12 FISHES AND FISHERIES OF THE IRISH SEA. 



determines the food.* Many animals feed upon the deposit, others browse upon the polyzoa 

 and zoophytes which can only attach themselves and grow where there are sufficiently large 

 objects, such as shell valves, from which they can get the necessary stability ; while others, 

 again, feed upon their neighbours, which subsist on the deposit or are attracted by the 

 zoophytes, &c. ; for example, soles are frequently caught upon ground (known to fishermen as 

 "sole ground") where Flustra foliacea lives in abundance, and the probable connection is that 

 the fish are dependent upon the numerous amphipoda and other small animals which frequent 

 the tufts of Flustra. The same locality may vary so much from time to time in the tempera- 

 ture, the salinity, and the transparency of the water, that it is probable that none of these 

 factors so long as the variations do not exceed certain limits have so much influence upon 

 the bottom fauna as the nature of the deposit has. It is therefore quite to be expected that the 

 fauna should vary from place to place with the nature of the bottom, and that is what we 

 have observed frequently in our work round the Isle of Man. In practically the same water, 

 identical in temperature, salinity, and transparency, at the same depth, with, so far as we 

 know, all the other surrounding conditions the same, the fauna varies from place to place with 

 changes in the bottom mud, sand, nullipores, and shell beds, all have their characteristic 

 assemblages of animals. 



As to the further, and very important, question of the origin of the deposits, that 

 is to a great extent a purely geological inquiry, and one which cannot, until we have 

 accumulated a much larger series of observations, be fully discussed ; but there are a few 

 matters which may be briefly pointed out as giving- some idea of the range and bearing 

 of the question. 



1. It is necessary to make a most careful examination of the deposits. For 

 example, all muds are not the same in origin. A deposit of mud may be due to the 

 presence of an eddy or a sheltered corner in which the finer particles suspended in the 

 water are able to sink, or it may be due to the wearing away of a limestone beach, or 

 to quantities of alluvium brought down by a stream from the land, or to the presence of 

 a submerged bed of boulder clay, or, finally, in some places, to the sewage and refuse from 

 coast towns. 



2. We have kept in view the possibility of some correlation between the geological 

 formations along the beach and the sub-marine deposits lying off the shore. There is no 

 doubt that the nature of the rock forming the shore has a great influence upon the marine 

 fauna, and has sometimes some effect upon the neighbouring deposits. For example, the 

 contrast between the deposits lying off the two prominent headlands, the Great Orme, in 

 North Wales, and Bradda Head, in the Isle of Man, is well marked. The Great Orme is 

 composed of mountain limestone, and the result of its weathering and erosion is that 

 large blocks are found lying scattered outside its base on the fine sand ; but there is no 

 deposit of smaller stones, gravel, and resulting sand farther out, probably because in the 

 wearing of the rock and large detached blocks by the sea a great deal is removed in 

 solution and the rest in suspension as very fine mud this we have found to be the 

 case round Puffin Island, which is also mountain limestone. Bradda Head, on the other 

 hand, is a schistose metamorphic silurian rock, which breaks up into large fragments, 



* The only food supply quite independent of the bottom is dead plankton, from the water above, which 

 may reach the bottom uneaten ; and possibly a small amount of decayed vegetation and other org-anic matters 

 brought down by rivers from the land, and some of which may reach the sea-bottom, 



