Ca XVni.l LOWER GREENSAND. 341 



CHAPTER XVIH. 



LOWER CRETACEOUS AND WEALDEN FORMATIONS. 



Lower Greensand — Term "Neocomian" — Atherfield section, Isle of Wight — Fos- 

 sils of Lower Greensand — Palceontological relations of the Upper and Lower Cre- 

 taceous strata — Wealden Formation — Freshwater strata intercalated between two 

 marine groups — Weald Clay and Hastings Sand — Tunbridge rocks — Fossil shells, 

 fish, and plants of Wealden — Their relation to the Cretaceous type — Geographi- 

 cal extent of Wealden — Movements in the earth's crust to which the Wealden 

 owed its origin and submergence. 



The term " Lower Greensand " has hitherto been most commonly 

 applied to snch portions of the Cretaceons series as are older than 

 the Gault. But the name has often been complained of as incon- 

 venient, and not without reason, since green particles are wanting in 

 a large part of the strata so designated, even in England, and wholly 

 so in some European countries. Moreover, a subdivision of the 

 Upper Cretaceous group has likewise been called Greensand, and to 

 prevent confusion the terms Upper and Lower Greensand were intro- 

 duced. Such a nomenclature naturally leads the uninitiated to sup- 

 pose that the two formations so named are of somewhat coordinate 

 value, which is so far from being true, that the Lower Greensand, in 

 its widest acceptation, embraces a series nearly as important as the 

 whole Upper Cretaceous group, from the Gault to the Maestricht beds 

 inclusive ; while the Upper Greensand is but one subordinate member 

 of this same group. Many eminent geologists have, therefore, pro- 

 posed the term " Neocomian " as a substitute for Lower Greensand ; 

 because, near Neufchatel (Xeocomum), in Switzerland, these Lower 

 Greensand strata are well developed, entering largely into the structure 

 of the Jura mountains. By the same geologists the Wealden beds are 

 usually classed as " Lower Neocomian," a classification which will not 

 appear inappropriate when we have explained, in the sequel, the inti- 

 mate relation of the Lower Greensand and Wealden fossils. 



Dr. Fitton, to whom we are indebted for an excellent monograph 

 on the Lower Cretaceous (or Greensand) formation as developed in 

 England, gives the following as the succession of rocks seen in parts 

 of Kent : 



No. 1. Sand, white, yellowish, or ferruginous, with concretion 



of limestone and chert, - - - - 70 feet. 



2. Sand with green matter, - - - - 70 to 100 feet 



3. Calcareous stone, called Kentish rag, - - ' - 60 to 80 feet. 



