The Causes of Variegation in Keuper Marls and in other Calcareous Rocks

Abstract

I t 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 speculation 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 called attention to the fact that many noncalcareous 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