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.]
As the most apparent of the rocks of the system are sandstone, shale, limestone, and coal, it will be necessary to consider how these were deposited in the waters of the carboniferous ages, and this we can best do by considering the laws under which strata of a similar nature are now being deposited as sedimentary beds.