﻿Vol. 6 1.] THE CAUSES OF VARIEGATION IN KEUPER MAULS, ETC. 431 



20. The Causes of Variegation in Keuper Marls and in other 

 Calcareous Rocks. By Gerald Tattersall Moody, D.Sc, 

 F.C.S. (Communicated by Prof. W. W. Watts, M.A., M.Sc, 

 F.R.S., Sec.G.S. Read June 21st, 1905.) 



It has long been known that the colouring of stratified rocks is 

 chiefly due to the presence of compounds of iron, although the 

 conditions under which deposition of iron has taken place have not 

 been fully determined. The occurrence of red and green masses of 

 rock in juxtaposition, such as is observed at the junction of the 

 Tea-Green and Red Marls of the Keuper, has given rise to specula- 

 tion regarding the origin of variegation, but no hypothesis affording 

 a satisfactory explanation of the chemical changes which must 

 accompany such phemonena has been hitherto advanced. 



It is generally taken for granted that the red colour of a 

 variegated rock is determined by the presence of ferric oxide, while 

 the green colour is attributed to iron in the ferrous state ; and the 

 suggestion has been made that the condition of the iron in the 

 distinct parts of a variegated rock may, in some cases, be dependent 

 on the relative amount of calcium-carbonate present. As far back 

 as 1868, George Maw L called attention to the fact that many non- 

 calcareous rocks are red in colour, and he advanced the opinion that 

 the lighter-coloured bands in variegated rocks have been formed 

 from the red or brown rock by addition of calcareous matter from 

 without. This view, that the green portion has been derived 

 from the red portion, appears to be generally accepted by geologists 

 at the present time; and so it seems desirable to quote at 

 length the concluding paragraph of that section of Maw's com- 

 munication which deals especially with the discoloration of red 

 rocks by lime and magnesia, and runs as follows : — 



'Although an increase, from extraneous sources, of the calcareous element in 

 any red bed seems to induce bleaching and discoloration, it, does not hear a 

 perfectly-regular ratio to the proportion of lime present, as discoloration has 

 taken place in some red beds connected with the infiltration of a less amount 

 of carbonate of lime than that originally present in others that have not 

 been so changed, and a blood-red colour pervades the base of the Ohalk at 

 Hunstanton. It may, however, be generally stated that a bright-red colour 

 is a character of non-calcareous strata, and dun-colour, or grey, of calcareous 

 beds, quite irrespectively of the amount of iron present.' (Op. cit. p. 386.) 



On reference to the published analyses of red rocks and of their 

 so-called 'bleached' or discoloured parts, it is seen that the lighter- 

 coloured portion usually contains a percentage of iron considerably 

 smaller than the adjacent red-coloured portion. On the other hand, 

 the aggregate percentage of calcium -carbonate and magnesium- 



1 Quart. Journ. Geo]. Soc. vol. xxiv (1868) pp. o51 ct seqq. 



