In South Wales the coal-bearing strata have been estimated at between 11,000 and 12,000 feet, yet amongst this enormous thickness of strata, the whole of the various coal-seams, if taken together, probably does not amount to more than 120 feet. This great disproportion between the total thickness and the thickness of coal itself shows itself in every coal-field that has been worked, and when a single seam of coal is discovered attaining a thickness of 9 or 10 feet, it is so unusual a thing in Great Britain as to cause it to be known as the "nine" or "ten-foot seam," as the case may be. Although abroad many seams are found which are of greater thicknesses, yet similarly the other portions of the formation are proportionately greater. It is not possible therefore to realise completely the significance of the coal-beds themselves unless there is also a knowledge of the remaining constituents of the whole formation. The strata found in the various coal-fields differ considerably amongst themselves in character. There are, however, certain well-defined characteristics which find representation in most of the principal coal-fields, whether British or European. Professor Hull classifies these carboniferous beds as follows:-- UPPER CARBONIFEROUS. _Upper coal-measures._ Reddish and purple sandstones, red and grey clays and shales, thin bands of coal, ironstone and limestone, with _spirorbis_ and fish. _Middle coal-measures._ Yellow and gray sandstones, blue and black clays and shales, bands of coal and ironstone, fossil plants, bivalves and fish, occasional marine bands. MIDDLE CARBONIFEROUS. _Gannister beds_ or _Lower coal-measures._ _Millstone grit._ Flagstone series in Ireland. _Yoredale beds._ Upper shale series of Ireland. LOWER CARBONIFEROUS. _Mountain limestone_. _Limestone shale_. Each of the three principal divisions has its representative in Scotland, Belgium, and Ireland, but, unfortunately for the last-named country, the whole of the upper coal-measures are there absent. It is from these measures that almost all our commercial coals are obtained. This list of beds might be further curtailed for all practical purposes of the geologist, and the three great divisions of the system would thus stand:-- Upper Carboniferous, or Coal-measures proper. Millstone grit. Lower Carboniferous, or Mountain limestone. In short, the formation consists of masses of sandstone, shale, limestone and coal, these also enclosing clays and ironstones, and, in the limestone, marbles and veins of the ores of lead, zinc, and antimony, and occasionally silver. [Illustration: FIG. 18.--Sigillarian trunks in current-bedded sandstone. St Etienne.]