| Upper Carboniferous |  | Coal Measures Millstone Grit | | Ardwick Stage Pennant Stage Gannister Stage |
| Lower Carboniferous | | Carboniferous (Mountain) Limestone Series. | ||
The Lower Carboniferous beds have been further subdivided into:—
Yoredale Series or Upper Limestone Shales,
Mountain Limestone,
Lower Limestone Shales, with Sandstones and Conglomerates,
but as these lithological types are found to be very variable when traced laterally for comparatively short distances, it is found more satisfactory to use the terms in a purely lithological sense rather than with chronological significance.
The somewhat abnormal development of the higher portions of the Carboniferous rocks of Britain renders the local classification only partially applicable in other regions, and as our knowledge progresses, a palæontological classification will probably be adopted. This has already been done with the more purely open-water sediments of Russia and Eastern Asia, where the development of the beds is more normal. There the rocks are classified as under:—
Upper Carboniferous or Gshellian,
Middle Carboniferous or Moscovian,
Lower Carboniferous,
and as this classification has already been found to be applicable over rather wide areas, it is almost certain that, as in the case of the rocks of other systems, it will prove more serviceable than one which is mainly (though not quite exclusively) based upon vertical variation of lithological characters, especially as the Carboniferous rocks over large tracts in North America possess faunas which are similar to those which have been discovered in Russia, Eastern Asia and North Africa.
Description of the strata. The variations in the lithological characters and fossil contents of the British Carboniferous strata when traced from north to south have been so frequently described, and utilised as a means of illustrating the indications as to local variations in physical conditions which are supplied by those strata, that little need be said upon the subject. The restoration of the physical geography of Carboniferous times over the British area will be found in a chapter by the late Professor Green in the work upon Coal by various professors at the Yorkshire College of Science and also in Prof. Hull's Physical History of the British Isles. Some modifications must be made in these restorations as the result of recent research, the principal being caused by discoveries amongst the Carboniferous rocks of Devonshire.
Taking the strata in vertical succession, we find evidence of the occurrence of a complete marine period (the second great marine period) between the second and third continental periods. The first shallow-water phase over a great portion of the British Isles is marked by thin terrigenous sediments, indicating that the period was a brief one; it was followed by the deep-water phase, probably of some length, lasting through the greater part of the remainder of Lower Carboniferous times; while the concluding shallow-water phase was lengthy as compared with that of the beginning of the period, and is marked by the accumulation of the great thickness of deposits belonging to the Millstone Grit and Coal Measures. There is no doubt, however, that in some parts of the British area minor changes produced local terrestrial conditions during the period, and accordingly we find that the deepest water deposits of the system in Britain are succeeded by an unconformable junction with the sediments of the upper portion of the system.