles not merely surface-coated, 

 but red throughout their mass, prove the detrital derivation of their 

 red colour from an earlier red rock. The red colour of the pebbles 

 is clearly not the result of a change of colour aff'ecting the entire 

 mass of the old red beds ; for they are associated in the red matrix 



* See Analysis of Keuper marls, given at p. 370, Carboniferous marls, 

 p, 365, and Marnes Irisees, p. 384. 



