160 HISTORICAL PALASONTOLOGY. 
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 generalised 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 /fe 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 characterised by 
the remains of marine animals; whilst the Upper Carbon- 
iferous (or Coal-measures) is characterised 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 1s 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- 
wise altered vegetable matter, and that it is of the highest 
economic value to man, the Coal-measures have been more 
thoroughly explored than any other group of strata of equiva- 
lent thickness in the entire geological series. Hence we have 
already a very extensive acquaintance with the Alans of the 
Carboniferous period ; and our knowledge on this subject is 
