MIDDLE CHALK—CONDITIONS OF DEPOSIT. 
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CHAPTER XLVI. 
THE PHYSICAL AND GEOGRAPHICAL CONDITIONS 
OF THE MIDDLE CHALIC PERIOD. 
Geographical Conditions. 
Our conception of the geogratphy of the Anglo-Gallic region 
during the formation of the Middle Chalk must be based mainly 
on the conclusions at which we have previously arrived respecting 
the relative positions of sea and land in the Lower Chalk or Ceno¬ 
manian period. Owing to subsequent detrition and denudation 
there are very few places in the British Isles where the Middle 
Chalk shows any sign of near approach to land. In England there 
are no such places, and we can only infer, from the purely calcar¬ 
eous character of the chalk of which this part of the formation 
consists, that it was formed during the progress of a continued 
subsidence, whereby the area of the neighbouring lands was 
still further reduced and their coast-lines were still further 
removed from the central portions of the sea. 
In France, however, there are much greater differences in the 
lithological characters of the deposits which correspond with our 
Middle Chalk. Thus in the north-east of France (Nord, Aisne, and 
Ardennes) the deposits of this age are argillaceous marls and marly 
clays, which sometimes contain as much as 70 per cent, of clay, 
and are clearly terrigenous deposits; they attest the continued 
existence of land in the south of Belgium and the country of the 
Rhine. 
In the central and south-eastern parts of the Paris basin the 
beds present a deep-water chalky facies closely resembling that of 
southern England, and this facies extends as far west as the neigh¬ 
bourhood of Rouen and Fecamp. 
In the western part of the basin, however, in the departments 
of Calvados, Orne, and Sarthe, and in the valley of the Loire, the 
Lower Turonian chalk passes into a soft marl containing 15 or 16 
per cent, of clay, a deposit resembling our “ Chalk Marl/’ and 
doubtless formed under similar conditions. Moreover, the higher 
beds in the south-west ( Angoumien ) are not chalks at all, but 
siliceous and quartzose limestones, with some calcareous sands.* 
The varying lithological aspects of the Turonian and Senonian 
deposits in France are shown in a tabular view by M. Cayeux 
*See Contribution a 1 etude Micrographique des Terrains Sedimentaires, 
par L. Cayeux, p. 405, Lille, 1897. 
