CHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 91 



Atlantic ooze — Leda, Verticordia, Neggra, 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 1,140 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 Scandinavian highlands, ex- 

 tending to Northern Germany and North-western Russia, 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 extend- 

 ing 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 an outlet to the Arctic Ocean 

 between the Ural range and Finland. South of the Alps there 

 was probably another sea, which may have communicated with 

 the northern one just described, and there was also a narrow 

 strait across Switzerland, north of the Alps, but, as might be 

 expected, in this only marls, clays, sandstones, and limestones 

 were deposited instead of true chalk. It is also a suggestive 

 fact that both above and below the true chalk, in almost all the 

 countries where it occurs, are extensive deposits of marls, clays, 



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

 1877. 



