176 MARINE DEPOSITS 



(5) Volcanic Muds. — In the deeper water surrounding vol- 

 canic islands are deposits of fine mud made from the disinte- 

 gration of volcanic rocks, mixed with considerable clay, and also 

 calcareous matter derived from organisms. 



b. The Pelagic Deposits are those, the materials of which are 

 not directly derived from the land, but consist of matters carried 

 to the sea in solution and extracted from the sea-water by the 

 agency of organisms, together with volcanic substances in a more 

 or less advanced stage of decomposition. Only rarely are terrige- 

 nous materials found in these deposits, as, for example, off the 

 west coast of Africa, where fine sand, carried by the wind from the 

 Sahara, is found in deep water, and ice-borne fragments are common 

 in high latitudes. The pelagic deposits are found far from land, 

 and to a great extent, in the deepest oceanic abysses. In these 

 profound depths the rate of accumulation is almost inconceivably 

 slow, and the remains of extinct animals still lie exposed, or but 

 slightly covered, upon the ocean floor. 



(1) Foraminiferal Ooze. — The Foraminifera are minute ani- 

 mals, each one a tiny speck of jelly, most of which, in spite of their 

 extreme simplicity of structure, have the power of secreting very 

 beautiful and complex shells of CaC0 3 . The species which are 

 of importance in this connection are those which live in infinite 

 multitudes at the surface of the ocean, and the most abundant at 

 the present time are those which belong to the genus Globigerina, 

 whence this deposit is frequently called Globigerina ooze. These 

 surface Foraminifera flourish best in warm water and follow the 

 warm currents, often into quite high latitudes. Their shells, which 

 drop to the bottom as the occupants die, are present in almost all 

 marine deposits, but near land the terrigenous materials prepon- 

 derate to such a degree that the Foraminifera make up but a slight 

 proportion of the deposit. In deeper water, where the wash from 

 the land does not come, the foraminiferal shells grow relatively 

 much more abundant, and when 30% or more of a given sample 

 of the bottom consists of them, it is classed as a foraminiferal ooze. 

 Other organisms which secrete calcareous shells or tests always 

 contribute more or less to these oozes (coral mud, echinoderms, 



