Like the lower coals of Scotland, the Russian coals are found in the carboniferous limestone. This may also be said of the coal-fields in the governments of Tula and Kaluga, and of those important coal-bearing strata near the river Donetz, stretching to the northern corner of the Sea of Azov. In the last-named, the seams are spread over an area of 11,000 square miles, in which there are forty-four workable seams containing 114 feet of coal. The thickest of known Russian coals occur at Lithwinsk, where three seams are worked, each measuring 30 feet to 40 feet thick. An extension of the Upper Silesian coal-field appears in Russian Poland. This is of upper Carboniferous age, and contains an aggregate of 60 feet of coal. At Ostrau, in Upper Silesia (Austria), there is a remarkable coal-field. Of its 370 seams there are no less than 117 workable ones, and these contain 350 feet of coal. The coals here are very full of gas, which even percolates to the cellars of houses in the town. A bore hole which was sunk in 1852 to a depth of 150 feet, gave off a stream of gas, which ignited, and burnt for many years with a flame some feet long. The Zwickau coal-field in Saxony is one of the most important in Europe. It contains a remarkable seam of coal, known as Russokohle or soot-coal, running at times 25 feet thick. It was separated by Geinitz and others into four zones, according to their vegetable contents, viz.:-- 1. Zone of Ferns. 2. Zone of Annularia and Calamites. 3. Zone of Sigillaria. 4. Zone of Sagenaria (in Silesia), equivalent to the culm-measures of Devonshire. Coals belonging to other than true Carboniferous age are found in Europe at Steyerdorf on the Danube, where there are a few seams of good coal in strata of Liassic age, and in Hungary and Styria, where there are tertiary coals which approach closely to those of true Carboniferous age in composition and quality. In Spain there are a few small scattered basins. Coal is found overlying the carboniferous limestone of the Cantabrian chain, the seams being from 5 feet to 8 feet thick. In the Satero valley, near Sotillo, is a single seam measuring from 60 feet to 100 feet thick. Coal of Neocomian age appears at Montalban. When we look outside the continent of Europe, we may well be astonished at the bountiful manner in which nature has laid out beds of coal upon these ancient surfaces of our globe. Professor Rogers estimated that, in the United States of America, the coal-fields occupy an area of no less than 196,850 square miles.