1868.] 



MAW VAKIEGATED STRATA. 



?>\ 



coloured rock {h), the bleaching of which and loss of red colour 

 appears, as in the case of the Southwold section, to be connected 

 with infiltration from the uppermost bed charged with humic and 

 ulmic acids. 



A somewhat similar case is given in fig. 17, Plate XIII., of a 

 specimen (in the Museum of the Jardin des Plantes, Paris) of a car- 

 bonaceous fossil in red Carboniferous sandstone at Ardenay, in which 

 the colour immediately adjacent has been bleached. 



The bleaching-power of carbonaceous matter is further illustrated 

 in fig. 2Q, Plate XIIL, of the Lower Green Sand near Folkestone, 

 where some carbonaceous spots are Yig. bO .—Bleached patch hi yel- 



loiu ferruginous sands accom- 

 panying the Qrag, Wang ford 

 Crag -pit, Suffolk. 



surrounded by discoloured zones, 

 and these again concentrically sur- 

 rounded by bright-yellow rings of 

 hydrous sesquioxide of iron, which 

 appears to have been displaced 

 from the central area. Similar 

 bleached patches frequently occur 

 in the sands of the Crag district ; 

 and these, as in fig. 50, are ahvays 

 surrounded by a zone darker than 

 the general colour of the sands, and 

 coloured apparently by the iron 

 withdi^awn from the lighter area. 



These phenomena present two 

 distinct points for consideration : 

 — first, simple chemical reaction and the mechanical washing out of 

 the iron in a soluble condition ; secondly, the rearrangement of the 

 colouring oxide, which cannot be explained by simple chemical and 

 mechanical agencies. 



Both of these processes seem to have operated in the production 

 of those variegations of ferruginous strata which are connected with 

 the presence of organic matter. 



The Southwold, Clive Hill, and Codsall examples of bleaching 

 appear to be the result of simple dissolution, and mechanical removal 

 of the iron brought into a soluble condition. The generally ac- 

 cepted explanation, and that originally suggested by De la Beche, 

 is, first, the reduction of the colouring-power of the iron by the 

 conversion of the sesquioxide to protoxide, from deoxidizing contact 

 with organic matter, and, secondly, the dissolution of the protoxide 

 thus formed by carbonated water. 



The experiments by Kindler and Bischof (Chemical and Physical 

 Geology, vol. iii. page 1, English edition) establish the possibility of 

 these reactions ; but it is remarkable that in none of the cases that 

 have been made the subject of the foregoing analyses does the bleach- 

 ing appear due to simple deoxidation ; and, furthermore, the propor- 

 tion of protoxide to sesquioxide of iron is not increased in the bleached 

 areas of red beds. 



In grey beds accompanying carbonaceous deposits the iron is 

 almost wholly in a state of carbonate of protoxide, and the nodules of 



