Relation of the Cretaceous Strata to the Systems above and below.—In central and northern Europe the boundary between the Cretaceous and Tertiary strata is sharply defined by a fairly general unconformity, except in the Danian and Montian beds, where there is a certain commingling of Tertiary with Cretaceous fossils. The relations with the underlying Jurassic rocks are not so clearly defined, partly because the earliest Cretaceous rocks are obscured by too great a thickness of younger strata, and partly because the lowest observable rocks of the system are not the oldest, but are higher members of the system that have overlapped on to much older rocks. However, in the south of England, in the Alpine area, and in part of N.W. Germany the passage from Jurassic to Cretaceous is so gradual that there is some divergence of opinion as to the best position for the line of separation. In the Alpine region this passage is formed by marine beds, in the other two by brackish-water deposits. In a like manner the Potomac beds of N. America grade downwards into the Jurassic; while in the Laramie formation an upward passage is observed into the Eocene deposits. There is a very general unconformity and break between the Lower and Upper Cretaceous; this has led Chamberlin and Salisbury to suggest that the Lower Cretaceous should be regarded as a separate period with the title “Comanchean.”

Physiographical Conditions and Types of Deposit.—With the opening of the Cretaceous in Europe there commenced a period of marine transgression; in the central and western European region this took place from the S. towards the N., slow at first and local in effect, but becoming more decided at the beginning of the upper division. During the earlier portion of the period, S. England, Belgium and Hanover were covered by a great series of estuarine sands and clays, termed the Wealden formation (q.v.), the delta of a large river or rivers flowing probably from the N.W. Meanwhile, in the rest of Europe alternations of marine and estuarine deposits were being laid down; but over the Alpine region lay the open sea, where there flourished coral reefs and great banks of clam-like molluscs. The sea gradually encroached upon the estuarine Wealden area, and at the time of the Aptian deposits uniform marine conditions prevailed from western Europe through Russia into Asia. This extension of the sea is illustrated in England by the overlap of the Gault over the Lower Greens and on to the older rocks, and by similar occurrences in N. France and Germany.

Almost throughout the Upper Cretaceous period the marine invasion continued, varied here and there by slight movements in the opposite sense which did not, however, interfere with the quiet general advance of the sea. This marine extension made itself felt over the old central plateau of France, the N. of Great Britain, the Spanish peninsula, the Armorican peninsula, and also in the Bavarian Jura and Bohemia; it affected the northern part of Africa and East Africa; in N. America the sea spread over the entire length of the Rocky Mountain region; and in Brazil, eastern Asia and western Australia, Upper Cretaceous deposits are found resting directly upon much older rocks. Indeed, at this time there happened one of the greatest changes in the distribution of land and water that have been recorded in geological history.

We have seen that in early Cretaceous times marine limestones were being formed in southern Europe, while estuarine sands and muds were being laid down in the Anglo-German delta, and that beds of intermediate character were being made in parts of N. France and Germany. During later Cretaceous times this striking difference between the northern and southern facies was maintained, notwithstanding the fact that the later deposits were of marine origin in both regions. In the northern region the gradual deepening and accompanying extension of the sea caused the sandy deposits to become finer grained in N.W. Europe. The sandy beds and clays then gave way to marly deposits, and in these early stages glauconitic grains are very characteristically present both in the sand and in the marls. In their turn these marly deposits in the Anglo-Parisian basin were succeeded gradually and somewhat intermittently by the purer, soft limestone of the chalk sea, and by limestones, similar in character, in N. France, extra-Alpine Germany, S. Scandinavia, Denmark and Russia. Meanwhile, the S. European deposits maintained the characters already indicated; limestones (not chalk) prevailed, except in certain Alpine and Carpathian tracts where detrital sandstones were being laid down.

The great difference between the lithological characters of the northern and southern deposits is accompanied by an equally striking difference between their respective organic contents. In the north, the genera Inoceramus and Belemnitella are particularly abundant. In the south, the remarkable, large, clam-like, aberrant pelecypods, the Hippuritidae, Rudistes, Caprotina, &c., attained an extraordinary development; they form great lenticular banks, like the clam banks of warm seas, or like our modern oyster-beds; they appear in successive species in the different stages of the Cretaceous system of the south, and can be used for marking palaeontological horizons as the cephalopods are used elsewhere. Certain genera of ammonites, Haploceras, Lytoceras, Phylloceras, rare in the north, are common in the south; and the southern facies is further characterized by the peculiar group of swollen belemnites (Dumontia), by the gasteropods Actionella, Nerinea, &c., and by reef-building corals. The southern facies is far more widespread and typical of the period than is the chalk; it not only covers all southern Europe, but spreads eastwards far into Asia and round the Mediterranean basin into Africa. It is found again in Texas, Alabama, Mexico, the West Indies and Colombia; though limestones of the chalk type are found in Texas, New Zealand, and locally in one or two other places. The marine deposits are organically formed limestones, in which foraminifera and large bivalve mollusca play a leading part, marls and sandstones; dolomite and oolitic and pisolitic limestones are also known.

European Classification.Britain. Germany, &c., several
other parts of Europe.		 Hippurite
limestones
of
Southern
France
and
Mediterranean
Region
Stages. Sub-stages.
Upper
Cretaceous.
Montian.
Danian.
Aturian.
  Senonian.
Emscherian.
Turonian.
Cenomanian.

(placed by some
in the Tertiary).
Maestrichtian
 (Dordonian).
Campanian.
Santonian.
Coniacian.
Angoumian.
Ligerian.
Carentonian.
Rothomagian.

Chalk of Trimingham.
Upper Chalk with
 Flints.
Middle Chalk without
 Flints.
Grey Chalk.
Chalk marl.
Cambridge Greensand.
Marls and pisolitic
 Limestone of Meudon.
Limestone of Saltholm
 and Faxö (Denmark).
Upper Quader Sandstone.
Quader Marls and
 Pläner Marls.
Upper Pläner.
Lr. Pläner and Lr.
 Quader.
Tourtia of Mons, &c.
Lower
Cretaceous.
Albian.
Aptian.
Barremian.
Neocomian.

Gault.
Gargasian.
Bedoulian.
Hauterivian.
Valangian.
Berriasian.
Selbornian.
  Gault and Upper
   Greensand.
__________________
Lower Greensand.
Weald Clay
  and
Hastings sands.
   Marine
   Beds of
   Specton.
Flammen mergel. Clay
 of N. Germany.
Urgonian
Requienia
 (caprotina) Kalk
 or Schrattenkalk.
  North
  German
  Hills
  formation
Upper Cretaceous.Lower Cretaceous.
Alpine Region.Aptychenkalk in E. Alps ... Cretaceous Flysch...
Biancone of S. Alps.		... Cretaceous Flysch ...
Carpathian and Vienna Sandstones,
  Gosau formation of E. Alps.
Seewan beds of N. Alps.
Scaglia of S. Alps.
Africa.Nubian Sandstone of ...
Uitenhage Beds S. Africa.		... N. Africa and Syria.
Pondoland Beds S. Africa.
India.Oomia and Utatur Group.Arialoor Beds (Deccan Trap).
Australia.Rolling Down Formation.Desert Sandstone.
New Zealand.Thick conglomeratic Series with Bitumous coals.Waipara Beds and Limestones, Chalk,
  with Flints, Marls and Greensand.
S. America.Puegiredon Series.        Belgrano ...... Series. San Martin Series.
Japan.Torinosa Limestone and Ryoseki Series.Izumi Sandstone and Hokkaido Series.
Greenland.Kome Group.Atani Group. Patoot Group (part).

Note to Table.

MontianfromMons in Belgium.
Danian”Denmark = Garumnien of Leymerie.
Aturian”Adour.
Maestrichtian”Maestricht.
Campanian”Champagne.
Emscherian”Emscher river in Westphalia.
Santonian”Saintonge.
Coniacian”Cognac.
Senonian”Sens in department of Yonne.
Turonian”Touraine.
Angoumian”Angoumois.
Ligerian”the Loire.
Cenomanian”Le Mans (Cenomanum).
Carentonian”Charente.
Rothomagian”Rouen (Rothomagus).
Albian”dept. of Aube.
Selbornian”Selborne in Hampshire.
Aptian”Apt in Vaucluse.
Gargasian”Gargas near Apt.
Bedoulian”la Bedoule (Var) = Rhodanien of Renevie
Barremian”Barrême in Basses Alpes.
Hauterivian”Hauterive on Lake of Neuchâtel.
Valangian”Château de Valangin near Neuchâtel.
Neocomian”Neuchâtel (Neocomum).
Berriasian”Berrias (Ardéche) near Besseges.
Urgonian”Orgon near Arles.