|
Levelling Process
This summary account of the levelling process which went on
during the Triassic and Jurassic will prepare us to expect a
return of warm climate and luxurious life, and this the record
abundantly evinces. The enormous expansion of the sea--a great
authority, Neumayr, believes that it was the greatest extension
of the sea that is known in geology--and lowering of the land
would of itself tend to produce this condition, and it may be
that the very considerable volcanic activity, of which we find
evidence in the Permian and Triassic, had discharged great
volumes of carbon-dioxide into the atmosphere.
Whatever the causes were, the earth has returned to paradisiacal
conditions. The vast ice-fields have gone, the scanty and scrubby
vegetation is replaced by luscious forests of cycads, conifers,
and ferns, and warmth-loving animals penetrate to what are now
the Arctic and Antarctic regions. Greenland and Spitzbergen are
fragments of a continent that then bore a luxuriant growth of
ferns and cycads, and housed large reptiles that could not now
live thousands of miles south of it. England, and a large part of
Europe, was a tranquil blue coral-ocean, the fringes of its
islands girt with reefs such as we find now only three thousand
miles further south, with vast shoals of Ammonites, sometimes of
gigantic size, preying upon its living population or evading its
monstrous sharks; while the sunlit lands were covered with
graceful, palmlike cycads and early yews and pines and cypresses,
and quaint forms of reptiles throve on the warm earth or in the
ample swamps, or rushed on outstretched wings through the purer
air.
It was an evergreen world, a world, apparently, of perpetual
summer. No trace is found until the next period of an alternation
of summer and winter--no trees that shed their leaves annually,
or show annual rings of growth in the wood--and there is little
trace of zones of climate as yet. It is true that the sensitive
Ammonites differ in the northern and the southern latitudes, but,
as Professor Chamberlin says, it is not clear that the difference
points to a diversity of climate. We may conclude that the
absence of corals higher than the north of England implies a more
temperate climate further north, but what Sir A. Geikie calls
(with slight exaggeration) "the almost tropical aspect" of
Greenland warns us to be cautious. The climate of the
mid-Jurassic was very much warmer and more uniform than the
climate of the earth to-day. It was an age of great vital
expansion. And into this luxuriant world we shall presently find
a fresh period of elevation, disturbance, and cold breaking with
momentous evolutionary results. Meantime, we may take a closer
look at these interesting inhabitants of the Middle Ages of the
earth, before they pass away or are driven, in shrunken
regiments, into the shelter of the narrowing tropics.
The principal change in the aspect of the earth, as the cold,
arid plains and slopes of the Triassic slowly yield the moist and
warm ow-lying lands of the Jurassic, to consists in the character
of the vegetation. It is wholly intermediate in its forms between
that of the primitive forests and that of the modern world. The
great Cryptogams of the Carboniferous world--the giant
Club-mosses and their kindred--have been slain by the long period
of cold and drought. Smaller Horsetails (sometimes of a great
size, but generally of the modern type) and Club-mosses remain,
but are not a conspicuous feature in the landscape. On the other
hand, there is as yet-- apart from the Conifers--no trace of the
familiar trees and flowers and grasses of the later world. The
vast majority of the plants are of the cycad type. These-- now
confined to tropical and subtropical regions--with the surviving
ferns, the new Conifers, and certain trees of the ginkgo type,
form the characteristic Mesozoic vegetation.
|
