176 NEOZOIC TIME. [CHAP. X. 



Lower Chalk sea. They may have been driven away by 

 the coldness of the water, for few of the Mollusca or 

 Echinoderms range up into the Middle Chalk above. 



In Ireland the Pecten asper sand is succeeded by yellowish 

 calcareous sandstones with nodules of chert, and varying 

 from five to thirty feet in thickness. These sandstones 

 are probably of the age of our Grey Chalk, the beds below 

 being the equivalent of our Chalk Marl (see ante, p. 173). 



The Middle Chalk has at its base a hard nodular rock, 

 which passes up into firm rocky chalk that consists largely 

 of broken and triturated fragments of Inoceramus shells, 

 with many cells of Foraminifera. Higher up the chalk is 

 softer and whiter, and microscopical examination shows 

 that the shell-fragments become fewer, and the Foramini- 

 fera become less robust, having smaller and thinner shells ; 

 the mass of the rock consisting of a fine amorphous cal- 

 careous sediment. Toward the top of this division there 

 is something like a reverse change, the Foraminifera be- 

 coming more abundant, and at the same time rather more 

 robust. 1 



This soft white chalk is evidently the deposit of a deep 

 sea at a considerable distance from land, but it contains 

 thin interstratified seams of marly shale, and it is impor- 

 tant to observe that the argillaceous element in these seams 

 increases northwards, till in Lincolnshire and Yorkshire 

 they are veritable clays, dark grey or black, and yet only a 

 few inches thick. Again, therefore, we seem to have evi- 

 dence of the occasional influx of currents from the north. 



The Middle Chalk is surmounted throughout the greater 

 part of England by one or more beds of hard cream-coloured 

 limestone, which is known as the Chalk Rock, and has every 



1 For these particulars I am indebted to my friend, W. Hill, F.G.S., 

 whose microscopical studies have greatly added to our knowledge of 

 the minute structure of the Chalk, and who has found that the successive 

 zones exhibit constant lithological differences. 



