Yol. 6S.'] SUCCESSION IN THE NOETH-WEST OF ENGLAND. 455 



The Bands. 



These bands, at first sight, appear to indicate periods especially 

 favourable for the development of certain organisms, but it is very 

 doubtful whether this was really the case. It is equally probable 

 that the bands are in many cases rich in certain species, not 

 because the conditions were especially suitable for the growth of 

 these organisms ; but, on the contrary, paradoxical as it may seem, 

 because the conditions were changing and becoming unfavourable : 

 in consequence of which the organisms died more rapidly, and 

 accumulated in greater relative abundance in a given thickness of 

 deposit. The abundance of certain forms in a band would thus be 

 due to a relatively sudden and drastic change of conditions, these 

 forms being more affected than others : the change from a muddy 

 to a sandy bottom, the variation in depth and temperature, the 

 change from salt to brackish water, and the shifting of currents 

 causing a change in the amount and character of the food-supply, 

 would have different effects on different organisms according to 

 their adaptability, their powers of locomotion, and their natural 

 habitat. This view appears to be borne out by the fact that the 

 bands are usually characterized by the abundance of certain species 

 which occur also, though not so plentifully, in the underlying beds, 

 but do not occur again above the bands which they characterize ; 

 and these are usually accompanied by other species which occur 

 abundantly in the band, but are not again found above it. Further, 

 the collection of species occurring in a band usually contains a large 

 proportion of small immature individuals. 



The division of the series into beds with and beds without 

 LitJiostrotion, given in fig. 2 (p. 452), is not intended so much as a 

 stratigraphical subdivision, as a statement of fact. The names 

 Yisean and Tournaisian are introduced out of deference to a wish 

 expressed during the discussion which followed the reading of 

 this paper ; but, while I admit the priority of the subdivisions 

 established for the Continental equivalents of these beds, these 

 have, in my opinion, no real significance when applied to the rocks 

 of the jSTorth-Western Province. There the Lower Carboniferous 

 strata form a continuous series of marine deposits uninterrupted 

 by any unconformity, the relative dates of submergence in the 

 different districts depending essentially on the contour of the pre- 

 Carboniferous land-surface. The sandstone episode which masks 

 the palaeontological succession in the Shap District is due, not to 

 interrupted deposition, but to the introduction of detrital material 

 from the east, consequent upon the submergence of the Pennine 

 District at this period. 



The palseontological break between the Michelinia Zone and the 

 overlying Productus corrugato-hemisphericus Zone is no greater than 

 that between the Michelinia Zone and the underlying AtJiyris- 

 glahristria Zone, or between the Lower and the Upper Dihuno- 

 ^liyUum Sub-zones. 



