896 
ELEMENTS OF GEOLOGY. 
In central England, where the sedimentary beds are re¬ 
duced to about 3000 feet in all, the Carboniferous Limestone* 
attains an enormous thickness, as much as 4000 feet at Ash¬ 
bourne, near Derby, according to Mr. Hull’s estimate. To a 
certain extent, therefore, we may consider the calcareous 
member of the formation as having originated simultaneous¬ 
ly with the accumulation of the materials of grit, sandstone, 
and shale, with seams of coal; just as strata of mud, sand, 
and pebbles, several thousand feet thick, with layers of veg¬ 
etable matter, are now in the process of formation in the cy¬ 
press swamps and delta of the Mississippi, while coral reefs 
are forming on the coast of Florida and in the sea of the Ber¬ 
muda islands. For we may safely conclude that in the an¬ 
cient Carboniferous ocean those marine animals which were 
limestone builders were never freely developed in areas where 
the rivers poured in fresh water charged with sand or clay; 
and the limestone could only become several thousand feet 
thick in parts of the ocean which remained perfectly clear for 
ages. 
The calcareous strata of the Scotch coal-fields, those of Lan¬ 
arkshire, the Lothians, and Fife, for example, are very insig¬ 
nificant in thickness when compared to those of England. 
They consist of a few beds intercalated between the sand¬ 
stones and shales containing coal and ironstone, the combined 
thickness of all the limestones amounting to no more than 
150 feet. The vegetation of some of these northern sedi¬ 
mentary beds containing coal may be older than any of the 
coal-measures of central and southern England, as being co¬ 
eval with the Mountain Limestone of the south. In "Ireland 
the limestone predominates over the coal-bearing sands and 
shales. We may infer the former continuity of several of the 
coal-fields in northern and central England, not only from the 
abrupt manner in which they are cut off at their outcrop, 
but from their remarkable correspondence in the succession 
and character of particular beds. But the limited extent to 
which these strata are exposed at the surface is not merely 
owing to their former denudation, but even in a still greater 
degree to their having been largely covered by the New Red 
Sandstone, as in Cheshire, and here and there by the Permian 
strata, as in Durham. 
It has long been the opinion of the most eminent geologists 
that the coal-fields of Yorkshire and Lancashire were once 
united, the upper Coal-measures and the overlying Millstone 
Grit and Yoredale rocks having been subsequently removed; 
but what is remarkable, is the ancient date now assigned to 
this denudation, for it seems that a thickness of no less than 
