ANNIVERSARY ADDRESS OF THE PRESIDENT. XCVII 



was then exercised at levels corresponding to the position of the 

 pebbles, and before any portion of the chalk had been raised suffi- 

 ciently to come within the sphere of its action, as no flint or chalk 

 pebbles are raised with this detritus. Mr. Austen further assumes that 

 the gravel was deposited before the surface had been intersected by its 

 present deep combes and broad valleys, which is at least a reasonable 

 supposition ; as soon however as elevation had removed the crystalline 

 rocks from the action of the sea, and brought any portion of the chalk 

 within it, the gravel would assume a different character, and be com- 

 posed of chalk and flint pebbles, capping the tertiary deposits which 

 had been in progress of formation, and filling up some of the indenta- 

 tions which had been previously formed in the softer strata, and which 

 had become the mouths of land-valleys. These gravels were again 

 cut through by fluviatile action on further elevation, and the remains 

 of elephants and other large mammalia brought into view ; affording 

 proofs, not only of the existence of dry land upon which such animals 

 must have lived, but also of a vegetation suitable to their existence. 

 Again, the beds of peat with timber trees which are seen at low water 

 opposite the openings of these valleys are further indications of dry 

 land, though, as Mr. Austen observes, these are not proofs of recent 

 change, but simply of the removal of some barrier which had before 

 shut out the sea from them. 



Up to this point it will be observed that the phsenomena would 

 appear to be explicable on the theory of one continuous and 

 gradual elevation, but Mr. Austen adduces other facts which in- 

 dicate both elevations and depressions. The section of one of the 

 docks at Portsmouth, taken from a paper by Colonel James, 

 shows, for example, stumps of trees in place on the London Clay, 

 and covered with a bed of mud and vegetable matter 2 feet thick, 

 this again being covered by an estuary mud with shells, 4 feet 

 thick, then by a bed of sand and shingle, and finally by the 

 mud of the present estuary, the whole indicating a considerable 

 amount of subsidence, following an elevation which had brought 

 the London Clay to the surface and left it there for a time 

 sufficiently long, first, to have prepared it for vegetable growth, 

 and then to have permitted the development and growth of large 

 trees. This section is the more remarkable, as three artesian borings 

 have been made in the neighbourhood, forming a triangle, neither 

 angle of which can be more than I ^ mile from it, without discovering 

 any trace of a laud-surface. At the one made at Blockhouse Fort, 

 a bed of oysters was found under a shingle deposit more than GO feet 

 thick, so that the surface of the London Clay appears to have shelved 

 off from the position of the docks outward, the superficial deposits, 

 including the " sand and shingle," having been much thinner there 

 than at any of the artesian borings, so that upon elevation it must 

 have been a bank, whilst its surface at the other points remained 

 submerged. 



Pagham Harbour affords another striking example of oscillation of 

 the surface, as a silt-deposit with bands of Cardium edule and Mactra 

 solida overlies an old terrestrial surface bearing trees which are rooted 



VOL. XIII. g 



