384 Correspondence — S. S. Biickman. 



characterizing each local (Oxfordshire) stratum have been collected 

 from the beds in situ. Lists of fossils are given. 



4. " The Causes of Variegation in Keuper Marl and in other 

 Calcareous Eocks." By Gerald Tattersall Moody, D.Sc, F.CS. 

 (Communicated by Professor W. W. Watts, M.A., F.E.S., Sec.G.S.) 



Analyses of a large number of specimens of Keuper Marl bring 

 to light the fact that the red portion of a variegated rock contains 

 a considerably higher percentage of iron and lower percentages of 

 calcium-carbonate and magnesium-carbonate than the green portion. 

 This is in agreement with earlier observations that red strata are 

 usually less calcareous than adjacent green strata ; but the author 

 is unable to confirm the hypothesis advanced by Maw,^ that the 

 lighter-coloured bands in variegated rocks are produced by bleaching 

 or discoloration of red rocks through addition from extraneous 

 sources of lime and magnesia. On the contrary, it is found that 

 the removal of iron existing as ferric oxide from a homogeneous 

 rock-mass is inhibited if calcium-carbonate, or magnesium-carbonate, 

 or both of these substances be present. When, however, a chalybeate 

 water (ferrous bicarbonate solution) permeates a rock containing 

 magnesium-carbonate or calcium -carbonate, iron is precipitated 

 as ferrous carbonate, and an equivalent weight of magnesium or 

 calcium passes into solution. Subsequent access of air oxidizes the 

 ferrous carbonate, and red or yellow ferric oxide is produced. The 

 author finds that when a green marl is subjected to the action of 

 chalybeate water in the manner indicated, it is readily converted 

 into a material closely approximating, both in appearance and in 

 composition, to the natural red marl of the same rock-mass. 



The author concludes that the variegation of the Keuper Marls 

 and of other calcareous rocks has been brought about by the 

 percolation of chalybeate water through the light-coloured mass, the 

 more porous parts of which have in consequence become stained 

 with ferric oxide, while the harder and more crystalline parts, 

 being non-porous, have remained unchanged. The uniformity in 

 distribution of ferric oxide in sonie red rocks, such as the New Red 

 Sandstone, suggests that the iron contained in them has probably 

 been derived from chalybeate water in a similar manner. 



The next Meeting of the Society will be held on Wednesday, 

 November 8th, 1905. 



C0E,i^:ESi30iNrx).H3isrc:E. 



WHERE ARE THE TYPES? 



Sir, — May I ask if any of your readers can kindly give me the 

 information as to where may be found the types of S. P. Pratt 

 (Kelloway Ammonites from Christian Malford) and of Young & 

 Bird (Lias Fossils in "Geology of Yorkshire") ? The information 

 is required for the purposes of the Palc&ontologia Universalis. 



Westfield, Thame, Oxon. S. S. BuOKMAN. 



1 Quart. Journ. Geol. Soc, vol. xxiv (1868), pp. 35 et seqq. 



