For we may safely conclude that in the ancient 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 Lanarkshire, the
Lothians, and Fife, for example, are very insignificant in thickness when
compared to those of England. They consist of a few beds intercalated between
the sandstones 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 sedimentary beds containing coal may be older than any of the
coal-measures of central and southern England, as being coeval 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.
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