The tertiary rocks contain numerous deposits of plastic clays, which are especially abundant in the Paris basin, where they are found above the chalk.
The Thanet stage of this system presents, at Lille, a bed of grey or black plastic clay, 15 to 20 metres thick. But it was particularly after the invasion of the Thanet Sea (from the Thanet promontory to the mouth of the Thames) that the plastic clays were formed in consequence of a period of lagoons and estuaries which forms the sparnacian stage.
Fig. 4. Diagram showing the Tertiary Deposits under Paris (1 in 10,000).
I, chalk and pisolithic limestone; 2, plastic clay; 3, limestone gravel; 4, Beauchamp sands and Saint-Ouen limestone; 5, gypsum and green clays; 6, Fontainebleau sands.
At Varengeville, near Dieppe, a plastic grey clay is worked for the manufacture of tiles and bricks; in Artois, in Flanders, in Picardy, in Hainault, plastic clay is often found collected in holes and irregular veins in the midst of sand. These clays bear signs of the conflict between the fresh water and the sea at the sparnacian period; but when we come to the Paris basin, there is no longer anything but fresh-water deposits. The thickness of the plastic clays of this district is very diverse; it is slight at Meudon, but below Saint-Denis extends to 50 metres.
At the base there is found a bed of clay streaked with red and grey, then the true bluish grey plastic clay divided into two strata by a fine, clayish, and ligneous quartz sand, called Auteuil sand. Everywhere in the neighbourhood of Paris these clays are worked for tiles, bricks, pipes, etc.
At the extremity of the Ile-de-France, in the GÆ'tinais country, a plastic clay, variegated with bright tints, occupies pockets in the chalk; it is used in the neighbourhood of Montereau for the manufacture of porcelain.
West of the Paris basin, on the Eure plateaux, there are found white sands streaked with yellow and red with veins of clay which are frequently red and usually form pockets in the chalk. In these pockets, peculiar chemical reactions have taken place which in certain places have caused the formation of halloysite (a variety akin to kaolin), and this is employed at Breteuil - sur - Iton for the manufacture of stoneware. The refractory clay of Abondant, near Dreux, also forms part of the sparnacian stage.
After this stage follows th ypr*sian (from Ypres in Flanders), which includes the London clay, a brown or brown-grey clay reaching a thickness of 250 metres, and known in Flanders by the name of Ypres clay or Flanders clay. Its thickness is about 100 metres; it is plastic and of a bluish grey colour, like the Roubaix clay which appears to belong to the same stratum.
As type of the eocene series, we will mention also the flint clay of le Perche. It is a conglomerate of natural flints coming from the chalk, and encrusted in a red and white clay which appears to have a chemical origin. This conglomerate is found also in the Sologne and Blaisois districts, where it extends in places to a thickness of 30 metres. In the forests of ChÆ'teau-neuf and Senonches, the conglomerate of plastic clay passes under quartz sands with lustred sandstones mingled with coarse kaolin sands and white or variegated pure clays. This variety of the plastic clay stage is continued in the neighbourhood of ChÆ'teaudun and round Evreux, where the clay sometimes becomes refractory, while the lustred sandstones are accompanied by iron ore formerly worked in the valley of the Iton.
To this series belong the clays or green marls of the Paris basin, the depth of which does not exceed 4 to 5 metres. They extend to the neighbourhood of Âtampes and Fert*-Alais, and are worked everywhere for brick and tile manufacture at Fresne-lez-Rungis, Sannois, and Orgemont. They contain no pyrites, and consequently give out no sulphurous acid in baking like certain plastic clays from the western side of Paris. This is what distinguishes the Vaugirard brick-earth from the clay called M*nilmontant or Belleville.
To the oligocene series are attached the Marseilles clays, the order of which is as follows: -
Yellowish Marseilles clays with pudding-stones.
Red clays of Saint-Henri and Lestaque.
White lacustrine limestone.
Blackish ligneous clays.
The refractory clay of Andenne (Belgium) is also of the same series,as is too the sometimes sandy,sometimes clayey,deposit which covers a large part of North Germany to a height of 160 metres.
Some clays of special formation belong to the oligocene period Under the plain of la Bresse at Bourg (Ain) they form a continuous layer of 28 metres thick and of various colours: white, yellow, marbled, red, blue, or green. They are of siderolithic origin and are found above iron ore.
The sands and clays of la Sologne are of this series; they reach a thickness of 40 metres on the right bank of the Loire, and join the so-called kaolin sands of the Eure plateaux, which are formed of grains of quartz, combined by means of a clay cement easily loosened by water. In company with plastic clays, chemically very pure, they fill cracks or funnels.
This is represented, in the Morbihan, by blue or grey clays found at Saint-Jean-la-Poterie, Redon (llle-et-Vilaine), Saint-Gildas-des-Bois (Loire-Inf*rieure).
In the neighbourhood of Chalon-sur-Sa*ne, fluvial sands are found mingled with variegated clays, which are often refractory, and are worked at Saint-L*ger-sur-Dheune and Montchanin (Sa*ne-et-Loire).
To this period belongs the lehm (brick-earth or tableland slime), and the loess (calcareous lehm). Generally these alluvial deposits are in terraces of different heights from the bottoms of valleys to the highest points of a watershed, as can be seen from Fig. 5. The sands and gravels accompanying the lower alluvions become rarer as one ascends; but more or less angular flints are always to be met with. The ooze is hollowed out, or sometimes covered with another reddish brown slime, the formation of which we have explained.
Fig. 5. Normal Section of the Valleys in the North of France. g,g',g", gravels at the bottom of the different levels; a,a',a", lehm ; l,l',l", red ooze with splinters of flint.
On lower ground the composition of the alluvions is fairly constant (Fig. 6).
The distribution of the lehm is independent of the altitude, and is found from the lew! of the sea to a height of 1500 metres in Europe, and in China to a height of 3500 metres.
Fig. 6. Section of the Alluvions of the Somme Valley.
I, chalk; 2, sands and gravels ; 3, rich sand; 4, brown ooze with angular splinters of flint; 5, brick-earth.
In Europe it is especially abundant in the valleys of the Rhine, Danube, and their tributaries.
Hainault, Brabant, and Limburg are completely covered with it. Although less thick in the north of France, it is found over a large part of the tablelands between the valley of the Loire and Flanders, where it reaches its maximum of thickness. It is found in Hungary, Moravia, and Roumania; but not in Russia, or on the Baltic or North Sea coasts.
The pampas of la Plata and the basin of the Mississippi are covered with it, and, finally, in China it attains a thickness of 400 to 500 metres.
Wherever it exists, lehm is used in the making of bricks, as also is loess when it is not too calcareous.