1865.] DAWSON — COAL-FORMATION". 101 



their thickness and in the materials associated with them, in tracing 

 them a few hundreds of feet from the top of the cliff to low-water 

 mark on the beach. I have no doubt that, could we trace them 

 over sufficiently large areas, they would all be found to give place to 

 sandstones, or to run out into bituminous shales and limestones, 

 according to the undulations of the surfaces on which they were 

 deposited, just as the peaty matter in modern swamps thins out 

 toward banks of sand, or passes into the muck or mud of inun- 

 dated flats or ponds. 



3. Geological Cycles. — The foregoing considerations bring, in: a very 

 distinct manner, before us two difierent, and at first sight irreconcile- 

 able, general views which we may take of any given geological period. 

 First^e must regard every such period as presenting during its whole 

 continuance the diversified conditions of land and water with their 

 appropriate inhabitants ; and secondly ^ we must consider each such 

 period as forming a 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 geo- 

 logical 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 exis- 

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

 characterized by swampy flats supporting land -plants. In like man- 

 ner 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 them 

 throughout the epoch of the deep-sea limestones. Regarded 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 oiF 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 re- 

 appear 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 themf. 



^ Acad. Greol. p. 183; Quart, Journ. G-eol, Soc. vol. ii. p. 133. 

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

 VOL. XXII. PART I. I 



