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 (_Sénonien_ 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. 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 undoubtedly 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, _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 _Faxöe 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:-- [Illustration: Fig. 185. GENERALIZED SECTION OF THE CRETACEOUS SERIES OF BRITAIN.]