appear to have
obtained so completely in any other period. The climate, we may assume
from the distribution of land and water, was generally moist, and it
was probably mild if not warm; conditions favourable to the growth of
certain types of plants. But there is no good evidence for an excess
of carbon dioxide in the atmosphere--an assumption founded on the
luxuriance of the vegetation, coupled with the fact that vulcanicity
was active and wide-ranging. Carbon dioxide may have been present in
the air in greater abundance in earlier periods than it is at present,
but there is no reason to suppose that the percentage was appreciably
higher in the Carboniferous period than it is now.

The occurrence of _red deposits_ in western Australia, Scotland, the
Ural mountains, in Michigan, Montana and Nova Scotia, &c., associated
in some instances with the formation of gypsum and salt, clearly
points to the existence of areas of excessive evaporation, such as are
found in land-locked waters in regions where something like desert
conditions prevail. The xerophytic structures found in some of the
plants might seem to corroborate this view; but similar structures are
assumed by many plants when dwelling in brackish marshes and morasses.

The abundance of corals in some of the Carboniferous seas and possibly
also the large size of some of the Productids and foraminifera may be
taken as evidence of warm or temperate waters.

In spite of the bulk of the evidence being in favour of geniality of
climate, it is necessary to observe that certain deposits have been
recognized as glacial; in the culm of the Frankenwald, in the coal
basins of central France, and in central England, certain
conglomeratic beds have been assigned, somewhat doubtfully, to this
origin. They have also been regarded as the result of torrential
action. Glacial deposits certainly do exist in the Permo-carboniferous
formations, which are described under that head, but in the true
Carboniferous system glaciation may be taken as not proven. The
foreign boulders of granite, gneiss, &c., found in the coal-measures
of some districts, are quite as likely to have been dropped by rafts
of vegetation as to have been carried by floating icebergs.

_Economic Products._--Foremost among the useful products of the
Carboniferous rocks is the coal (_q.v._) itself; but associated with
the coal seams in Great Brit