MAEINE BIOLOGICAL STATION AT POET EEIN. 59 



seems little doubt that the bulk of the material on the 

 sea-bottom over this area has been derived from the 

 breaking up of pre-existing Glacial deposits. This may 

 occur ax a depth of several fathoms through the gradual 

 washing away of the muddy and sandy matrix of a boulder 

 clay or Glacial gravel. Coarse gravel is thus caused to 

 accumulate at a spot where the currents may be too 

 feeble to transport anything but sand. 



" This sub-marine origin of angular gravel deposits 

 should not be forgotten, for it affects the lithological 

 character of the sea-bottom over most of the area which 

 was formerly glaciated, even as far south as Cornwall. 

 On the other hand, it does not affect, except to a small 

 extent, the sea-bed beyond the former limit of the ice, and 

 it does not affect pre-Glacial deposits. Thus we must 

 always expect to find at similar depths the same fauna 

 associated with deposits of finer texture as soon as we 

 leave the glaciated area, or when we go back into Ter- 

 tiary times. 



" It is also worth noting that the occurrence of a stony 

 bottom at 20 or 30 fathoms — where normally there would 

 be no deposit coarser than sand — will probably lead to a 

 disproportionate increase of all incrusting organisms, and 

 of all organisms needing a solid base. This has certainly 

 taken place, as anyone studying our shoal-water Tertiary 

 deposits will have observed. They contain few stones, 

 and though each stone or dead shell may be covered with 

 incrusting organisms, yet the relative proportion of these 

 to the free forms is far smaller than seems commonly to 

 be the case in the seas that now wash our shores. The 

 sole exception to this rule among the British Tertiary 

 strata is found in the Coralline Crag, in which the con- 

 temporaneous consolidation of the limestone was sufficient 

 to provide the necessary solid base for the incrusting and 

 fixed organisms so abundant in that deposit." 



