248 XIXEEAL COMPOSIIIOX OF THE HECTIC BED. [Junei9I2, 



Black Sea sulphides are said to be due to the action of microbes, 

 Bacterium hydrosiilj^lmricvm ponticum and others, which disengage 

 hydrogen sulphide both from decaying organic matter and from 

 alkaline sulphate solutions. Part of the hydrogen sulphide evolved 

 combines with iron to form ferrous or ferric sul^jhide, the remainder 

 effectually poisons the water against the higher forms of life up to 

 within 100 fathoms of the surface. On the coast of Bornholm ^ a 

 yellow film of ferric disulphide is formed on the pebbles on the 

 sea-bottom through the action of decaying seaweed on iron-bearing 

 spring water. In the ' !Mineralmooren ' (chalybeate bogs) of 

 Pranzensbad and Marienbad ^ beds of pyrite and marcasite occur, 

 for the most part enveloping plant-stems. The great development 

 of iron pyrites here is partly due to the numerous chalybeate springs 

 which rise in the moors. 



The Arctic Bed of the Lea Yalley seems to have formed in a 

 marsh, or in a series of pools of stagnant water, subject to periodical 

 floods. In these pools remains of terrestrial plants and marsh vege- 

 tation by their decay, accompanied by bacterial action, abstracted 

 the dissolved oxygen from the water and evolved hydrogen sulphide^ 

 which combined with iron salts to form ferrous sulphide or 

 pyrrhotite. The ferric disulphide (pyrites) may have been formed 

 under less perfect reducing conditions, or by the subsequent altera- 

 tion of pyrrhotite. The ferric hydrate (limonite) resulted from 

 further oxidation. 



YI. Derivation of the Detrital Minerals. 



With the exception of the sulphides and hydrate of iron, formed 

 in situ, all the minerals observed may well have been derived from 

 the Eocene beds over which the Piver Lea flows. The foraminifera, 

 moreover, by their perfect preservation, appear not to have travelled 

 far and come probably from the London Clay, Plint and glauconite,. 

 derived from Cretaceous rocks, are abundant in many Eocene 

 deposits. The heavy minerals, too, are such as are constantly met 

 with in examining the Eocene sediments of the London Basin. 



The occurrence of andalusite perhaps deserves special attention., 

 Mr. Herbert H. Thomas ^ has inferred, from the absence of records 

 of detrital andalusite in sedimentary rocks of greater antiquity than 

 the Pliocene, ' with the possible exception of the Chalk of Beer 

 Head,' that andalusite cannot survive as such in a sand of pre- 

 Pliocene age. If we accept that theory, we must look to the 

 Boulder Clay or the Crags as the immediate source of the andalusite 

 of the Arctic Bed. There seems, however, no reason to doubt Dr.. 

 W. F. Hume's diagnosis of the mineral in the Chalk of Beer Head, ^ 



"^ G. Biscbof, ' Lehrbucli der Chemischen & Plijsikalischeu Geologie ^ 

 1st ed. vol. i (1847) p. 926. 



2 E. Palla, ' Pezente Bildung tou Markasit im Moore ron Marienbad " 

 NeuRS Jahrb. (1887) vol. ii, p. 5. 



3 Min. Mag. vol. xv (1909; pp. 241-44. 



^ ' The Cretaceous Eocks of Britain' Mem. Geol. Surv. vol. ii (1903) p. 510.. 



