2 TatterSALL & Coward, Fmma of Rostheme Mere. 



special reference to its periodicity and to the seasonal 

 changes in the organisms constituting it. 



No complete account of the fauna and flora of any of 

 the English lakes has, however, yet been published, so far 

 as we are aware. The proximity of Rostherne Mere to 

 Manchester seemed to us to present a favourable oppor- 

 tunity for undertaking work of this kind, since it was 

 possible to arrange for the collection of samples with some 

 attempt at regularity. This mere, one of the largest and 

 most important of the characteristic sheets of water which 

 occur throughout the Cheshire plain, presents two features 

 which render the elucidation of its biology of added in- 

 terest, (i) the mode of its formation and (2) the geological 

 strata upon which it lies. Rostherne Mere appears to be 

 what has been described as a dissolution basin, that is, a 

 depression on the surface of the earth caused by the 

 gradual removal of underlying soluble rock as the result 

 of the chemical and mechanical action of underground 

 water. In this case, the bed rocks are themselves of a 

 more or less impermeable character, but are underlaid at a 

 greater or lesser depth by soluble rock salt. The gradual 

 removal of this salt by underground water has resulted in 

 subsidence of the surface and the consequent formation of 

 a lake basin. The majority of the lakes of Scotland, 

 the English Lake District and Wales are glacial basins, 

 depressions on the earth's surface scooped out by the 

 action of moving glaciers. Lough Neagh, on the other 

 hand, is a tectonic basin, that is, a basin formed by de- 

 formation of the earth's crust by the subsidence or faulting 

 of rock material deposited as the result of volcanic action. 

 Rostherne Mere has thus been formed in quite a different 

 way from any of the other British lakes whose biology 

 has been investigated. The majority of the English, 

 Welsh and Scottish lakes lie on the Pre-Cambrian or 



