PART II. CHAPTER XV. 



193 



Green-sand Formation External Configuration of Chalk. 



sits of mud, sand, and pebbles, must subsequently have ceased 

 to act ; for it is evident that no similar sediment disturbed the 

 clear waters of the sea in which the white chalk accumulated. 

 The only hypothesis which seems capable of explaining such 

 changes is the gradual submergence of land which had been pre- 

 viously exposed to aqueous denudation. This operation may 

 have gone on with such slowness as to allow time for consider- 

 able fluctuations in the state of the organic world, so that differ- 

 ent sets of strata, beginning with the lower green-sand, and end- 

 ing with the upper white chalk, may each contain some peculiar 

 remains of animals which lived successively in the sea ; while 

 some species may have continued to exist throughout the whole 

 period, and are therefore common to all these formations. 



It will be seen in the next chapter, when we treat of the strata 

 called the Wealden, that such a general subsidence of land as is 

 'here supposed to explain the manner in which the chalk succeeds 

 the green-sand, may be inferred from other independent proofs 

 to have taken place throughout large areas. 



It cannot however be assumed, that all the green-sand in 

 Europe had ceased to be deposited before any chalk began to 

 accumulate. Such indeed was the order of events in parts of 

 England, France, Belgium, and Denmark ; but if we compare 

 different countries, and some of these not far distant from each 

 other, we find reason to believe that sand and clay continued to 

 be thrown down in one place, while pure chalk was forming in 

 another. In Westphalia, for example, strata containing the same 

 fossils as the white chalk of England, consist of sand and marl 

 with green grains like the upper green-sand. Similar facts have 

 been observed in Hungary in the Carpathian mountain chain. 

 Such variations would occur if the supposed sinking down of 

 land did not take place simultaneously everywhere ; and for this 

 reason the minor subdivisions of the cretaceous group, however 

 persistent and uniform in their mineral characters in some 

 regions, vary rapidly, and change entirely in other directions. 



Exteimal configuration of Chalk. — The smooth rounded out- 

 line of the hills composed of white chalk is well known to all 

 who have travelled in the south-east of England. The chalk 

 downs, being free from trees or hedge-rows, afford us an oppor- 

 tunity of observing the manner in which the upper valleys unite 

 with larger ones, and how these become wider and deeper as 

 they descend. For the most part they are dry, yet occasionally 

 they afford a perfect system of drainage, when a sudden flood is 

 caused by heavy rains or the melting of snow. We may con- 

 ceive their excavation to have been caused by the action of the 

 waves and currents while the chalk was gradually emerging from 



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