﻿Yol. 6 1.] VAKIEGATION IN KETJPEE MAELS, ETC. 437 



calcareous strata. Subsequently, when conditions were favour- 

 able, owing to subsidence of water, air has been introduced, and the 

 ferrous carbonate has been converted into ferric oxide. That the 

 Keuper Marls have been variegated in this way admits of no doubt* 

 The pale blotches, stringers, and spherical masses occurring in the 

 red marl, represent not parts where reduction of ferric oxide has 

 taken place, but portions of the rock-mass which at the time of 

 percolation were hard, crystalline, and impervious to the chaly- 

 beate solution. As bearing on this, it is noteworthy that both the 

 red and the green parts of the variegated marl contain practically 

 the same amount of carbon present as organic matter. This even 

 distribution of organic carbon through the variegated rock is scarcely 

 to be interpreted in favour of the view that the green marl has 

 been derived from the red marl, by reduction of ferric oxide owing 

 to percolation of water containing organic matter in solution. 1 



Tho continuous passage of chalybeate water through strata 

 containing calcium and magnesium would, in time, rob them entirely 

 of these constituents; and, therefore, it may be properly inferred that 

 rocks which are saturated with and are exuding chalybeate water 

 are devoid of calcareous matter. 



The even distribution of ferric oxide in the Triassic rocks of 

 England makes it probable that the colour of the red sandstones, as 

 well as of the red marls, has been caused by the action of chalybeate 

 water, which it must be assumed permeated the whole of the New 

 Red Sandstone and part of the overlying marls. Since the pale 

 blotches in the red Bunter Sandstone contain only about one-third 

 of the iron present in an equal mass of the red matrix, it is likely 

 that careful investigation will show that the red matrix contains 

 ferrous oxide equivalent to the whole of the iron in the pale blotches ; 

 and should this be the case, additional evidence of the deposition of 

 ferric oxide from external sources will be afforded. 



To what extent the colour of other red rocks, such as the Red 

 Chalk of Hunstanton, is to be attributed to the action of chalybeate 

 waters, is an interesting subject for enquiry ; but it must be 

 accepted that the ease with which ferrous carbonate is precipitated 

 from solution, and converted into ferric oxide, makes it highly 

 probable that chalybeate waters have played a very important part 

 in the colouring of rocks. 



Discussion. 



The President congratulated the Author in that he was able, by the 

 aid of chemistry, to throw light upon the origin of rocks concerning 

 which palaeontology was almost silent. It would be interesting to 

 know how the water was introduced which produced a change in a 

 great thickness of rocks now coloured red, leaving a thin mass of 

 Tea-Green Marls above. He hoped that the Author would investigate 



1 SeeH. G. Madan, Proc. Oottesw. Nat. F.-C. vol. xiv (1901-03) p. 132. 



