THE CRETACEOUS PERIOD. 259 



Greensand " is to be regarded as a name and not a description. 

 The group consists, in Britain, of sands and clays, sometimes 

 with bands of calcareous grit or siliceous limestone, and occa- 

 sionally containing concretions of phosphate of lime, which are 

 largely worked for agricultural purposes. 



V. White Chalk. The top of the Upper Greensand be- 

 comes argillaceous, and passes up gradually into the base of 

 the great formation known as the true Chalk, divided into 

 the three subdivisions of the chalk-marl, white chalk without 

 flints, and white chalk with flints. The first of these is sim- 

 ply argillaceous chalk, and passes up into a great mass of 

 obscurely- stratified white chalk in which there are no flints 

 (Turonien of D'Orbigny; Mittelquader of Germany). This, in 

 turn, passes up into a great mass of white chalk, in which the 

 stratification is marked by nodules of black flint arranged in 

 layers (Senonien of D'Orbigny ; Oberquader of Germany). The 

 thickness of these three subdivisions taken together is some- 

 times over 1000 feet, and their geographical extent is very 

 great. White Chalk, with its characteristic appearance, may 

 be traced from the north of Ireland to the Crimea, a distance 

 of about 1140 geographical miles; and, in an opposite direction, 

 from the south of Sweden to Bordeaux, a distance of about 

 840 geographical miles. 



VI. In Britain there occur no beds containing Chalk fossils, 

 or in any way referable to the Cretaceous period, above the 

 true White Chalk with flints. On the banks of the Maes, 

 however, near Maestricht in Holland, there occurs a series of 

 yellowish limestones, of about 100 feet in thickness, and un- 

 doubtedly superior to the White Chalk. These Maestricht 

 beds (Danien of D'Orbigny) contain a remarkable series of 

 fossils, the characters of which are partly Cretaceous and 

 partly Tertiary. Thus, with the characteristic Chalk fossils, 

 Belcmnites, Baculites, Sea-Urchins, &c., are numerous Univalve 

 Molluscs, such as Cowries and Volutes, which are otherwise 

 exclusively Tertiary or Recent. 



' Holding a similar position to the Maestricht beds, and 

 showing a similar intermixture of Cretaceous forms with later 

 types, are certain beds which occur in the island of Seeland, 

 in Denmark, and which are known as the Faxb'e Limestone. 



Of a somewhat later date than the Maestricht beds is the 

 Pisolitic Limestone of France, which rests unconformably on 

 the White Chalk, and contains a large number of Tertiary 

 fossils along with some characteristic Cretaceous types. 



The subjoined sketch-section exhibits the general succession 

 of the Cretaceous deposits in Britain : 



