178 NEOZOIC TIME. [CHAP. X. 



Above the Chalk rock of England there are a set of 

 rough nodular and shelly beds of a yellowish tint, in which 

 Echinoderms of the genus Micraster are extraordinarily 

 abundant, and these pass up into soft white chalk with 

 numerous layers of flints. Still higher is a considerable 

 thickness of chalk without flints, surmounted by other 

 beds in which they abound. The thickness of this Upper 

 Chalk increases from west to east, its total thickness near 

 Dorchester being estimated by Dr. Barrois at 500 feet, while 

 in the Isle of Wight it is about 700, and in Norfolk pro- 

 bably 900 feet ; even here, however, the actual summit is 

 not reached, the very highest or Maestricht chalk not 

 occurring in England. 



t In Ireland the base of the Upper Chalk is a hard lime- 

 stone full of green grains and quartz pebbles, but holding 

 Echinoderms of Upper Chalk species. It appears, in fact, 

 to be the condensed equivalent of the two lowest zones of 

 the English Senonien, which are 250 feet thick in the south 

 of England, whereas the Irish bed is less than three feet. 

 Above there is hard white chalk which, however, is less 

 than 100 feet thick, though it must represent some 600 

 feet of the English Chalk. The same hard white limestone 

 is found at a few places on the west coast of Scotland, in 

 Mull and Argyleshire. 



2. Geographical Restoration. 



Lower Cretaceous Time. We have seen that at the close 

 of the Jurassic period the area of deposition was limited 

 to a region south of the latitude of Devizes and Chatham. 

 Everywhere to the north of this line there is a gap and un- 

 conformity between the Jurassic and Cretaceous rocks ; and 

 we may conclude that the greater part of the British area 

 was then in the condition of dry land. The areas occupied 

 by the Palaeozoic rocks formed hilly and mountainous 



