THE CRETACEOUS PERIOD. 259 
Greensand ” is to be regarded as a mame 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 Cha/k, 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 
(Zuronien of D’Orbigny ; ALittelguader of Germany). This, in 
turn, passes up into a great mass of white chalk, in which the 
stratification 1s marked by nodules of black flint arranged in 
layers (.Sézonten of D’Orbigny ; Oberguader 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 1oo feet in thickness, and un- 
doubtedly superior to the White Chalk. These JZaestricht 
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, 
Belemnites, 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 Faxde 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 :— 
