94 



lying parts of Scotland and Ireland. Chalk still occurs in County 

 Antrim, where it has been preserved from the complete denudation 

 of the rest of Irish deposits by being covered in middle Tertiary 

 times by sheets of basaltic lava. Patches of cretaceous rocks occur 

 also in the Isle of Mull, and a fragment of chalk, with charac- 

 teristic fossils, has been preserved in the plug now exposed by 

 denudation, of a Tertiary volcano in Arran. 



Chalk was deposited in a wide sea, which at its greatest ex- 

 tent had its shores to the west on an ancient land of which Ireland 

 and the Hebrides are only relics, its southern shores were in what 

 is now Central France and its eastern in the middle of Germany. 

 In the early times of the Cretaceous Period the crust of the earth 

 in this part of the globe gradually sank and various distinct forma- 

 tions of rocks were laid down during the different phases. First 

 the Weald Clay accumulated in a wide freshwater lake which was 

 formed above the sinking deposits of the Purbeckian formation ; 

 this was followed by the sands and silt of the Lower Greensand 

 as the sea burst in and deepened, and these in their turn were 

 covered by the fine blue clays of the Gault and the glauconitic 

 sand, often richly calcareous, of the Upper Greensand. The crust 

 of the earth hollowed to still greater depths, and in these the 

 calcareous deposits of the Chalk accumulated. The conditions 

 throughout the period of deposition of the Chalk were on the 

 whole remarkably persistent. A closer study of the fossil con- 

 tents and the mineralogical composition of the Chalk, however, 

 shows that the period was marked by several distinct physio= 

 graphic stages which corresponded to various depths as the floor 

 of the sea underwent elevation or subsidence. 



The Chalk has long been divided into three main sub-divisions 

 of a petrographical character ; the Upper Chalk with Flints (the 

 Senonian of Continental terminology), the Middle Chalk (the 

 Turonian), and the Lower or Grey Chalk (the Cenomanian). At 

 the base of each of these three divisions there is a more or less 

 well marked bed: the " Chalk Rock" beneath the Upper Chalk, 

 the " Melbourne Rock " (at the base of the Middle Chalk), and 

 the so-called " Chloritic Marl," forming the basement bed of the 

 Chalk as a whole. Locally the Chalk Rock and Melbourne Rock 

 are not characteristically developed. The Chloritic Marl varies 

 both in character and in horizon with reference to the Upper 

 Greensand as between the Isle of Wight and Dorset, in places 

 resting without any sign of unconformitv on the Upper Green- 

 sand, while elsewhere containing man}' derived fossils and rolled 

 fragments. 



Another, and more trustworthy, manner of division, the 

 paleontological, is based on a sequence of zones which are each 

 characterised by some particular species of cephalopod or echino- 

 derm. The type species of the zone is one which in some wav 

 distinguishes it from others, as being particularly abundant in it 

 or wholly restricted to it, or representative of a small but constant 

 assemblage of species confined to it over a wide' area. It is thus 



