542 HISTORICAL PALAEONTOLOGY. 



varia. It never exceeds TOO feet in thickness ; but it con- 

 tains many fossils, usually in a state of beautiful preservation. 



IV. The Gault is succeeded upward by the Upper Green- 

 sand, which varies in thickness from three up to 100 feet, and 

 which derives its name from the occasional occurrence in it of 

 green sands. These, however, are local and sometimes want- 

 ing, and the name " Upper Greensand ;J 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 occasionally containing concretions of 

 phosphate of lime, which are largely worked for agricultural 

 purposes. 



V. 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. 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. 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 direc- 

 tion, 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 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-Ur- 

 chins, &c., are numerous Univalve Molluscs, such as Cowries 

 and Volutes, which are otherwise exclusively Tertiary or Re- 

 cent. 



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 Faxoe Limestone. 



VII. In North America, the Lower Cretaceous Rocks are 



