378 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



segregation are also carbonate of iron. Ferruginous nodules of 

 segregation connected with the variegation and depletion of red beds, 

 however, invariably consist of the sesquioxide. Carbonic acid per se 

 is impotent in either reducing sesquioxide of iron to the lower oxide 

 or in dissolving it. If, therefore, interbedded carbonaceous matter has 

 operated as a solvent of sesquioxide in red beds by its reduction to 

 protoxide, its effect must be limited to the parts in immediate contact ; 

 and i^ seems difficult to explain on this theory how, as in the case 

 of the bleached beds at South wold and Clive Hill, the operation can 

 have extended to a distance often or twenty feet from the carbonaceous 

 matter. 



The direct solvent action of humic, ulmic, and other acids the 

 product of organic decomposition, appears a more probable agent 

 of dissolution, and does not necessarily involve the reduction of the 

 sesquioxide to protoxide. According to Bischof (Chemical and Phy- 

 sical Geology, English edition, vol. i. p. 166), humic acid occurring 

 in vegetable mould forms a compound with sesquioxide of iron 

 soluble in 2300 parts of water and crenic acid, a combination so- 

 luble in ammonia. The formation of limonite or bog-iron-ore ap- 

 pears to be the result of such dissolution ; it consists of sesquioxide 

 of iron in combination with variable amounts of humic acid (Dana's 

 System of Mineralogy, p. 178, fifth edition). The aggregation of 

 sesquioxide of iron around roots and other vegetable remains in bogs 

 may thus be incidental to its temporary dissolution by the acids of 

 organic decomposition. 



These simple chemical reactions, though coordinate with them, 

 will not, however, fully account for such phenomena of variegation 

 as those presented by the Argile plasti(|ue and Lower Greensand, in 

 which the variegation has resulted not from the loss of the iron in a 

 soluble condition, but from its rearrangement, — in the one case centri- 

 petally, resulting in its aggregation to concentrated nuclei surrounded 

 by a depleted area ; in the other centrifugally, the rearranged oxide 

 of iron circumscribing the depleted area from which it was dispersed. 



8. On Variegation due to the Decomposition of Bisidphide of Iron. 

 — Among the secondary changes of colour from an altered state of 

 combination, must be noticed that due to the oxidation of bisulphide of 

 iron, which frequently occurs mechanically disseminated in sedimen- 

 tary strata. Its simple oxidation, as is well known, results fii'st 

 in the production of sulphate of protoxide of iron and free sulphur, 

 and ultimately, by the further decomposition of the protosulphate, of 

 a rusty deposit of the hydrous sesquioxide. "When iron pyrites 

 occurs in mechanical association with grey beds charged with car- 

 bonate of protoxide of iron, as in the case of the London clay, the 

 ultimate result of the decomposition appears to be limited to the pro- 

 duction of the sesquioxide ; but in a case which has come under my 

 observation, a more complicated change appears to have taken place, 

 involving the bleaching of beds charged with the anhydrous ses- 

 quioxide. 



At the base of the Ashdown ("Wealden) sands under the East Cliff, 

 Hastings, occurs a bed of light bluish-grey tenacious clay, here and 



