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 A/ustra. 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 
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 organic matters 
brought down by rivers from the land, and some of which may reach the sea-bottom, 
