SEA-FISHERIES LABORATORY. 181 



tated as ferrous sulphide. So one often finds a dead cockle- 

 shell in clean sand with some black sand in the shell cavity. 

 Wet ferrous sulphide very easily oxidises, of course, and there- 

 fore the upper layers of sand are always clean. But the black 

 stuff underneath exists under conditions that are nearly 

 anaerobic and so the oxidation is confined to the upper layer. 

 Near the shore there is a continual access of organic matter, 

 in the form of land drainage, which penetrates into the sand 

 and renders the process of formation of ferrous sulphide a 

 continuous one. Further out, over the littoral zone, there is 

 not so much of the organic matter for it is the more easily 

 distributed by the tide, and so the bleached layer is thicker and 

 the underlying discoloured stratum is not so black. 



The same process, as was pointed out by J. Y. Buchanan,* 

 occurs on the sea-bottom in deep water and is responsible 

 for the formation of blue and black muds. Such sulphide- 

 containing muds and sands do not (or, at least, need not) be 

 found among sedimentary strata, for oxidation would generally 

 have led to the bleaching. But, in such strata, the presence 

 of much iron in the form of Fe 2 3 may usually indicate that 

 the sediments from which they have been formed have been 

 the seats of putrefactive processes. 



* See "Accounts Rendered/" Cambridge University Press, 1919j 

 pp. 133-158. 



