perhaps are those that flourish in New Zealand. In New
Zealand, too, are found many species of ferns, both those which are
arborescent and those which are of more humble stature. Add to these the
numerous conifers which are there found, and we shall find that a forest
in that country may represent to a certain extent the appearance
presented by a forest of carboniferous vegetation. The ferns, lycopods,
and pines, however, which appear there, it is but fair to add, are mixed
with other types allied to more recent forms of vegetation.

There are many reasons for believing that the amount of carbonic acid gas
then existing in the atmosphere was larger than the quantity which we now
find, and Professor Tyndall has shown that the effect of this would be to
prevent radiation of heat from the earth. The resulting forms of
vegetation would be such as would be comparable with those which are now
reared in the green-house or conservatory in these latitudes. The gas
would, in fact, act as a glass roof, extending over the whole world.

CHAPTER II.

A GENERAL VIEW OF THE COAL-BEARING STRATA.

In considering the source whence coal is derived, we must be careful to
remember that coal itself is but a minor portion of the whole formation
in which it occurs. The presence of coal has indeed given the name to the
formation, the word "carboniferous" meaning "coal-bearing," but in taking
a comprehensive view of the position which it occupies in the bowels of
the earth, it will be necessary to take into consideration the strata in
which it is found, and the conditions, so far as are known, under which
these were deposited.

Geologically speaking, the Carboniferous formation occurs near the close
of that group of systems which have been classed as "palaeozoic," younger
in point of age than the well known Devonian and Old Red Sandstone
strata, but older by far than the Oolites, the Wealden, or the Cretaceous
strata.

In South Wales the coal-bearing strata have been estimated at between
11,000 and 12,000 feet, yet amongst this enormous thickness of strata,
the whole of the various coal-seams, if taken together, probably does not
amount to more than 120 feet. This great disproportion between the total
thickness and the thickness of coal itself shows itself in every
coal-field that has been worked, and when a single seam of coal is
discovered attaining a thickness of 9 or 10 feet, it is so unusual a
thing in Great Britain as to caus