The group consists, in Britain,
of sands and clays, sometimes with bands of calcareous grit or
siliceous limestone, and occasionally containing concretions of
phosphate of lime, which are largely worked for agricultural
purposes.
V. _White Chalk_.--The top of the Upper Greensand becomes
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 simply 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 sometimes 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.
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