e chalk--Abrupt
transition from the secondary to the tertiary fossils--Accession of
land, and elevation of mountain chains, after the consolidation of
the secondary rocks--Explanation of Map, showing the area covered by
sea, since the commencement of the tertiary period--Astronomical
theories of the causes of variations in climate--Theory of the
diminution of the supposed primitive heat of the globe.

In the sixth chapter, I stated the arguments derived from organic
remains for concluding that in the period when the carboniferous strata
were deposited, the temperature of the ocean and the air was more
uniform in the different seasons of the year, and in different
latitudes, than at present, and that there was a remarkable absence of
cold as well as great moisture in the atmosphere. It was also shown that
the climate had been modified more than once since that epoch, and that
it had been reduced, by successive changes, more and more nearly to that
now prevailing in the same latitudes. Farther, I endeavored, in the last
chapter, to prove that vicissitudes in climate of no less importance may
be expected to recur in future, if it be admitted that causes now active
in nature have power, in the lapse of ages, to produce considerable
variations in the relative position of land and sea. It remains to
inquire whether the alterations, which the geologist can prove to have
_actually taken place_ at former periods, in the geographical features
of the northern hemisphere, coincide in their nature, and in the time of
their occurrence, with such revolutions in climate as might naturally
have resulted, according to the meteorological principles already
explained.

_Period of the primary fossiliferous rocks._--The oldest system of
strata which afford by their organic remains any evidence as to climate,
or the former position of land and sea, are those formerly known as the
_transition rocks_, or what have since been termed Lower Silurian or
"primary fossiliferous" formations. These have been found in England,
France, Germany, Sweden, Russia, and other parts of central and northern
Europe, as also in the great Lake district of Canada and the United
States. The multilocular or chambered univalves, including the Nautilus,
and the corals, obtained from the limestones of these ancient groups,
have been compared to forms now most largely developed in tropical seas.
The corals, however, have been shown by M. Milne E