I58 THE STORY OF THE EARTH. 



under the hammer. Between these limits the 

 Middle Chalk is about 350 feet thick in Bucking- 

 hamshire and Bedfordshire, and more than 200 

 feet thick in Cambridgeshire. It is thinner in 

 the North Downs. It contains some thin beds 

 of flint in the Upper part, which is known as the 

 zone of Holaster planus. The Middle Chalk is a 



it lenticular deposit, which is frequently marly, 

 as though a quantity of clay had been deposited, 

 while the chalk was being built up, by the growth 

 of its characteristic organisms. This portion of 

 the chalk has been thought to derive its clay 

 from the old land, of which the Cambridge Green- 

 sand is evidence in the previous period of time, 

 the intermixed day being the finer river mud 

 carried out into the ocean from a region of an- 



I and crystalline rocks removed to a greater 

 distance in the depression of land. On this hori- 

 zon there are multitudes of cup-sponges with 

 siliceous skeletons, of the genus Ventriculites* 

 Middle Chalk is chiefly conspicuous for 

 wanting most of the peculiar Cephalopods oi the 

 1 halk, and for wanting the sea-urchins and 

 ishes of the Upper Chalk. 



The Upper Chalk, which is only about 250 

 • thick m the North Downs near Croydon, at- 

 tains its maximum thickness north and south. At 



,\ich it is alxuit 500 feet thick; and in the 



[s e ' M ght its thickness may be 1000 feet. At 



Lyn - its thickness appears to be reduced 



It is whiter and softer than the chalk 



■x. The Hint winch characterise curs 



'•'inic- in horizontal tabular layers, in the 



of bedd Sometimes tl 



ary nodules, fantastically irregular in form, 

 wh;< i< >t abs< ilutely limited to the planes " ( 



