THE CARBONIFEROUS PERIOD. 163 



tive coal-fields of the world being in Great Britain, North 

 America, and Belgium. Wherever they are found, with 

 limited exceptions, the Coal-measures present a singular 

 general uniformity of mineral composition. They consist, 

 namely, of an indefinite alternation of beds of sandstone, 

 shale, and coal, sometimes with bands of clay-ironstone or beds 

 of limestone, repeated in no constant order, but sometimes 

 attaining the enormous aggregate thickness of 14,000 feet, or 

 little short of 3 miles. The beds of coal differ in number and 

 thickness in different areas, but they seldom or never exceed 

 one-fiftieth part of the total bulk of the formation in thickness. 

 The characters of the coal itself, and the way in which the 

 coal-beds were deposited, will be briefly alluded to in speaking 

 of the vegetable life of the period. In Britain, and in the Old 

 World generally, the Coal-measures are composed partly of 

 genuine terrestrial deposits such as the Coal and partly of 

 sediments accumulated in the fresh or brackish waters of vast 

 lagoons, estuaries, and marshes. The fossils of the Coal- 

 measures in these regions are therefore necessarily the remains 

 either of terrestrial plants and animals, or of such forms of 

 life as inhabit fresh or brackish waters, the occurrence of strata 

 with marine fossils being quite a local and occasional phe- 

 nomenon. In various parts of North America, on the other 

 hand, the Coal-measures, in addition to sandstones, shales, 

 coal-seams, and bands of clay-ironstone, commonly include 

 beds of limestone, charged with marine remains, and indicating 

 marine conditions. The subjoined section (fig. 107) gives, in 

 a generalized form, the succession of the Carboniferous strata 

 in such a British area as the north of England, where the series 

 is developed in a typical form. 



As regards the life of the Carboniferous period, we naturally 

 find, as has been previously noticed, great differences in dif- 

 ferent parts of the entire series, corresponding to the different 

 mode of origin of the beds. Speaking generally, the Lower 

 Carboniferous (or the Sub-Carboniferous) is characterized by 

 the remains of marine animals; whilst the Upper Carbon- 

 iferous (or Coal-measures) is characterized by the remains 

 of plants and terrestrial animals. In all those cases, how- 

 ever, in which marine beds are found in the series of the 

 Coal-measures, as is common in America, then we find that the 

 fossils agree in their general characters with those of the older 

 marine deposits of the period. 



Owing to the fact that coal is simply compressed and other- 



