chap, vj.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 
91 
Atlantic ooze — Leda, Verticordia, Nesera, and the Bulla family 
— are either very rare or entirely wanting in the ancient 
Cretaceous deposits. 1 
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. 
