158 NEOZOIC TIME. [CHAP. IX. 



of corallian limestones in the deeper water of the English 

 sea, were due to one and the same cause. 



During the formation of the Kimeridge Clay the subsi- 

 dence reached its farthest extent, and eventually a reverse 

 movement set in, the supply of mud grew scantier, and 

 the bluish argillaceous sands which underlie the Portland 

 stone were doubtless formed during upheaval. Finally, a 

 large part of the sea-bottom was raised into dry land, and 

 the sea was contracted into two separate branches or gulfs, 

 the separation of which seems to have resulted from the 

 upheaval of a tract across the centre of England a tract 

 which had formed a submarine ridge ever since the time 

 of the Lias. The existence of this ridge was mentioned on 

 p. 139, and the influence it had on deposition can be seen 

 from the following table of thicknesses, which shows that 

 the amount of sediment laid down over it was about 1,000 

 feet less than the accumulation in the basin to the south, 

 and 450 feet less than that in the area to the northward : 



Wiltshire. Oxford and Lincolnshire. 



Bucks. 



Portland Beds . .. 100 ... 50 . . absent? 



Kimeridge Clay .. 500 ... 150 ... 500 



Corallian .... 200 . . absent . . absent. 



Oxford Clay ... 500 ... 300 ... 350 

 Lower Oolites . 350 .. 100 . 200 



Feet 1,650 600 1,050 



This central tract now became an isthmus, uniting the 

 eastern and western Palaeozoic areas to one another, and 

 probably also to the southern end of the Pennine chain. 

 The emergence of this isthmus began in Portland times, the 

 sea of the Portland Limestone lying wholly to the south of 

 the latitude of Bedford, and opening southward through 

 France, while a northern gulf extending from Germany 

 reached into Yorkshire and Lincolnshire (see Plate VIII.). 



