1 6 FISHES AND FISHERIES OF THE IRISH SfiA. 



extremely varied polyzoon fauna, we cannot yet match in British seas :* it was probably 

 formed, as the mollusca indicate, in a sea several degrees warmer than ours. 



" It was hoped that in the course of these dredgings some light might be thrown 

 on the Tertiary strata underlying the bed of the Irish Sea, for in the North Sea the dredge 

 occasionally brings up hauls of Tertiary fossils. This expectation has not yet been realized ; 

 but possibly, by dredging in the channels, where the submarine scour is greatest, such 

 deposits may yet be reached. It is very important to obtain some knowledge of the 

 Tertiary bed of the Irish Sea, for Irish Pleistocene deposits contain a considerable admix- 

 ture of extinct forms, which may be derived from Tertiary deposits below the sea level. 

 The Glacial Drift of Aberdeenshire contains Pliocene Volutes and Astartes, derived from 

 some submarine deposit off the Aberdeenshire coast. The so-called 'Middle Glacial Sands' 

 of Norfolk are full of shells which I now believe to be derived from some older deposit, 

 probably beneath the sea." 



One point which this collection of deposits from comparatively shallow shore waters 

 seems to bring out is that the classification of submarine deposits into "terrigenous" and 

 "pelagic," which was one of the earliest oceanographic results of the "Challenger" 

 Expedition, and which is still adhered to in the latest "Challenger" volumes as an 

 accepted classification, does not adequately represent or express fully the facts. Terrigenous 

 deposits are supposed to be those formed round continents from the waste of the land, 

 and are stated to contain on the average 68 per cent, of silica. Pelagic deposits are 

 those formed in the open ocean from the shells and other remains of animals and plants 

 living on the surface of the sea above, and they are almost wholly free from quartz particles. 



Ordinary coast sands and gravels and muds are undoubted terrigenous deposits. 

 Globigerina and Radiolarian oozes are typical pelagic deposits. But in our dredgings 

 in the Irish Sea, where the deposits ought all, from their position, to be purely terrigenous, 

 we meet with several distinct varieties of sea-bottom which are not formed mostly from 

 the waste of the land, and do not contain anything like 68 per cent, of silica ; but, on the 

 contrary, are formed very largely of the remains of bottom-haunting plants and animals, 

 and may contain as little as 17 per cent, of silica. Such are the nullipore bottoms, and 

 the shell sand and shell gravel met with in some places, and the sand formed of comminuted 

 spines and plates of echinoids which we have found off the Calf Island. These deposits 

 are really much more nearly allied in their nature, and in respect of the kind of rock which 

 they would probably form if consolidated,! to the calcareous oozes amongst pelagic 

 deposits, than they are to terrigenous deposits, and yet they are formed on a continental 

 area close to land in shallow water. Moreover, although agreeing with the pelagic 

 deposits in being largely organic in origin, they differ in being derived not from surface 

 organisms, but from plants (the nullipores) and animals which lived on the bottom. 

 Consequently the division of deposits into "terrigenous" and "pelagic" ought to be 

 modified or replaced by the following classification : 



i. Terrigenous (Murray's term, restricted) where the deposit is formed chiefly (say, 

 at least two-thirds, 66 per cent.) of mineral particles derived from 

 the waste of the land. 



* See, however, the deposit described on p. 18, where nearly 60 species of Polyzoa are recorded 

 from one haul. W. A. H. and R. A. D. 



t They seem closely comparable with the Coralline and Red Crag formations of Suffolk. 



