.1892. 



DEEP-SEA DEPOSITS. 19 



The pelagic deposits are essentially made up of the remains of 

 pelagic organisms — pteropods, heteropods, foraminifera, diatoms, and 

 radiolaria — together with the products of volcanic eruptions and 

 secondary products resulting from the decomposition of the latter. 

 Uniformity over large areas is a special characteristic of these 

 deposits. The authors do not admit that any formations identical 

 with the typical pelagic deposits are to be found in the rocks of 

 which the land is composed. They refer, however, in a footnote, to 

 the oceanic deposits of Barbados, recently described by Messrs. 

 Harrison and Jukes-Browne, and to the radiolarian cherts of the 

 south of Scotland described by Dr. Hinde. 



The first three chapters deal with the collection and examination 

 of deep-sea deposits, the composition of the samples obtained at 

 354 stations (this being given in a tabular form) and the general 

 nature and distribution of the different kinds of deposit. The last 

 three chapters treat of the constituents of the deposits, and v/e will 

 refer to these first. They are classified as follows: — (i) Materials 

 of organic origin ; (2) mineral substances of terrescrial and extra- 

 terrestrial origin ; and (3) chemical products formed in situ on the 

 floor of the ocean. 



Nearly all the deep-sea deposits contain traces of albuminoid and 

 other organic matters. These substances give rise to sulphuretted 

 hydrogen by decomposition, and thus lead to the formation of 

 sulphides, especially sulphide of iron, in the blue muds. The food 

 of the deep-sea animals is the organic matter which falls from the 

 surface and intermediate waters. The stomachs of echinoderms, 

 annelids, and other organisms living on the bed of the sea, were always 

 found to be full of the surface layers of the ooze. Thus a large 

 proportion of marine deposits must have passed througli the intestines 

 of marine animals. That the same kind of action took place in past 

 time is shown by the extraordinary abundance of worm-casts in some 

 geological formations, as, for example, in the Cambrian rocks of the 

 North-west of Scotland. 



The different classes of organisms whose remains occur in the 

 deposits are considered in detail, and the relative frequency of these 

 remains in the different kinds of deposit is described. The division 

 of the oceanic organisms into those which live in the waters and 

 those which dwell on the floor of the ocean — the Plankton and 

 Benthos of Haeckel — is of primary importance from the point of view 

 of the origin of deep-sea deposits. The "Challenger" observations 

 show that 90 per cent, of the carbonate of lime in these deposits is 

 due to the remains of pelagic organisms. The nature of the deposit 

 varies with the latitude, and depends upon the character of the 

 surface fauna and flora. The calcareous organic remains belong 

 principally to pelagic foraminifera, moUusca (pteropods and 

 heteropods), and algae (coccoliths and rhabdoliths). 



The general absence of the exoskeletons of the higlier crustacea 



c 2 



