Dr. Wheelton Hind — Zones of the Carboniferous. 259 



of marine origin ; and a calcareous deposit differs essentially from 

 those of sandstone and clays, in being to no great extent due to 

 terrestrial denudation, and therefore, it may be presumed, that Car- 

 boniferous Limestone was laid down in a sea into which the muds 

 and sands brought down by river action were not, or were prevented 

 from being, deposited. 



The Carboniferous series of Northumberland and Scotland are 

 essentially characteristic of those deposited near a shore which 

 occasionally became land, and occasionally was depressed deep 

 enough to allow limestones to accumulate on its floor. The land 

 which was the source of the muds and sandstones was probably of 

 no great elevation, and consequently river action was not strong 

 enough to carry the sediments far out to sea. As conditions changed 

 the sea became shallower, and consequently the shore-line retreated 

 south, with occasional intervals and retrogressions : so that in the 

 north, marine, littoral, and terrestrial deposits are found superimposed 

 in varying and recurring sequence, with the faunas peculiar to each 

 also recurring at several horizons in a vertical deposit. Further 

 south, however, constant marine conditions obtained at first for a 

 long time. Deposition of sediment, however, seems to have been 

 in the long run always in excess of the depression, for even in 

 those areas where marine deposits are most constant there is distinct 

 evidence that the sea was gradually becoming shallower, for we find 

 the massive limestones gradually replaced by shale containing thin 

 limestones, and these in turn covered by shales, and sandstones, 

 and clays, during the deposit of which those terrestrial conditions 

 obtained necessary for the growth of such a flora as was to result 

 in the many beds of coal. 



We may take it, therefore, that marine and terrestrial conditions 

 existed contemporaneously during the whole of the Carboniferous 

 epoch in Great Britain, and that there was a constant migration 

 of species backwards and forwards, according as conditions slowly 

 changed, and that many forms of life existed in areas where a steady 

 uniformity of environment obtained throughout the whole of that 

 period. Under such conditions zones of biological succession would 

 be of little value as a means for the correlation of rocks over any 

 great areas, with the possible exception of limited belts along the 

 line of strike which would be the equivalents of isobathymetrical 

 zones. 



The few fossils quoted as characteristic of a certain succession 

 of the North of England, by Messrs. Garwood and Marr, however 

 indicative they may be of the sequence in a certain locality, seem 

 to me to be singularly unfortunately chosen with regard to the 

 British Carboniferous beds as a whole. The sequence given is — 



Beds with Productus cf . Edclburgensis ) yoredale series. 

 ,, P. latissimus j 



,, P. gigcmteus 



, , Chonetes papilionacea { M ^^ Limes tone series. 



,, Vhcetetes septosus 

 ,, Spirifer octoplicata 



