14 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The effect of gentle subsidence of the shore has been 

 shown to be the lapping of successive members of a rock-series 

 over one another, each thus encroaching on the land area. 

 Overlap of this type is found in the buried Mesozoic rocks 

 under the Wealden area, where the Lias extends beyond and 

 unconformably covers the Trias, and is in turn overlapped by 

 the Bathonian (and possibly the Bajocian) which creeps up 

 over the ancient and eroded surface of Coal Measures. Overlaps 

 of the Purbeck rocks by the Wealden, and of the Wealden by 

 the Lower Greensand also occur. In these examples, the 

 subsidence appears to have been fairly gentle, although there 

 is evidence of elevation and erosion at many levels, and the 

 successive members of the Jurassic maintain approximately 

 the characters they bear in the north-east of France where 

 some of them crop out. Each also extends only a short 

 distance beyond its underlying deposit, and little overstepping 

 of the Palaeozoic rocks occurs. 



Somewhat different is the case of the great overlap which 

 began with the Gault, continued with the Upper Greensand, 

 and culminated in the great spread of the Lower Chalk sea, 

 termed " the Cenomanian Transgression." The Gault and 

 Upper Greensand (Selbornian) sea, extending beyond the limits 

 of the Lower Cretaceous, Jurassic, Triassic, and other rocks, 

 washed the shores of the old Palaeozoic land now underlying 

 the east of England. The Upper Greensand sea swept rapidly 

 westward forming a plane of marine denudation and trans- 

 gressing the various divisions of the secondary strata until 

 it deposited its sands and gravels upon the Palaeozoic rocks 

 of the west (Haldon Hills, Devon). The Cenomanian sea 

 marked an even greater subsidence, which carried it beyond 

 the north of Ireland and north-west of Scotland. The deposits 

 of Greensand and Chalk (both of Cenomanian age) rest upon 

 Jurassic rocks in these areas, and whilst denudation may have 

 since removed them in part, they probably indicate by their 



