320 



NA TURE 



\Ang. 15, 1872 



is the great freshwater Wealden series, next the entire Cretaceous 

 group, then portions of the Nummulitic group, including the 

 unique fossi'iferous beds of Bracl<Iesham. At Selsey is to Ije 

 seen a remnant of a definite Tertiary period, of which at no other 

 place in England is there any record. Lastly, the evidence as to 

 local conditions during the Glacial period is peculiarly interest- 

 ing. This rich field has not wanted for competent labourers, 

 foremost amongst whom must be named Dr. Gideon Mantell, 

 who in his day did so much by his zeal and knowledge to diffuse 

 a taste for his favourite pursuit. There must also be added the 

 names of Mr. Martin, of Pulborough, and Mr. Dixon, of Bognor. 

 It might perhaps be a fitting preliminary to the local communi- 

 cations which we may expect in the course of this meeting, 

 should I here give a summary of what has been already done 

 with reference to the geology of this S.E. of England ; but to 

 many who meet now in this section, very much of such a survey 

 would be familiar. Instead of this I propose to call attention to 

 what is the peculiar feature of our local geology, namely, its 

 great Wealden formation, the product of that vast lake or sound, 

 which at a time before a particle of the chalk hills of Sussex had 

 been formed, covered an area larger than the whole of the south- 

 east of this island. What I shall endeavour to put before you, a 

 point not generally understood, is with reference to the place of 

 formations akin to our Wealden, in the records of past time, and 

 enable you to realise what were then the geographical conditions 

 of the northern hemisphere, what the distribution and extent of 

 other areas of fresh water, the equivalents of our Wealden. 



Place of the Fresh- and Bracklsh-iijater fonnations oji tht 

 Geological Scale 



If a general view be taken of the successive physiographical 

 conditions of bygone geological periods, it is seen in respect of 

 each, such as those of the Palaeozoic period, or of the Mesozoic 

 such as for the Jurassic, Cretaceous, and Nummulitic, which all 

 represent distinct periods of past time, and are all the products 

 of purely marine conditions, that what is at present terrestrial 

 surface, was at those times to a great extent covered by water, 

 and that the great geological formations are merely old sea-beds. 



When on a projection of the northern hemisphere, the known 

 extent of each of these old seai is represented, as on the accom- 

 panying maps, it is also seen to how great an extent at those 

 times the area of water exceeded what it has at present ; at each 

 of these great periods the northern hemisphere must liave pre- 

 sented just sucti a preponderance of water which the southern 

 hemisphere does at present ; and it is further to be remarked how 

 closely the area of one period of northern geological submergence 

 corresponds with the others, as the Nummulitic with the Cre- 

 taceous, and the Cretaceous with the Jurassic. Whatever tlie 

 course, there is to be seen in this a recurrence of like conditions 

 at enormously long intervals of time. 



If next the internal evidence to be derived from these Mesozoic 

 formations be taken, it is to be seen, as is familiar to most geolo- 

 gists, that each, when most complete, presents a like order of 

 change, from its older to its newer portions. 



Over the mid-European area shallow-water accumulations, such 

 as shingle and sand-zones (infra-Liassic), preceded the deeper 

 water Lias-shales and limestones. Jurassic oolites follow ujion 

 these, indicating somewhat decreased depths for the Middle 

 Jurassic series. Oscillations of surface mark this period ; and 

 with respect to its physiography, Mr. Darwin has given his 

 opinion that the Malay Archipelago, with its numerous large 

 islands separated by wide and shallow seas, probably represents 

 the former state of Europe, when the Middle Jurassic beds were 

 accumulating. Next foUjw deep-water depositions, when the 

 widely spread Kinimeridge series was formed, ending upwards 

 with the Portland beds. 



The Cretaceous group, as it is exhibited here in the South of 

 England, where its vertical thickness is very great, presents in its 

 lowerbeds (Neocomian) a marine fauna which indicated to Edward 

 Forbes a limited sea, with depths not exceeding eighteen 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 conditions as to depth of water. To which 

 succeeds the widely-spread oceanic depositions of the white 

 chalk. Here recurring conditions come about in like order as in 

 the Jurassic series ; and a corresponding illustration might be de- 

 rived from the physical changes indicated in the course of the 

 Nummulitic period. 



In respect to none of these marine geological formations is 



there any indication whatever that one passed into, or was in con- 

 tinuous sequence with, another, either stratigraphically or geo- 

 logically ; on the contrary, wherever there is apparent continuity, t 

 eitlier upwards or downwards, it is by change or transition from \ 

 one set of conditions to another wholly different. The purely 5 

 Marine Upper Silurian beds of the Welsh border are followed , 

 conformably by the Old Red Sandstone, which last is now i 

 universally accepted as a lacustrine formation, the place of which, j 

 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 subor- 

 dinate. 



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

 indicating widely spread marine conditions, that represented by 

 the Devonian and Mountain limestone formations, sets in, as in 

 North Devon, with shallow-water sands, and a marine faun a 

 (Lower Devonian) in sequence in " Old Red " depositions, w ith 

 fresh-water fishes and crustaceans. There is no continuity from 

 " Old Red " into the earliest Devonian beds, any more than from 

 uppermost Silurian into Lower " Old Red " (Phillips's Geology 

 of Oxford, pp. 77 — 79). 



The later Palxozoic ocean-floor, now one mountain limestone, 

 in turn become terrestrial surface on which the Coal-measurei 

 were accumulated, and over which the abundant vegetation of 

 that period established itself. The Coal-measures represent so 

 much of the surface of their time, as from position favoured ex- 

 panses of fresh and brackish waters, and of alterations 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 Perm-Trias period presents true 

 Aralo-Caspian conditions, physically defined, 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 physio- 

 graphy 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 continental land, on which the fresh-water for- 

 mations 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 corre- 

 sponding conditions, and, as already noticed, of a progress of 

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

 corresponding order. Great periods, during which wide marine 

 conditions prevailed, alternated with others of wide terrestrial 

 surfaces. The marine periods, as we measure them by the pro- 

 ducts of the agents which seas and oceans call into action, must 

 have been of vast duration. In like manner we may feel as- 

 sured that the great fresh-water 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 IVealilen Formation 



Mr. Martin proposed this designation for the assemblage of 

 fresh-water depositions exhibited in the counties of Kent, Surrey, 

 and Sussex, and which may be described generally as consisting of 

 thick accumulations of sands and sandstones, for a lower or earlier 

 part, surmounted by a great argillaceous deposit (Weald clay). Mr. 

 Webster suggested the propriety of uniting the Purbeck beds 

 Hastings sands, and Weald clay into one group, the whole being 

 mainly a consecutive fresh-water series. It must be understood, 

 however, that there is not a definite line separating the Hastings 

 sands from the Weald clay ; all that is signified is that sands 

 predominate for the lower, and clays for the upper portion of the 

 Wealden depositions ; but just as thick bands of clay occur in 

 the lower series, so bands of sandstone occur in the upper. 



The arrangement adopted by the Geological Survey, in de- 

 scending order, is Weald clay, Tunbridge Wells sand, Wad- 

 hurst clay, Ashdown sands, Ashburnham beds, which in Sussex 

 are the equivalent of the Purbeck beds of Dorsetshire. 



The Lower Sands are well seen on the coast at Hastings, 



