the Purbeck and Wealden Deposits of England and France, 283 



age. The existence of the north-east opening during the oolitic 

 age, in the form of a strait not caused by any abrupt dislocation 

 of the land surrounding the basin to which it gave access, fur- 

 nishes presumptive evidence that the bed of it was shallow and 

 capable of being converted into land by a very moderate eleva- 

 tion, resembling in this respect the Sound entrance to the Baltic 

 at the present day. The absence of all appearance of convul- 

 sive movement in that region at the close of the oolitic age would 

 also show that the opening became converted into land by merely 

 participating in the general elevation which so uninterruptedly 

 marked that age until its close, becoming by the process a low- 

 lying tract. The apparent discrepancy, that this low-lying tract 

 should not have been submerged while the rest of the palaeozoic 

 barrier and more central part of the basin were undergoing a 

 depression of some 1200 or 1400 feet, is removed by the proofs 

 which exist that the extremity of the barrier which abutted 

 on the midland counties did not undergo any material depres- 

 sion until the upper cretaceous age, and that even during that 

 age it marked the north- westernmost limit of the depression 

 then proceeding. 



In realizing, therefore, the process of depression which marked 

 the formation of the Wealden, we have to conceive the depres- 

 sion taking place along the line from Dorsetshire to Valenciennes 

 to have been at its maximum over the palaeozoic barrier, diminish- 

 ing in amount at the western extremity of the line, and losing 

 its force as the distance southwards from the line increased ; so 

 that by the time the central part of that line had subsided to the 

 extent of 1200 feet and upwards, the low-lying land of the north- 

 east opening was again submerged to an extent probably only 

 sufficient to permit the ingress of the sea-water to the basin, 

 and not of the deposit of any thickness of sediment over the 

 opening itself. On the sea re-entering, it would be only the 

 most depressed area that would be occupied by it, those portions 

 of the basin lying west and south of the line of depression 

 remaining land until the continuance of the process of depression 

 or attenuation brought them successively below the sea-level. 

 Hence the absence of lower cretaceous marine deposits in Dor- 

 setshire, and hence the successive thinning out of the lower 

 cretaceous deposits through the departments of Calvados, Orne, 

 and Sarthe. Into the deeper or more central portions of a land- 

 locked gulf the greater quantity of sediment would be swept ; 

 and hence the maximum thickness of the lower greensand occurs 

 at Atherfield, diminishing north-eastwards towards the palaeozoic 

 barrier, and disappearing or thinning out westwards in Dorset- 

 shire, and southwards in the Calvados. 



The whole grouping of the Purbeck formation would have 



