CONDITIONS OF DEPOSITION OF TUP. HF.DS. 135 



But the strict conformability of all the members of the Carboniferous 

 wriee in the. great majority of cases, shows that these instances of 

 Unconformability are exceptional. In the section at the Joggins 

 more especially, the whole scries presents a regular dip, diminishing 

 gradually from the margin to the middle line of the trough, where the 

 beds become horizontal. 



The most gradual and uniform oscillations of level must, however, 

 be accompanied with irregularities of deposition and local denudation; 

 and phenomena of this kind are abundantly manifest in the Carbon- 

 iferous strata of Nova Scotia. I have described a bed in the Pictou 

 Coal-field which seems to be an ancient shingle-beach, extending 

 across a bay or indentation in the coast-line of the Carboniferous 

 period.* At the Joggins, many instances occur of the sudden running 

 out and cutting off of beds,-}- and Mr Brown has figured a number of 

 instances of this kind in the Coal formation of Sydney.;}: They are 

 of such a character as to indicate the cutting action of tidal or fluviatile 

 currents on the muddy or sandy bottom of shallow water. In some 

 instances the layers of sand and drift-plants filling such cuts suggest 

 the idea of tidal channels in an estuary filled with matter carried down 

 by river-inundations. Even the beds of coal are by no means uniform 

 when traced for considerable distances. The beds which have been 

 mined at Pictou and the Joggins show material differences in quality 

 and associations ; and small beds may be observed to change in a 

 remarkable manner, in 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. 



Geological Cycles. 



The foregoing considerations bring, in a very distinct manner, before 

 us two different, and at first sight irreconcilcablc, general views which 

 we may take of any given geological period. First, we must regard 

 every such period as presenting during its whole continuance the 

 diversified conditions of land and water with their appropriate inhabi- 

 tants ; and secondly, we must consider each such period as forming a 



* Quart. Journ. Geol. Soc, vol. x. p. 45. 



t Ibid., vol. x. p. 12. \ iW&i vol. vi. p. 12f> ct seq. 



