226 THE SEA FLOOR 



10. Coral Mud. — This deposit occurs off coral 

 islands and coral reefs, and is chiefly made up of 

 fragments of organisms living in the shallow waters 

 and on the reefs, such as calcareous Algae, Corals, 

 Molluscs, Polyzoa, Annelids, Echinoderms, and Fora- 

 minifera. In the shallower waters near the reefs 

 these calcareous fragments are larger, and the more 

 finely divided calcareous matter is less abundant, than 

 in the deeper waters further removed from the reefs ; 

 the deposits are then called Coral Sands. These de- 

 posits may contain, at times, much volcanic material. 



Coral Mud is estimated to cover an area of about 

 2,700,000 square miles (or 7,000,000 square kilo- 

 metres) . 



General Remarks. 



The Black Sea is far removed from the great oceans, 

 and its deeper water is cut off by submarine barriers 

 at the Bosphorus from the Mediterranean, which in 

 its turn is cut off from the Atlantic by an additional 

 barrier at the Straits of Gibraltar. Vertical circulation 

 is much restricted by this and by the fact that the 

 surface layers are often much less saline than the 

 deeper layers ; hence the deeper waters of the Black 

 Sea become more or less saturated with sulphuretted 

 hydrogen ; no living organisms (other than bacteria) 

 are met with in depths greater than 100 fathoms. 

 Amorphous carbonate of lime is precipitated from the 

 water of the Black Sea, and makes up a considerable 

 part of the deposits now in process of formation ; this 

 is the only place where this is known to occur in the 

 seas of the present day. In all partially enclosed seas 

 — like the Mediterranean and the Red Sea — circulation 

 is generally cut off by submarine barriers, and this 

 results in thermal and other conditions which are 



