Ixviil PROCEEDINGS OF THE GEOLOGICAL sociETy. {May 1902, 
supplies the necessary vital energy, and converts the carbonate 
present into sulphate with the liberation of carbon-dioxide. Thus 
under anaérobic conditions, such as those which exist in the deeper 
portions of the Black Sea, carbonates are formed at the expense of 
sulphates, while under aérobic conditions, such as those which exist 
at a depth of about 200 fathoms in the same sea, sulphates are 
formed at the expense of carbonates. 
The deposits found in the Black Sea are such as would be expected 
under these conditions. It is only in shallow water that oxides of 
iron and manganese are found. ‘The deposits which underlie the 
deeper stagnant waters are analogous with the blue muds of the 
open ocean, from which they differ in containing a variable amount 
—in some cases over 40 per cent.—of amorphous carbonate of lime, 
and a much greater amount of sulphide of iron, The last-mentioned 
ey is of special interest. It is found in all the deposits 
from 220 fathoms down to the greatest depths, usually as minute 
spherules, disseminated through the mud or deposited in the diatoms, 
but sometimes occurring as larger spherules or in elongated irregular 
forms suggestive of minute twigs. These observations throw im- 
portant light on the origin of the pyritized diatoms of the London 
Clay, and the minute spherules and twig-like forms of pyrites so 
common in the washings of Kimmeridge and many other fine- 
erained argillaceous deposits. 
In regions where there is a free vertical circulation the chemical 
conditions which prevail throughout the greater portion of the 
Black-Sea basin are found only in the deposits themselves, and 
chiefly in the blue muds, the waters of which often contain 
sulphuretted hydrogen. The chemical changes which go on both 
in and at the surface of these muds have been worked out in great 
detail by Murray & Irvine, and the results are published in the 
papers to which I have already referred. Iron and manganese are 
often found in solution in the head-waters of streams as carbonates, 
especially in those streams which drain peat-bogs; but they are 
soon thrown down as oxides on pebbles and other objects on the 
beds of the streams. These oxides may be rubbed off the pebbles, 
carried out to sea, and deposited along with the fine argillaceous 
material. Owing to the presence of organic matter in the blue 
muds, reduction takes place. The iron is fixed in the deposit as a 
sulphide, but the manganese-sulphide is decomposed by carbonic 
acid, which is necessarily present, thus giving rise to sulphuretted 
hydrogen and a soluble bicarbonate of manganese, which diffuses 
., 
} 
