THE CHALK PERIOD 



posed of the remains of organised beings the White 

 Chalk is in Norfolk quite 1200 feet thick must have 

 been very great. If we allow that the tiny shells of 

 the foraminifera may have accumulated at the rate 

 of two feet in thickness in a century, then it would have 

 required 50,000 years to form the chalk of the south- 

 east of England, whose thickness we have estimated at 

 1000 feet." 



Every one who has been on a chalk cliff or hill has 

 found, and perhaps thrown, chalk flints. Flints are made 

 of mineral called silica, and very often these flints, 

 or nodules of silica, surround some organism like a 

 sponge or a shell. During the formation of the chalk 

 the sea floor appears to have been covered at intervals 

 by a growth of sponges, which were composed of siliceous 

 matter, and their death and decay produced most of the 

 flint. Sometimes flint is found in bands, in which case 

 it may have been deposited by siliceous water trickling 

 through fissures or cracks in the chalk. 



In the sea which thus existed the Plesiosaurs and 

 Ichthyosaurs still pursued the even tenor of their way, 

 growing larger and larger. They were of many shapes, 

 and probably of many habits. Some were certainly 

 fish-eaters, and with their enormous jaws must have been 

 most undesirable neighbours. Probably, however, they 

 had plenty of diversity in their lives, and may have had 

 many a bitter struggle with equally ferocious sea animals 

 of other types. The scaly saurians, for example, were 

 beginning to come on ; and began in this era to assume 

 the size and appearance that have occasionally since been 



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