In the Cretaceous period the waters with-drew 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 granulites, 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 Peters-burg 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.