160 THE DEPTH AND MARINE DEPOSITS OF THE PACIFIC OCEAN. 
GENERAL REMARKS AND CONCLUSIONS. 
The character of a deep-sea deposit is determined not only by the depth 
and conditions of the overlying waters but also by the geographical environ- 
ment of those waters, including volcanic phenomena which may take place 
on the floor of the ocean. It may here be proper to follow out the general 
changes in the nature of the deposits which may be expected on proceeding 
from the remotest land-locked branches of the ocean to its midmost areas. 
The Black Sea, to take an extreme instance of enclosed seas, is far re- 
moved from the great oceans, and the deeper water is cut off by submarine 
barriers at the Bosphorus and the Straits of Gibraltar. Vertical circulation 
is much restricted by this and other physical conditions, and 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 pulverulent carbonate of lime is 
precipitated from the sea water of the Black Sea, and makes up a consider- 
able part of the deposits now in process of formation ; this is the only place 
where this reaction is known to occur in the seas of the present day, with 
one doubtful exception in the Pacific (see Station 4531). In all partially 
enclosed seas—like the Mediterranean and the Red Sea— circulation is 
generally cut off by submarine barriers, and this produces temperature and 
other conditions which are less favorable to organisms than the conditions 
existing in the open ocean, where free communication with polar waters 
obtains, and where there is consequently a more abundant supply of oxygen. 
The deposits in those seas which interpenetrate the continental masses are 
terrigenous in origin, although there may be an approach to pelagic condi- 
tions towards the more central parts, as for instance in the Red Sea. 
The continental shelf facing the great oceans is continually swept by waves, 
tides, and oceanic currents, down to an average depth of 100 fathoms. 
Just beyond this depth all the minute mineral and organic fragments torn 
from the continental shelf come to rest on the bottom, and form what Sir 
John Murray has called the mud-line. This constitutes the great feeding- 
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