﻿438 . DR. G. T. MOODY ON THE CAUSES OF [Aug. 1905, 



other cases, such as the green spots in the red and purple Cambrian 

 slates of North Wales, and the curious concentric shells of alternating 

 red and green colour in the Houghton Whetstones of the West 

 Hiding of Yorkshire. 



Prof. H. E. Armstrong spoke of the value of the paper, as showing 

 how numerous were the points of contact between geology and the 

 other sciences. The explanation advanced by the Author that the red 

 coloration of many rocks was due to the displacement of magnesium 

 and calcium by ferrous iron, which subsequently became oxidized, 

 was both rational and reasonable ; but it remained to ascertain 

 what were the physical peculiarities which affected the percolation 

 of the clay by chalybeate waters. It was not merely a question of 

 this or that distinct layer being penetrated: at Seaton, for example, 

 little blue masses occur interspersed throughout the red clay in the 

 most irregular manner possible. The speaker also referred to the 

 interest attaching to the determination of the nature of the mineral 

 which was the essential constituent of the blue clay. 



Prof. Seeley said that the alternation of red and green clays and 

 marls, though not limited to freshwater deposits, was characteristic 

 of the Headon Beds, the Woolwich and Reading Beds, and especially of 

 the Keuper Harls, which were presumably freshwater deposits. In 

 no case did these beds show evidences of infiltration. In the Reading 

 Beds the clays in several localities showed current-bedding, with 

 alternations of lamination of scarlet clays with green clays. This 

 was some evidence that the clays were coloured when originally 

 deposited. When the scarlet beds were followed for some distance 

 they became plum-coloured, and the green beds became grey, and 

 more or less mottled. The only condition of alternation which 

 would affect the oxidation of iron in the sediments of all deposits 

 of all ages, was the alternation of the seasons ; and he suggested 

 that the green marls were formed in cold winter-seasons, and the 

 thicker red clays in warmer wet seasons. 



Dr. J. W. Evans referred to the replacement of carbonate of lime 

 by iron-ore in some of the Jurassic limestones, as a parallel to the 

 changes shown by the Author to have taken place in the variegated 

 marls. In the latter case, he suggested that the chalybeate waters 

 had ascended from the red Triassic sandstones. He thought that, in 

 some instances, the occurrence of red bands or patches might be 

 determined by the previous distribution of carbonate of lime or 

 magnesia which caused the deposition of iron. He mentioned that 

 the ' silver-amber ' phlogopite-mica of Brazil showed brown patches 

 in the neighbourhood of cracks, and that these on analysis proved 

 to contain more iron and water and less magnesia than the unaltered 

 portions of the mica. 



Hr. 0. A. Shrtjbsole remarked that he had observed instances, 

 in the Gres de Hay of Normandy, where a greenish quartzite had 

 been partly stained red by infiltration. He agreed, however, that 

 the explanation hardly covered other cases, such as the mottled clay 

 of the Reading Beds. 



Hr. J. V. Elsden asked whether the Author's attention had been 



