Dr. Wheelton Hind — Zones of the Carboniferous. 257 



sandstones, shales, and indurated clays, with thin beds of coal and 

 many plant-remains. Such a succession points to complete changes 

 in the conditions of deposition ; and the beds of coal afford evidence 

 of a suspension of the deposition, and point even to terrestrial 

 conditions. 



It would be inadmissible to suppose that the fauna of a marine 

 bed died out when its environment no longer was suitable to its 

 needs, owing to bathymetric change, and that its reappearance at 

 a higher horizon was due to a redevelopment from primordial 

 forms ; because it is not probable that many forms would be again 

 evolved on precisely the same lines. 



The presence of a fauna at several horizons, with the complete 

 absence of the same forms of life in the intervening beds, points to 

 the continued survival of that fauna in some outside area during 

 the whole of the time that the series of beds containing it were 

 laid down, whence it could migrate in any direction in which the 

 environment became suitable. The change of conditions from 

 marine to littoral and littoral to terrestrial, and vice versa, must have 

 been very gradual, giving time to the faunas to retreat and invade 

 fresh localities, and allowing them to remain in an environment 

 suitable for their survival. How long some species did survive, in 

 some outside area, in Carboniferous times, may be judged from the 

 reappearance of the typical Lower Carboniferous species Productus 

 semistriatus and Streptorhynchus crenestria, amongst others, in the 

 Coal-measures of North Staffordshire. These species are, of course, 

 common enough in the Carboniferous Limestone of the district; 

 but they recur only at one single horizon in the Yoredale series, 

 and once in the Coal-measures. This fact affords undoubted evidence 

 that, at some area, even towards the close of the Carboniferous 

 period, a marine fauna was in existence. This fact at once raises 

 the question as to whether the Carboniferous rocks of the world are 

 contemporaneous. It is quite possible that a slow wave of depression, 

 with occasional shorter waves of return, passed gradually over 

 the surface of the world, that faunas migrated with these waves, 

 and that deposits characterized by similar faunas in the different 

 parts of the earth were anything but contemporaneous. But to 

 follow this question would lead me too far astray for the limits of 

 this paper ; still, if American palaeontologists are correct in recog- 

 nizing very few of their fossils as belonging to European species, 

 though generically the faunas are almost identical, such an interval 

 of time may have been the main cause in the gradual change of 

 specific form by the accumulated variations of successive generations. 



The beds of the Carboniferous deposits are, as a rule, characterized 

 by repeated alternations and recurrences of similar lithological strata, 

 with recurrences, after great intervals, of similar floras and faunas ; 

 and it appears to me that such a series of strata is not as likely to 

 be differentiated by well-marked and isolated zones of life as might 

 obtain in a series of rocks formed by a steady deposit, continued for 

 a long time. In the British Isles it is only in Central England, 

 North and South Wales, Somersetshire, and parts of Ireland 



DECADE IV. VOL. III. NO. VI. 17 



