PHYSICAL FEATURES OF THE IRISH SEA. 



2. Neritic* -where the deposit is largely of organic origin, its calcareous matter 



being derived from the shells and other hard parts of the animals 

 and plants living on the bottom. 



3. Planktonic (Murray's pelagic) where the greater part of the deposit is formed 



of the remains of free-swimming animals and plants which lived in 

 the sea above the deposit. 



The last group is Murray's " pelagic" unchanged, and thai, there can be no doubt, is a 

 natural group of deposits ; but " terrigenous " in the usual sense is an unnatural or heterogeneous 

 assemblage containing some deposits, such as the gravel off Bradda Head and the sand of the 

 Liverpool Bar, which are clearly terrigenous in their origin, along with others such as shelly 

 sands and nullipore deposits which have much less to do with the waste of the land, but are 

 very largely organic in origin and formed by animals and plants in situ. The proposal is then 

 to recognise this latter group of deposits by separating them from the truly terrigenous under 

 the name " Neritic." Probably some of the Coral sands described by Murray and Renard in 

 their Challenger Report on Sea-Deposits would also fall into this category. 



Dr. C. Kohn has kindly analysed a series of fair samples of deposits from different parts 

 of the Irish Sea, with the following results : 



NERITIC. TERRIGENOUS. 



The localities and particulars are : 



A. i mile S.E. of Spanish Head, 16 fathoms, shell fragments, 



B. i mile W. of Calf of Man, 20 fathoms, shells and spines. 



C. i mile N. W. of Calf of Man, 18 fathoms, shell sand, spines. 



D. 2 miles W. of Dalby, 15 fathoms, nullipores. 



E. Liverpool Bar, 3 fathoms, sand. 



F. Off Bahama Bank, 13 fathoms, muddy sand. 



G. King William Bank, 5 fathoms, coarse sand. 

 H. North end of " Hole," 20 fathoms, mud. 



It will be noticed that the four terrigenous deposits (sands and muds) all show less than 

 10 per cent, of calcium carbonate ; while the four neritic have all more than 38 per cent. well 

 over a third of calcium carbonate, and one (A.) has over 79 per cent. The silica in these 

 neritic deposits may be less than 17 per cent., and does not rise in any to 55 per cent. In 

 round numbers it may be said that in these examples the silica makes up from 20 to 50 per 



* Adopted from Haeckel's term for the zone of shallow water marine fauna, see " Plankton Studien" 

 Jena, 1890; also Hickson's "Fauna of Deep Sea," 1894. 



I Shelly deposit. Contained i '09 per cent, of small stones not included in analysi-s. 

 Contained 4-82 per cent, of magnesium carbonate, in addition to calcium carbonate. 



O 



