136 THE CARBONIFEROUS SYSTEM. 



geological cycle, in which such conditions to a certain extent were 

 successive. As we give prominence to one or the other of these views, 

 our conclusions as to the character of geological chronology must vary 

 in their character ; and in order to arrive at a true picture of any given 

 time, it is necessary to have both before us in their due proportion. 



We know that the marine animals of the Lower Carboniferous seas 

 continued to exist in the time of the Coal formation, and that some of 

 them survived until the Permian period, proving to us the existence 

 of deep seas even in that age which we regard as specially character- 

 ized by swampy flats supporting land-plants. In like manner we 

 know that some of the species of land-plants found in the lowest coal 

 measures continued to exist in the time of the upper coal formation, 

 proving that there was some land suitable for tliem throughout the 

 epoch of the deep-sea limestones. Eegarded from this point of view, 

 any exceptional beds with land-plants in the marine parts of the 

 formation, or beds with sea-shells in the parts where land conditions 

 predominate, acquire a special interest ; and so likewise do regions in 

 which, as in some parts of the Appalachian Coal-field, the marine 

 limestones are absent, and those in which, as in some parts of the 

 Western States, marine conditions seem to have continued throughout 

 the whole period. In Nova Scotia, so far as my present knowledge 

 extends, the marine limestones of the Lower Carboniferous cut off the 

 flora of the Lower Coal measures, apparently by a long interval of 

 time, from that of the Middle Coal formation ; and in like manner the 

 fossils of the marine limestones cease at the time of the Millstone-grit, 

 and only in one instance, that of a small bed of limestone near 

 Wallace Harbour, partially reappear in the Upper Coal formation.* 

 I have, however, ascertained that the marine limestones may be 

 divided into an upper and a lower member, and that there is some 

 reason to suppose that in some parts of Nova Scotia where the true 

 coal measures are not developed, the upper member may, in part at 

 least, represent them. -J- On the other hand, I have not as yet been 

 able to bridge over the gulf which separates the flora of the Lower 

 Carboniferous coal measures from that of the Middle Coal formation, 

 an interval which may include much of the " Lower Coal Measures " 

 of Rogers in the Pennsylvania Coal-field. 



Turning to that broader view which takes the prevalent conditions 

 of each portion of the period as characteristic, notwithstanding the 



* Quart. Journ. Geol. Soc, vol. ii. p. 133. 



f Quart. Journ. Geol. Soc, vol. xv. pp. 63 et seq. My friend Mr C. F. Hartt, who 

 has more recently studied the marine limestones, has obtained facts which seem to 

 indicate the possibility of a more minute subdivision than any hitherto attempted of 

 these beds. Vide chapter on L. C. Limestones, infra. 



