lxxxiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9H r 



the Lower Greensand, but more definitely by the Nagelfluh of the 

 Alps. 



The deltaic deposits of the Upper Carboniferous are copied by the 

 Estuarine deposits of the Lower Oolites, and by both the Wealden 

 and the Oligocene deposits, although it is not easy to point to any 

 forerunners quite comparable with them. 



The British Rhaetic type of deposit is not altogether unlike the 

 lowest Carboniferous in Scotland and the Upper Eocene in Southern 

 England. 



The Elysch of the European Alpine regions may be compared 

 with the uppermost Silurian and with the problematical and 

 difficult rocks of the Permo-Carboniferous. 



The graptolitic shales of the Lower Palaeozoic are to some extent 

 copied by the clays of the Jurassic and Cretaceous. 



(e) Variability and Non-Recurrence. 



But in every case there are differences which go far to mask the 

 primitive resemblances. The likeness is perhaps strongest in the 

 Pebble-Beds with their exquisite rounding, their method of occur- 

 rence, and to some extent the nature of their material ; but the 

 Breccias differ in their massiveness, in the nature of the fragments 

 contained in them, and in the relation of the latter to the rocks 

 exposed to denudation at the time when they were formed. The 

 Rhaetic types of deposit vary in the smoothness and wide extent of 

 the deposition areas, in the slowness or otherwise of the incursion 

 of sea- water and marine organisms, and in the amount of oscillation 

 which the area was undergoing. The Fly sch -like deposits bridge 

 over periods of movement of varying length, and form a barren 

 type of deposit which encroaches upon different parts of one or 

 more Systems. They differ from one another according to the 

 duration and balance of conditions of shallow sedimentation ruled 

 by uplift and contemporaneous erosion. The Red Beds may or 

 may not be accompanied by deposits due to chemical reaction or 

 precipitation. The Shales and Clays agree in the fineness of their 

 sediment and in the presence of pelagic organisms ; but they differ in 

 consequence of the nature, depth, and extent of the great hollows 

 of the sea-bed in which they were laid down — in some cases perhaps 

 comparable with great ocean-depths, in others with those of smaller 

 land-locked basins. 



But the most profound difference is perhaps that seen between 

 the Coal-bearing strata and any correlative deposits that may 



