ox THE MARINE ZOOLOGY OK THE IRISH SEA. 331 



none of these factors^so long as the variations do not exceed certain 

 limits — have so much influence upon the fauna as the nature of the deposit 

 has. It is therefore quite to be expected that the fauna sliould 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 one can see, all the other surrounding con- 

 ditions the same, the fauna varies from place to place witli 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 partly a geological inquiry, and one which cannot, until 

 we have accumulated a much larger series of observations, be fully dis- 

 cussed ; but there are a few matters which may be briefly pointed out as 

 givdng some idea of the range and bearing of the question. 



1. It is necessaiy 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 submarine 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 some- 

 times some effect upon the neighbouring deposits. For example, the 

 contrast between the deposits lying oft" 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 the 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, and 

 these into smaller, and so forms deposits of dark slatey more or less 

 angular gravel, and then very coarse sand, extending for some way out from 

 the foot of the cliff. 



The influence of the shore rocks upon the littoral fauna is an important 

 subject upon which we have accumulated some observations ; but the 

 matter requires further work and detailed discussion, and must be left 

 over for a future report. 



3. Probably the great bulk of the siliceous sand which forms so large 

 a part of the floor of our sea is derived proximately — whatever may have 

 been its ultimate source ' — from the great deposits of drift which were 

 formed in the neighbourhood during the Glacial period, and large tracts of 

 which may since have been broken up by the sea. 



' Probably, to a great extent, Triassic sandstones. 



