50 GEOLOGICAL HISTORY. [CH. 



X. Cretaceous. 



In the south of England, and in some other districts, it is 

 difficult to draw any definite line between the uppermost strata 

 of the Jurassic and the lowest of the Cretaceous period. The 

 rocks of the so-called Wealden series of Kent, Surrey, Sussex, 

 and the Isle of Wight, are usually classed as Lower Cretaceous, 

 but there is strong evidence in favour of regarding them as 

 sediments of the Jurassic period. The Cretaceous rocks of 

 England are generally speaking parallel to the Jurassic strata, 

 and occupy a stretch of country from the east of Yorkshire 

 and the Norfolk coast to Dorset in the south-west. The Chalk 

 downs and cliffs represent the most familiar type of Cretaceous 

 strata. In the white chalk with its numerous flints, we have 

 part of the elevated floor of a comparatively deep sea, which 

 extended in Cretaceous times over a large portion of the east 

 and south-east of England and other portions of the European 

 continent. On the bed of this sea, beyond the reach of any 

 river-borne detritus, there accumulated through long ages the 

 calcareous and siliceous remains of marine animals, to be 

 afterwards converted into chalk and flints. At the beginning 

 of the period, however, other conditions obtained, and there 

 extended over the south-east of England, and parts of north 

 and north-west Germany and Belgium, a lake or estuary in 

 which were built up deposits of clay, sand and other material, 

 forming the delta of one or more large rivers. For these 

 sediments the name Wealden was suggested in 1828. Eventu- 

 ally the gradual subsidence of this area led to an incursion of 

 the sea, and the delta became overflowed by the waters of a 

 large Cretaceous sea. At first the sea was shallow, and in it 

 were laid down coarse sands and other sediments known as the 

 Lower Greensand rocks. By degrees, as the subsidence continued, 

 the shallows became deep water, and calcareous material slowly 

 accumulated, to be at last upraised as beds of white chalk. The 

 distribution of fossils in the Cretaceous rocks of north and 

 south Europe distinctly points to the existence of two fairly 

 well-marked sets of organisms in the two regions ; no doubt the 



