292 TEANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



identical in temperature, salinity, and transparency, at the 

 same depth, with, so far as one can see, 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 nmds 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. I 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 

 sojne effect upon the neighbouring deposits. For example, 

 the contrast between the deposits lying off the two pro- 

 minent 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 



