lxvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mayi9II r 



gradually obtains ingress to the lower lands and lake-areas, 

 converting them into gulfs and estuaries, and lays down in them 

 sheets of shoreward but ' estuarine ' deposits, smooth and flat 

 over the deposits of the preceding period, and made of debris 

 provided by the continued activity of the agents of erosion. But, 

 owing to the irregular contour of the drowning land, every sheet 

 so laid down will be of irregular shape, each one will pass at 

 its edges into coastal deposits, and each will transgress during- 

 the submergence with increasing overlap beyond the preceding 

 deposit and against the old land until that old land is at length 

 completely lost under the sea. 



(v) Finally, the old land sinks completely under the sea, deeper- 

 w r ater (' thalassic ') conditions recur, and the cycle is complete. 



(4) Difficulties and Exceptions. 



While the normal succession of events in a cycle of deposition 

 will be that just described, normal stratigraphical phenomena by 

 no means always obtain. For example : — 



(a) The uplift may be of epeirogenic rather than orogenic type ; 

 the sediments may not be much disturbed in their lie, and denu- 

 dation may be content only to strip off the upper portion of recently 

 formed deposits. Unconformity will be slight, the break in sedi- 

 mentation inconsiderable, and the newly formed sediments not 

 strikingly different from those which precede them. 



(b) The uplift, if orogenic, may be of such long duration that 

 an entire cycle of denudation is completed and the land -forms 

 smoothed down to a peneplain, so that the newer sediments become 

 laid down on an approximately level, plain-like, surface of older 

 rock-formations, the strata of which will, however, be separated 

 from those of the new-made formations by flagrant unconformity. 



(5) Details. 



In our own country excellent examples of the normal progression 

 and the normally associated phenomena are usually detected, as 

 well as examples of the two variants just noted. Of the normal 

 sequence, the progression from the time of deposition of the 

 Carboniferous Limestone through the time of the Pennine-Armorican 

 movement into Middle Jurassic time may be regarded as quite 

 typical, but numerous other examples will at once suggest them- 

 selves. We may consider the normal succession in a little fuller 

 detail. 



