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CEETACEOUS ERA. 



THE record of this period consists of a series of strata, in 

 which chalk beds make a conspicuous appearance, and which 

 is therefore called the Cretaceous System or formation. In 

 England, a long stripe, extending from Yorkshire to Kent, 

 presents the cretaceous beds upon the surface, generally lying 

 conformably upon the oolite, and in many instances rising into 

 bold escarpments towards the west. The celebrated cliffs of 

 Dover are of this formation. It extends into Northern 

 France, and thence north-westward into Germany, whence it 

 is traced into Scandinavia and Russia. The same system 

 exists in North America, and probably in other parts of the 

 earth not yet geologically investigated. Being a marine de- 

 posit, it establishes that seas existed at the time of its forma- 

 tion on the tracts occupied by it, while some of its organic 

 remains prove, that in the neighbourhood of those seas there 

 were tracts of dry land. 



The cretaceous formation in England presents beds chiefly 

 sandy in the lowest part, chiefly clayey in the middle, and 

 chiefly of chalk in the upper part, the chalk beds being never 

 absent, which some of the lower are in several places. In the 

 vale of the Mississippi, again, the true chalk is wholly, or all 

 but wholly absent. In the south of England, the lower beds 

 are (reckoning from the lowest upwards), 1, Shankland or green- 

 sand, " a triple alternation of sands and sandstones with clay ;" 

 2, Gauti, "a stiff blue or black clay, abounding in shells which 

 frequently possess a pearly lustre ;" 3, Hard chalk ; 4, Chalk 

 with flints ; these last two being generally white, but in some 

 districts red, and in others yellow. The whole are, in England, 

 about 1200 feet thick, showing the considerable depths of the 

 ocean in which the deposits were made. 



Chalk is a carbonate of lime, and the manner of its produc- 

 tion in such vast quantities was long a subject of speculation 

 among geologists. Some light seemed to be thrown upon the 

 subject a few years ago, when it was observed, that the detritus 

 of coral reefs in the present tropical seas gave a powder, undis- 

 tinguishable, when dried, from ordinary chalk. It then 

 appeared likely that the chalk beds were the detritus of the 

 corals which lived in the oceans of that era. Mr. Darwin, 

 who made some curious inquiries on this point, further sug- 

 gested, that the matter might have intermediately passed 



