CHAP. VI GEOGRAPHICAL AND GEOLOGICAL CHANGES 93 



authorities on shells, who has himself dredged largely both 

 in deep and shallow water and who has no theory to support, 

 has carefully examined this question. Taking the whole 

 series of genera which are found in the Chalk formation, 

 seventy-one in number, he declared that they are all com- 

 paratively shallow-water forms, many living at depths not 

 exceeding 40 to 50 fathoms, while some are confined to 

 still shallower waters. Even more important is the fact 

 that the genera especially characteristic of the deep 

 Atlantic ooze — Leda, Yerticordia, Nesera, and the Bulla 

 family — are either very rare or entirely wanting in the 

 ancient Cretaceous deposits."^ 



Let us now see how the various facts already adduced 

 will enable us to explain the peculiar characteristics of the 

 chalk formation. Sir Charles Lyell tells us that "pure 

 chalk, of nearly uniform aspect and composition, is met 

 with in a north-west and south-east direction, from the 

 north of Ireland to the Crimea, a distance of about 1,500 

 geographical miles ; and in an opposite direction it extends 

 from the south of Sweden to the south of Bordeaux, a 

 distance of about 840 geographical miles." This marks 

 the extreme limits within which true chalk is found, 

 though it is by no means continuous. It probably implies, 

 however, the existence across Central Europe of a sea 

 somewhat larger than the Mediterranean. It may have 

 been much larger, because this pure chalk formation 

 would only be formed at a considerable distance from land, 

 or in areas where there was no other shore deposit. This 

 sea was probably bounded on the north by the old Scan- 

 dinavian highlands, extending to Northern Germany and 

 North-western Eussia, where Palaeozoic and ancient 

 Secondary rocks have a wide extension, though now 

 partially concealed by late Tertiary deposits ; while on the 

 south it appears to have been limited by land extending 

 through Austria, South Germany, and the south of France, 

 as shown in the map of Central Europe during the 

 Cretaceous period in Professor Heer's Primeval World of 

 Switzerland, p. 175. To the north the sea may have had 



^ See Presidential Address in Sect. D. of British Association at Plymouth, 

 1877. 



