remarkable freedom from disturbances, either in the form of mountain
folding or of igneous intrusions. Over the greater part of the
country the strata are still nearly as flat as when they were first
laid down, and the deposits, even of the Cambrian period, are as
soft as those of the Mesozoic and Tertiary formations in England.
Only in the Urals, the Caucasus, the Timan Mountains, the region
of the Donets coalfield, and the Kielce Hills is there any sign of the
great folding from which nearly the whole of the rest of Europe has
suffered at one time or another.
In the early part of the Palaeozoic era only the gneissic region of Finland and Olonets and probably the Archean mass of S. Russia remained constantly above the sea; but there were several oscillations. Gradually, however, the sea retreated from W. Russia and in the Upper Carboniferous and Permian periods it was confined to the E.
At the beginning of the Mesozoic era the whole country became land, bearing upon its surface the salt lakes in which the Trias was laid down. During the Jurassic period the sea again invaded the region, both from the N. and from the S., but still the W. of Russia rose above the waves. In the Cretaceous period the waters withdrew from the N.E., but in the S. they spread W., covering the whole of Poland and finally uniting with the ocean in which the chalk of W. Europe was deposited. The Tertiary era was marked by a gradual extension S. of the N. land-mass. In the later stages arms of the sea were cut off and were converted at first into lagoons and then into brackish or fresh-water lakes which continued to occupy much of S. Russia until the beginning of the Quaternary period.
During the first part of the Glacial period Russia seems to have been covered by an immense ice-sheet, which extended also over central Germany, and of which the E. limits cannot yet be determined.
The Archean rocks have a broad extension in Finland, N. Russia, the Ural Mountains and the Caucasus. In S. Russia they form the floor upon which lies a thin covering of Tertiary beds, and they are exposed to view in the valleys of the Dnieper and the Bug. They consist for the most part of red and grey gneisses and granulates, with subordinate layers of granite and granitite. The Finland rappa-kivi, the Serdobol gneiss, and the Pargas and Rustiala marble (with the so-called Eozoon canadense) yield good building stone; while iron, copper and zinc-ore are common in Finland and in the Urals. Rocks regarded as representing the Huronian system appear also in Finland, in N.W. Russia, as a narrow strip on the Urals, and in the Dnieper ridge. They consist of a series of unfossiliferous crystalline slates.
The Cambrian is represented by blue clays, ungulite sandstones and bituminous slates in Esthonia and St Petersburg. The Ordovician and Silurian systems are widely developed, and it is most probable that, with the exception of the Archean continents of Finland and the S., the sea covered the whole of Russia. Being concealed, however, by more recent deposits, the deposits appear on the surface only in N.W. Russia (Esthonia, Livonia, St Petersburg and on the Volkhov), where all the subdivisions of the system have been found; in the Timan ridge; on the W. slope of the Urals; in the Pai-kho ridge; and in the islands of the Arctic Ocean. In Poland the rocks of these periods are met with in the Kielce Mountains, and in Podolia in the deeper ravines.
The Devonian dolomites, limestones and red sandstones cover immense tracts and appear on the surface over a much wider area. From Esthonia these rocks extend N.E. to Lake Onega, and S.E. to Mogilev; they form the central plateau, as also the slopes of the Urals and the Petchora region. In N.W. and middle Russia they contain a special fauna, and it appears that the Lower Devonian series of W. Europe, represented in Poland and in the Urals, is missing in N.W. and central Russia, where only the Middle and Upper Devonian divisions are found.
Carboniferous deposits occur over nearly the whole of E. Russia, their W. boundary being a line drawn from Archangel to the upper Dnieper, thence to the upper Don, and S. to the mouth of the last-named river, with a long narrow gulf extending W. to encircle the plateau of the Donets. They are visible, however, only on the W. borders of this region, being covered towards the E. by thick Permian and Triassic strata. Russia has three large coal bearing regions—the Moscow basin, the Donets region and the Urals. In the Valdai plateau there are only a few beds of mediocre coal. In the Moscow basin, which was a broad gulf of the Carboniferous sea, coal appears as isolated inconstant seams amidst littoral deposits, the formation of which was favoured by frequent minor subsidences of the seacoast. The coal is here confined to the lower division of the system; the Upper Carboniferous (corresponding with the English Coal-Measures) is exclusively marine, consisting chiefly of Fusulina limestone. The Donets Coal-Measures, containing abundant remains of a rich land-flora, cover nearly 16,000 sq. m., and comprise a valuable stock of excellent anthracite and coal, together with iron-mines. In this basin, as in W. Europe generally, the principal coal seams occur in the Upper Carboniferous, while the Lower Carboniferous is mainly composed of marine deposits, with, however, the first bed of coal near its summit. Several smaller coalfields on the slopes of the Urals and on the Timan ridge may be added to the above. The Polish coalfields belong to another Carboniferous area of deposit, which extended over Silesia.
The Permian limestones and marls occupy a strip in E. Russia of much less extent than that assigned to them by Murchison. The variegated marls of E. Russia, rich in salt-springs, but very poor in fossils, are now held by most Russian geologists to be Triassic. The Permian deposits contain marine shells and also remains of plants similar to those of England and Germany. But in the government of Vologda, on the rivers Sukhona and N. Dvina, Glossopteris, Noeggerathiopsis and other ferns characteristic of the Indian Gondwana beds have been found; and with these are numerous remains of reptiles similar to those which occur in the Indian deposits. In the Urals the marine facies is more fully developed and the fauna shows affinities with that of the Productus limestone of the Central Asian mountain belt.
During the Jurassic period the sea began again to invade Russia from S.E. and N.W. The limits of the Russian Jurassic system may be represented by a line drawn from the double valley of the Sukhona and Vytchegda to that of the upper Volga, and thence to Kieff, with a wide gulf penetrating towards the N.W. Within this space three depressions, all running S.W. to N.E., are filled up with Upper Jurassic deposits. They are much denuded in the higher parts of this region, and appear but as isolated islands in central Russia. In the S.E. all the older subdivisions are represented, the deposits having the characters of a deep-sea formation in the Aral-Caspian region and on the Caucasus.
Cretaceous beds—sands, loose sandstones, marls and white chalk—occupy nearly the whole of the region S. of a line drawn from the Niemen to the upper Oka and Don, and thence N.E. to Simbirsk. Over a large part of this area, however, they are concealed by the later Tertiary deposits, and they are absent over the Dnieper and Don ridge in the Yaila Mountains and in the higher parts of the Caucasus. They are rich in grinding stone, and in phosphatic deposits.
The Tertiary formations occupy large areas in S. Russia. The Eocene covers wide tracts from Lithuania to Tsaritsyn, and is represented in the Crimea and Caucasus by thick deposits belonging to the same ocean which left its deposits on the Alps and the Himalayas. Oligocene, quite similar to that of N. Germany, and contain in brown coal and amber, has been met with only in Poland, Courland and Lithuania. The Miocene (Sarmatian stage) occupies extensive tracts in S. Russia, S. of a line drawn through Lublin to Ekaterinoslav and Saratov. Not only the higher chains of Caucasus and Yaila, but also the Donets ridge, rose above the
