Изображения страниц

since the epoch when the Falls began their backward progress from Lake Ontario.

This epoch, however, is not of necessity the same as that of the origin of the river action, which may have gone on for some unknown time previously. It does not then give us the desired information of the length of the Postglacial period, or the date, as De Luc might have expressed it, of the birth of our continents. But it seems to point in the same direction as all the other natural chronometers, and to compress within a few thousand years the later part of the Pleistocene Period, when the main features of the Land, the Rivers and Lakes, and Plains and Mountains, had been finally redeemed from the power of the sea, and peopled by the now existing races of plants and animals.


Few inferences have obtained a more general assent among geologists than that which affirms the change of climate during the progress of life on the globe. The evidence on which reliance has been placed has been sometimes adopted on light grounds, sometimes rejected for fresh and better testimony; but the conviction of almost every writer has been deliberately recorded in favour of the prevalence of much higher temperatures during early geological

periods in the northern zones of the earth, varied by at least one great interval of remarkable cold in later times. Before proceeding to consider how such variations of climate might be possible, it will be useful to collect some points of the evidence which may be regarded as establishing the fact of their having really taken place.

There is only one kind of evidence-that to be obtained from organic remains; and as in existing nature some groups are more definitely related to and indicative of climate than others, so in the fossil world. In some modern genera and families the species are distributed over different latitudes; some being intra-tropical, others extra-tropical, some in the temperate, others in the arctic zones.

Such genera and families can only be employed in arguments on ancient climate, where they contain species which are both recent and fossil, and this occurs only in the Cænozoic Strata. But there are other fossil genera, families, and even larger natural groups, which on proper questioning yield satisfactory answers in regard to the main characters of the climate to which they were appointed, whether on land or in the sea.

To take our first examples from the vegetable kingdom, we may inquire what climate is indicated by the characteristic forms of land-plants buried in the Coal-formation. Ferns are usually in fragments, distributed by water on the successive surfaces of deposition. It can seldom be ascertained whether they belonged to arborescent or repent kinds, but two or three species of the former division under the title of Caulopteris are recognized in the Coalmeasures. The great abundance of Ferns is a further and good argument for great warmth and dampness. If Lepidodendra belong to the natural order of Lycopodiaceæ, their extraordinary size may be held to demand the extreme of the conditions favourable to that race, heat and moisture; if they include strong analogies to Araucariæ, that is an indication in the same direction. Sigillariæ, now commonly placed among the Gymnospermous Phanerogamia, near Cycadaceæ, have also been thought allied to Cacteaceæ, and to Tree-ferns, and thus follow on the same side as their companions the Lepidodendra.

Calamites, no longer referred to Equisetaceæ, but with Asterophyllites classed among Coniferæ, give no independent testimony to climate; but a few Palms (Flabellaria, Palmacites, Trigonocarpum), and Musaceæ (Musocarpum), concur with the Tree-ferns in requiring for the low shores of early time, where now extend the coal-deposits of America and Europe, a mean temperature of 64', which is 16° above that now experienced in the centre of the Coal-basin of Scotland, 20° higher than that of the Coal-field of Michigan, and not less than 30° above that of the northern part of Newfoundland, to which the American Coal-basin extends.

Again, in the Oolitic period, we find Ferns still the prevalent vegetation, with gigantic Equiseta, and stems and fronds of Zamioid and Cycadeoid plants and Pandanaceæ, all indications of an equally warm climate, prevalent as far north as Yorkshire and Bornholm. Thus a difference of 16° or more appears in favour of the ancient temperature.

In Cænozoic Strata, the natural orders of Cucurbitaceæ, Anonaceæ, and Nipadaceæ in the London basin, and Palmaceæ in that of Paris, carry on the inference to times nearer our own; but there is reason to suspect the influence of drift—like that of the modern gulf-stream-in transporting the numerous fruits now found in the clay of Sheppey?

Coral growing into masses comparable to modern reefs affords a valuable illustration of marine climate at several geological epochs ; for reefs having this origin, whether rising perpendicularly in the waters, or accumulated under the influence of sea-currents, are confined in modern nature to a limited breadth on either side of the equator. The Corals which fall

1 Bowerbank, Fossil Fruits and Seeds of Sheppey. R, L.


under this designation in the West Indian Archipelago are for the most part not of the same species as those which occur in the Indian and Pacific Oceans, and thus the argument acquires a generality and independence of specific forms and peculiarities; which suits it for application to the extinct races, and the earlier reefs constructed often by different genera, in the Silurian, Carboniferous, and Oolitic periods.

Keeping in mind that light, warmth, and proximity to the surface of pure sea-water, are essentials for the life of the reef-making animals, we shall understand the complete segregation of these remains from the great mass of argillaceous and arenaceous sediments in the several formations. The limestones of Wenlock and Aymestry, of Plymouth, of Mendip, Flintshire, Derbyshire, and Yorkshire, all of Palæozoic ages and full of Coral-beds and bands of corals in place and attitude of growth, appear to be little else than the accumulations of Polypean and Crinoidal reliquiæ, augmented by the shells and other exuviæ of the sea-animals naturally attracted to the growing calcareous accretion. Each of these limestones indicates an interval of rest in the accumulation of sediments, a pause of the depression of the sea-beda.


1 Memoirs of Malvern in Memoirs of Geological Survey, . 1.

« ПредыдущаяПродолжить »