DKEf-SEA DEPOSITS. 553 



exist some whicli form, as it were, an iutennediary between tliem and to 

 which the authors give the name littoral deep sea deposits. 



Terrigenous debris still form the principal part of these deposits. In 

 fact, among the products carried oif from terra firina, there are some 

 which remain suspended long enough in the air or in the sea to be trans- 

 ported at last into the domain of the deep seas. It is thus that par- 

 ticles of quartz and of other rocks, the continental origin of which is 

 easily recognized, have been met with at a depth of 7,000 meters. 



The blue muds must be specially noted : They are characterized by a 

 slate color which comes from the presence of organic matter in a state 

 of decomposition. They often exhale an odor of sulphuretted hydrogen 

 and in that case they are mixed with sulphide of iron. This often 

 happens in the vicinity of a continent where large rivers bring in sus- 

 pension reducing organic substances. Fragments of minerals such as 

 quartz, mica, and feldspar, in very fine grains of the diameter of half 

 a millimeter at most, often appear in them. 



ABYSSAL SEA DEPOSITS. 



Until recent times no ocean sediments had been explored, except 

 those formed as we have Just seen, in the vicinity of continents and 

 islands, and which border them, like belts usually of small width com- 

 ])ared with the vast dimensions of the sea. 



Beyond a depth of 500 or 000 meters the waves and currents seem 

 no longer to exercise an erosive influence. The agitation of the water 

 and the mechanical action witnessed in tlie vicinity of land are not 

 felt in the abysses unless in excei>tional cases. It appears from the 

 tliermometric observation of the Challenger^ it is true, that the cold 

 waters have a motion at the great ocean bottoms from the poles toward 

 the e(]uator, but the motion is very slow and could exercise but slight 

 influence upon the distribution of marine sediment. 



Thousands of kilometers may be sailed over in certain directions 

 across the Atlantic and the Pacific without seeing any land above the 

 surface, but finding everywhere depths of several thousands of meters. 

 What takes place in these vast regions where the waves which toss the 

 surface cannot exercise a mechanical action on any solid mass"? This 

 could not be known or even surmised before many soundings, made 

 over large areas, had furnished their contingent of observations. To 

 bring up specimens of the bottom from several thousand meters below 

 the surface was a difficult operation, and to do it successfully ingenious 

 and ])owerful apparatus, skillfully nianipuhxted, was needed. The expe- 

 dition of the Challenger surmounted all obstacles. 



The bottom deposits of the great oceanic basins which differ much 

 from the marginal, have received the name i)elagic. We prefer to use 

 here the term abyssal, which expresses with greater precision the greafc 

 depth which constitutes their essential character. - - - 



