634 DEEP-SEA DEPOSITS. 



Deep-Sea Deposits. — The materials composing the bed of the Ocean 

 may be divided into two classes, according as they have been derived 

 from the land or the sea. Terrigenous Deposits, or those derived from the 

 degradation of adjacent land surfaces, do not extend as a rule more than 

 300 or 400 miles from the shore, but powerful ocean or river currents 

 would carry them much farther ; in these, it is thought, that compound 

 rocks, such as now exist, can be recognised in process of formation. 



Pelagic Deposits are formed with extreme slowness, in the deep water 

 of the central regions of the great ocean basins, and consist of organic 

 oozes and a reddish clay. They are chiefly made up of the calcareous 

 and siliceous remains of organisms which have fallen from the surface 

 water, along with clay and decomposing volcanic debris. These do not 

 appear to bear any relation to the known marine stratified rocks. 



Between the depths of 400 and 2,000 fathoms, the floor of the Atlantic 

 Ocean is, in general, covered with a whitish mud, formed by the shells of 

 globigerina and other foraminifera, identical with those in chalk. These 

 are rarely found in depths beyond 2,500 fathoms. In the deepest parts, 

 the floor is a sort of red clay, with granules of manganese and iron, 

 supposed to be of meteoric origin. 



It is probable, however, that there is no invariable rule of the quality 

 of the bottom depending on the depth. For instance, in ten soundings 

 between 2,000 and 3,000 fathoms, off the United States coast, in 1884, 

 the Albatross found a bottom of globigerina ooze. The following is a 

 short description of the principal Pelagic Deposits. 



Bed Clay, with which it is conjectured that more than one-fourth of 

 the globe's area is covered, is found in great depths, remote from land. 

 It is soft and greasy to the touch, and a specimen from a depth of 3,150 

 fathoms, in the North Atlantic Ocean, was found to consist of 53-30 silica, 

 17*40 alumina, 11-70 ferric oxide, with traces of lime and magnesia. It 

 is considered to be derived mainly from the decomposition of volcanic 

 materials. The Manganese Nodules, found with this deposit, usually 

 contain about 35 per cent, peroxide of manganese, with some 25 per 

 cent, of ferric oxide ; cobalt, with copper, and a little nickel, are present 

 in all of them, according to Mr. Buchanan. 



Badiolarian Ooze is only found in the greatest depths, and has not been 

 found in the Atlantic. It consists of a basis of Ked Clay, with a large 

 admixture of siliceous organisms. 



Diatom Ooze is only found in the South Polar regions, and in the N.W. 

 part of the North Pacific Ocean. 



Globigerina Ooze appears to occupy all the medium depths of the Ocean, 

 away from land, and consists mainly of carbonate of lime, formed from 

 the dead shells of foraminifera, which, when living, abound on the 

 surface of the sea. --^ 



Pteropod Ooze is similar to the last, and is found on the central Atlantic 

 ridges, and contains shells of mollusca, which also live on the surface. 



In 1883, the Talisman examined the Sargasso Sea region, and obtained 

 from the bottom a collection of volcanic lava and scoriae, some appearing 

 to be of comparatively recent origin. 



