1915-] SCHUCHERT— BLACK SHALE DEPOSITION. 233 



occurs at greater depths. The shores are high and bold on the 

 northeast, east, and southwest, and flat on the north and northwest. 



Andrussow^ has described the physical and bionomic conditions 

 of the Black Sea as follows : Beyond the shallow marginal waters 

 of 600 feet depth there is no bottom-living life (benthos), while in 

 the surficial fresher waters down to about 750 feet there is a more 

 or less great abundance of floating, usually microscopic, open-sea 

 forms (plankton) and the larger, free-swimming life (nekton), col- 

 lectively also spoken of as the pelagic biota. This upper layer of 

 freshened water and its peculiar life conditions are brought about 

 by the enclosed nature of the deep basin, the inflowing of immense 

 quantities of less dense fresh water thaf remains at the surface or 

 is there evaporated, and a deep-seated, partially compensating cur- 

 rent of salt water from the Sea of Marmora through the strait of 

 Bosporus. It is estimated that it takes about 1,700 years to renew 

 the entire salt-water content of the Black Sea. 



Because of these differences between the lighter surface and 

 the heavier bottom salt waters, there is no vertical streaming nor 

 convection currents beyond 750 feet of depth, and therefore no re- 

 plenishing of the deeper marine waters with the oxygen that is so 

 necessary for the maintenance of benthonic life. At the depth of 

 600 feet, hydrogen sulphide begins to form (33 c.c. in 100 liters of 

 water) and increases rapidly with the depth to 3,000 feet (570 c.c.) 

 and then more slowly to the bottom of the sea. The formation of 

 the HoS is in the main due to the sulphur bacteria. Hand in hand 

 with the increase of the H^S goes the decrease of the sulphates in 

 the sea water and the precipitation of the carbonates and iron sul- 

 phides. 



That the aeration of marine waters, and also the generation of 

 sulphuretted hydrogen may be better understood, a digression into 

 the studies of oceanographers becomes necessary. The atmospheric 

 gases, oxygen and nitrogen, are absorbed at the sea surface more 

 abundantly in cold than in warm latitudes, and the quantity absorbed 

 is again variable under varying pressures and chemical conditions 

 of the water. This complex subject, too long to state here, may be 



2 " La Mer Noire," Guides des Excursions, VIP Cong. Geol. Internat, 

 St. Petersbourg, 1897, Art. XXIX. 



