128 GEOLOGY. 



the Lower and Upper Cretaceous systems are, in general, markedly 

 different, both in origin and distribution. 



In Europe. The general relations of the Lower Cretaceous of 

 western Europe have already been suggested. The lowest beds of the 

 system in different regions are probably not strictly contemporaneous, 

 for the basins in which they were deposited appear to have come into 

 existence at somewhat different times, some of them enduring from 

 the Jurassic period. The non-marine deposits of the early Neocomian 

 were later succeeded, in many places, by beds of marine origin, and of 

 greater extent. 



The slow encroachment of the sea over western Europe during the 

 Early Cretaceous period seems not to have been without interruptions, 

 for reverse changes, more or less local and temporary, took place now 

 and again, re-establishing lacustrine conditions, or severing marine 

 communications between regions which had been overspread by a 

 common sea. 



In general there is great diversity in the formations of the Lower 

 Cretaceous in central and western Europe. They embrace all sorts 

 of clastic rocks, from coarse to extremely fine (plastic clays); also 

 glauconitic beds, limestone, and beds of coal (northwestern Germany) 

 and iron ore. They embrace, indeed, about all varieties of sedimentary 

 rock except chalk, the rock from which the name " Cretaceous ;; was 

 derived. 



The iron ore which occurs locally in the Lower Cretaceous of Ger- 

 many, differs from most formations of this ore. It occurs in beds 

 made up of nodules of iron carbonate derived from the Jurassic beds. 

 These ore beds are, therefore, of clastic origin. They reach a maxi- 

 mum thickness of nearly 100 feet. In general the Lower Cretaceous 

 beds of Europe are more generally indurated than those of eastern 

 North America. 



In England, the Wealden formation is thought to have been accu- 

 mulated as a great delta, 20,000 or 30,000 square miles in extent, 

 in an inland body of water which occupied a part of England, and parts 

 of the continent to the east. The sediments are thought to have come 

 from the north. The later Neocomian beds of England contain some 

 greensand (glauconite). The succeeding Gault series, mainly clastic 

 and nearly 1000 feet (maximum) thick, is more widespread than the 

 Neocomian. 



