﻿432 DE. G. T. MOODY ON THE CAUSES OP [Aug. I905, 



carbonate expressed in equivalents is generally shown to be higher 

 in the light-coloured portion than in the red mass. It would 

 therefore appear that if the view expressed by Maw (loc. cit.) and 

 H. G. Madau, 1 that the light-coloured rock is formed by bleaching or 

 reduction of the iron in the red rock, be correct, such reduction 

 must have been accompanied by the carry ing-away from the red 

 rock of a considerable part of its iron, and by the introduction of 

 calcium-carbonate and magnesium-carbonate from without. In the 

 case of the Keuper and other variegated marls, this loss of iron is 

 difficult to understand, and appears on chemical grounds highly 

 improbable, owing to the inhibiting influence of the surrounding 

 calcium-carbonate and magnesium-carbonate on the solubility of 

 ferric oxide. To this protective action of calcium-carbonate and 

 magnesium-carbonate reference is made later (p. 434). 



In order to remove any possible doubts regarding the actual 

 composition of the adjacent portions of variegated rocks, it was 

 decided to submit typical examples to careful analysis. Through 

 the kindness of Prof. H. E. Armstrong, Dr. C. G. Cullis, and 

 Mr. L. Richardson, I have been supplied with a large number of 

 specimens of variegated Keuper Marl. Most of these come from 

 the Wainlode-Cliff section, on the left bank of the Severn, between 

 Gloucester and Tewkesbury, where Keuper Marls are exposed for 

 a thickness of 98 feet, 75 feet of which is red marl variegated 

 throughout. Light greenish-grey marls, weathering bluish-green 

 and white, cover the variegated marl ; and it is interesting to note 

 that these are in places coloured yellow, through the downward 

 passage of water carrying iron in solution. Other specimens came 

 from Aust Cliff, near Bristol, and from Seaton (South Devon). 



Each of the specimens examined showed the green and the red 

 marl in juxtaposition, and presented an appearance which clearly 

 indicated that the colour-change was caused by infiltration, though 

 no evidence was afforded as to whether the alteration was from 

 green to red by upward percolation, or from red to green by 

 downward percolation. The texture of the red and the green parts 

 was somewhat similar throughout, although the red mass appeared 

 in some specimens to be a little less coherent than the green. 

 The greater capillarity of the red part was subsequently made 

 evident, by the fact that it was found to hold a larger percentage 

 of moisture than the green part. Each specimen was broken into 

 pieces weighing from 2 to 5 grammes ; and the pieces were sub- 

 sequently sorted, so as to separate the green and the red portions 

 from each other and from the still variegated portions.' 2 The red 

 and green portions were then separately ground to fine powders, 

 and passed through a fine muslin-sieve. 



1 Proc. Cottesw. Nat. F.-C. vol. xiv (1903) p. 132. 



2 However carefully this separation was conducted, the powdered green, 

 portions always contained some red particles, and the powdered red rock some 

 green particles. 



