TRANSACTIONS OF THE SECTIONS. 91 



The Cretaceous group, as it is cxliibited here in the south of Euglantl, whore 

 its vertical thickness is very great, presents in its lower beds (Neocomian) a 

 marine fauna which indicated to Edward Forbes a limited sea, with depths not 

 (exceeding 18 fathoms. Sand-zones hundreds of feet in thickness overlie these. 

 The argillaceous Gault, in its composition and fauna, is a deep-water deposit, 

 followed by shallower-water sands (Upper Green Sand) indicating oscillating con- 

 ditions as to depth of water, to which succeeds the widely spread oceanic de- 

 position of the white Chalk. Here recurring conditions come aoout in like order 

 as in the Jurassic series ; and a corresponding illustration might be derived from 

 the physical changes indicated in the course of the Nummulitic period. 



With respect to none of these marine geological formations is there any indication 

 whatever that one passed into, or was in continuous sequence with, another, 

 either stratigraphically or geologically ; on the contrary, wherever there is apparent 

 continuity, either upwards or downwards, it is by change or transition from one 

 set of conditions to another wholly different. The purely marine Upper Silurian 

 beds of the Welsh border are followed conformably by the Old Red Sandstone, 

 which last is now universally accepted as a lacustrine formation, the place of 

 which, in time, was intermediate between the middle Palaeozoic group and the 

 upper or Carboniferous, which commenced with the so-called " Devonian." The 

 positions and extent of the " Old Red " lacustrine beds in all parts of the British 

 Islands indicate, even at this day, to what extent Silurian sea-bed had become 

 terrestrial surface, to which the lacustrine basins were subordinate. 



In the contrary direction, and in our own area, the next group, indicating widely 

 spread marine conditions, that represented by the Devonian and Mountain-Lime- 

 stone formations, sets in (as in North Devon) with shallow-water sands and a 

 marine fauna (Lower Devonian), in sequence to " Old Red " depositions with fresh- 

 water fishes and crustaceans. There is no continuity from " Old Red" into the 

 earliest Devonian beds, anv more than from uppermost Silurian into Lower "Old 

 Red." (Phillips's Geology of Oxford, pp. 77-79.) 



The later Palteozoic ocean-floor, now our Mountain Limestone, in turn became 

 terrestrial surface, on which the Coal-measures were accumulated, and over which 

 the abundant vegetation of that period established itself. The Coal-measures re- 

 present so much of the surface of their time as, from position, favoured expanses of 

 fresh and brackish waters, and alternations from one set of conditions to the other. 



Geologists are familiar with the amount of physical change which took place over 

 the European area after the coal-growth period. The subsequent condition of surface 

 which resulted is still distinctly traceable. The Permian-Trias period presents true 

 Aralo-Caspianconditions,physicallydefined,subordinate to the same continental area. 



The marine Jurassic series, next in sequence, was succeeded by that period of 

 terrestrial conditions to the more detailed physiography of which I here propose 

 to call your attention. It may suffice on this occasion to state that at the end of 

 the great Cretaceous period the area of those seas, in our hemisphere, down to 

 depths at which the great chalk floor had been deposited, became part of a con- 

 tinental land, on which the freshwater formations of the times which preceded 

 the marine Nummulitic were accumulated. 



These evidences of successive physical conditions over' the northern hemisphere 

 indicate an order of recurrence of corresponding conditions, and, as already noticed, 

 of a progress of change which, in the course of each period, came about in a corre- 

 sponding order. Great periods, during which wide marine conditions prevailed, 

 alternated with others of wide terrestrial surfaces. The marine periods, ns we 

 measure them by the products of the agents which seas and oceans call into action, 

 must have been of vast duration. In like manner we may feel assured that the 

 great freshwater formations are not, as some geologists have supposed them, mere 

 subordinate parts of the great marine groups, as our "Wealden " of the " Cretaceous," 

 but rather true intermediate groups, of equal geological value with them in the 

 estimate of past time. 



The WeaWcn Formaiion, 



Mr. ^Nfartin proposed this designation for the assemblage of freshwater deposi- 

 tions exhibited in the counties of Kent, Surrey, and Sussex, and which may be 



