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the white chalky-looking mud so named is found, when examined 

 by the microscope, to consist mainly of the shells of a minute 

 creature called a Globigerina, Avhich abounds in many seas both at 

 the surface and at various depths. It is one of the most widely 

 distributed forms of Foraminifera. The flint bands so common 

 in the Chalk, are made up of the sponges, foraminifera, and other 

 organisms, which secrete a flinty or siliceous shell or skeleton. 

 The hard parts of both the calcareous and flinty organisms have 

 sunk together to the bottom, and at some later period the flinty 

 particles have separated themselves from the calcareous ones by the 

 influence of "concretionary action." Concretionary action is found 

 to have taken place in many different kinds of rocks, and has 

 been noticed in laboratory experiments. 



It was once supposed that the ancient chalk sea was as deep 

 as that in which Globigerina Ooze is now being deposited. But 

 Mr. Alfred Kussel Wallace and Mr. Gwjti Jefi'reys have lately 

 been caUuig attention to evidence which tends to show that the 

 Chalk, though deposited in a broad open sea, is hardly to be 

 considered a deejj-sea. formation, as the Globigerina Oozes from 

 comparatively shallow water, much more resemble the Chalk than 

 those from a depth of 1000 fathoms or more. And the fossils of 

 the Chalk are all comparatively shallow water forms, many living 

 at depths not exceeding 40 to 50 fathoms ; while the genera 

 specially characteristic of the deeper Atlantic are very rare or 

 entirely wanting in the Chalk. 



Chalk is found here and there from the north of Ireland to 

 the Crimea, and from the south of Sweden to the south of 

 Bordeaux. This implies the former existence across what is now 

 Central Europe, of a sea rather larger than the Mediterranean. It 

 may have been much larger, as chalk would only be formed at a 

 considerable distance from land. 



Besides the microscopic fossils of the Chalk there are plenty of 

 larger size. Various kinds of sea-urchins are especially abundant. 

 According to the present state of our knowledge, the Chalk is the 

 latest formation containing Ammonites, and the remains of those 

 great extinct reptiles, the Ichthyosaurus, Plesiosaurus, and Ptero- 

 dactyle. The difference between the past and present condition of 



