﻿Vol. 6 1.] VARIEGATION IN KETJPEK MARLS, ETC. 435 



momentary excess of acid in one part of the beaker, might induce 

 temporary dissolution of the iron, but on stirring the sludge the 

 whole of the iron was immediately reprecipitated in the form of 

 oxide. As soon as the complete decomposition of the calcium- 

 carbonate and magnesium-carbonate was effected, the paste on 

 further addition of acid permanently assumed the colour of iron, and 

 gave the reactions characteristic of it. Precisely-similar results were 

 obtained on treating with acids intimate mixtures of ferric oxide 

 and chalk, of ferric oxide and magnesium-carbonate, and of ferric 

 oxide and dolomite. 



It must, therefore, be accepted that the removal of iron existing as 

 ferric oxide from a homogeneous rock-mass is inhibited, if either 

 calcium-carbonate or magnesium-carbonate, or both of these sub- 

 stances, be present; and it follows that the variegation of 

 marls is not to be explained by the assumption that 

 bleaching of the red rock has occurred through reduc- 

 tion of ferric oxide and loss of iron. 



The adjacent portions of red and green rock, while showing a 

 great difference in total iron, contain very nearly the same per- 

 centage of combined ferrous oxide. This agreement in ferrous- 

 oxide content can scarcely be accidental, and is in itself almost 

 suggestive that the red rock has been produced from the green by 

 the deposition within its pores of finely-divided ferric oxide. It is 

 noteworthy that, on natural weathering, the green marl does not 

 turn brown, but assumes a bluish-grey colour. Nor is any marked 

 change in colour produced on exposing the finely-divided marl, kept 

 moist, to the acid fumes of a laboratory for several months. There 

 is no indication in either case cited, of the production of ferric oxide, 

 such as would be expected if the green colouring-matter had been 

 formed by the reduction of ferric oxide in the red marl. Such 

 reduction would give rise to a ferrous compound, itself very sensi- 

 tive to the influence of atmospheric oxygen; whereas the colour of 

 the green marl is, under the conditions named, extraordinarily stable. 

 The ferrous compound in the green marl remains undissolved after 

 long-continued contact with either a solution of carbon-dioxide, 

 which dissolves ferrous carbonate, or a large excess of very dilute 

 hydrochloric acid ; the rock contains practically no pyrites, and it 

 seems most probable that the colour of the marl is due to ferrous 

 silicate, or to some double silicate containing ferrous oxide. Con- 

 firmation of this view is afforded by the fact that, in determining 

 ferrous iron in the marl, maximum and concordant results were 

 obtained only by a method involving rapid dissolution of the rock in 

 a mixture of sulphuric and hydrofluoric acids. 



Experiments were next made with the object of determining the 

 conditions under which the green marl could be converted into red 

 marl. Such a change involves the addition of ferric oxide, which 

 might be deposited from solutions of iron existing in the earth's 

 crust. In some parts, such as the Wealden area in the Isle of 



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