I.]

Lyell's Theory considered.

29

in the distribution of sea and land might have brought about the excessive cold which appears to have prevailed just before man came into existence.

Sir Charles Lyell has suggested, that the most favourable condition for diffusing throughout the globe a genial temperature, would be that, in which all the principal tracts of land were collected within the tropics, and all the water in more northern and southern latitudes, as the soil heated by the solar rays would communicate its warmth to the surrounding waters, and thus produce a northerly and southerly current, which would modify very materially the climate of the Arctic regions, whilst the latter, consisting of water, would, for the reasons above stated, undergo a less degree of refrigeration than at present.

This theory is developed by the author with his usual ability, but it may be questioned, whether it be not beset with unnecessary difficulties, owing to his adherence to his favourite principle of uniformitarianism, or, in other words, his reluctance to admit, that any progression can be traced in the order of physical events from the earliest to the most modern condition of our planet.

And yet the doctrine of Darwin, which he now espouses, would seem more in harmony with such a notion, if even it does not imply it, than that of the reversion of the earth's surface to its status quo, after the fulfilment of a certain cycle of revolutions; for if the globe had been equally fitted for the abode of man and the higher Mammalia from the earliest period known to us, as it is at the present time, one does not understand why such classes of animals should have been absent, and if they did exist, what then becomes of the theory which supposes the gradual evolution of more complex from more simple organisms, of man from the monad?

Sir Charles Lyell has been accused, most unjustly, of maintaining that the world never had a beginning, but this erroneous impression of his meaning has arisen, from his contending, that geology has never yet mounted far enough into the records of creation to be able to find traces of its dawn, or, in other words, that from the earliest to the latest of the deposits which compose the crust we recognise

30

Objections to Lyell's Theory.

[LECT.

only a series of alternating movements, of elevations of land in one region, and of depressions in another; so that in the course of an indefinite number of ages each portion of the globe will have been subjected at one time or another to the same internal commotions, occasioning the same amount of external change.

Now if this view be correct, one does not see, why periods of intense cold should not have been intercalated at many different epochs between those of high temperature, which the organic remains preserved in the rocks appear to indicate, and why evidences of glacial action anterior to the post-pliocene epoch should not have been traced repeatedly.

One such event, indeed, has been pointed out to us by Professor Ramsey, who contends, that towards the close of the palæozoic period a glacial sea was spread around several of the islands which occupied the place of what now constitutes our British and Scottish mountains, and that the remarkable conglomerate rock found in the Malvern and Abberley Hills, now pronounced to be of permean origin, is due to the floating of icebergs along the sea which washed the Langwyd, Abberley, and Malvern Hills.

Granting, however, the correctness of this deduction, it still remains to be explained why, if the sources of heat in former periods did not possess more intensity than at present, the local circumstances should have been, in so much the greater number of instances, more favourable to their operation than is the case under existing conditions.

It seems indeed contrary to all probability, that the very arrangement of sea and land, most favourable to the production of warmth, should have existed almost universally, till the commencement of the glacial epoch, which represents nearly the most modern date in our geological calendar.

But what would be the temperature of the entire globe, if it had been uniformly covered with sea, so that the flow of ́its warm currents proceeded continually from the equator towards the poles without let or hindrance from the in-. terposition of continents, and that at a time when, owing to the absence of land, no such radiation of heat into space occurred, as takes place from continents at present?

I.]

Former Preponderance of Water over Land.

31

Without pretending to calculate what the precise temperature in each parallel would be under such circumstances, as we have probably not sufficient data for determining this with exactness, it may at least be affirmed, that the climate everywhere would be much more uniform, and that even in the polar regions it would nowhere at the surface sink to the freezing point of water; nor would the case be materially altered, even when large islands started up in the midst of the abyss, provided the latter were not numerous enough to oppose a barrier to the circulation of the aqueous currents, and also presented no lofty chains of mountains to serve as centres of refrigerating influence.

It may be remarked, that the plants that have principally left their traces in the coal formation, namely the Ferns, Lycopodiacea, and the like, are such as abound principally on islands, and delight in an equable and humid, rather than in a scorching and sultry atmosphere. Even the Tree-ferns of the tropics thrive principally in the midst of deep forests, protected at once from drought and from excessive heat.

If therefore we suppose the globe to have been at that time covered with water, with only some islands occasionally protruding above it, it is conceivable, that the temperature even in these latitudes might have been exalted enough for the plants that have left their vestiges in the coal.

Now this gradual emergence of the land above the waters is rendered probable, not only from considering the nature of the plants and animals which came successively into being, but also from the character of the rocks themselves which predominated at each successive period.

With regard to the former, it may be remarked, that at the earliest epoch of which we find any traces, namely the palæozoic, fish and marine productions alone existed; at one somewhat later, during the carboniferous, reptiles were the only animals that peopled the land; whilst mammals of a low order first appeared in the oolite, and went on gradually increasing in number and in variety in proportion as the most recent period was approached.

And with reference to the rocks themselves, it seems unquestionable, that all those of igneous formation which have

32

The Volcanos chiefly submarine.

[LECT. I. been observed intruding themselves into the strata, appear to be submarine, for although it may be said, that all traces, of craters, of cones of scoriæ, and even of streams of lava, might have been obliterated by the series of catastrophes that had since occurred, yet vestiges of the description of rocks which had constituted them would nevertheless present themselves, if volcanos had at that period broken out frequently upon dry land.

Their débris, even if washed into the contiguous seas, and covered over with neptunian deposits, would have retained their vitreous and porous character--they would have partaken often of the structure belonging to obsidian and pumice, which indicates sudden cooling; whereas the igneous rocks we observe in the older formations, if cellular, have their interstices filled by crystaline matter, and possess when compact that lithoid aspect which is so generally absent from lavas erupted in the open air*.

Thus, whether we consider the nature of the rocks themselves, or that of the organic beings that existed upon their face, we are alike led to the conclusion, that the globe was at first covered with water,-that, next, only a few low islands gradually emerged from the abyss, suitable for the abode of reptiles and other of the lower animals,—that these gradually increased in number, and became more elevated in positionbut that until a later period no such extensive continents or such lofty chains of mountains existed, as would be sufficient to bring about that degree of cold which belongs to the higher latitudes of the globe at present.

This low temperature, since the introduction of man upon our planet, has been mitigated by causes already pointed out in this lecture, but it is quite conceivable that, supposing a different disposition of land and sea to have existed antecedently, a climate might have prevailed in these regions as severe as that of Siberia, and therefore corresponding to what we find traces of during the so-called Glacial Period.

k See my "Description of Active and Extinct Volcanos," 2nd Edit., 1848; especially chap. xl. p. 669, &c.

LECTURE II.

Local influences affecting temperature - the Gulf-stream increasing the warmth of the West of Europe-the Antarctic current increasing the cold of the Southern Hemisphere. Cause of the greater coldness of mountainous regions. Line of perpetual congelation. Temperature of the soil differs from that of the atmosphere-its influence on vegetation. Geothermal culture-instance of its advantages. Influence of solar light as distinct from solar heat. Instruments for measuring it. Humidity of a climate. Instruments for estimating it. Productive of three classes of phenomena: first, Fogs, mists, and clouds-nature of vesicular vapour; second, Rain and Snow-causes producing them-why rain does not fall in certain countries: third, Dew-theory of its formation. Winds. General laws affecting the aerial currents-arising from the difference in the temperature of different zones-coupled with the effect of the earth's rotation. Hence the Trade Winds. Why calms exist near the equator. Monsoons, typhoons, and hurricanes-their vorticose and progressive movement. Winds in temperate regions more variable and less violent, but apparently subject to the same laws-Admiral Fitzroy's method of forecasting their arrival. Influence of winds on vegetables. The Simoom. Pressure of the atmosphere varies in proportion to the elevation-hence the latter may be estimated by the barometer. The pocket aneroid, the most convenient and portable instrument for estimating heights in travelling. Pressure of the atmosphere dependent on height. Indications afforded by the barometer. Electrical state of the atmosphere during rains-hail dependent on the same cause-its effects on plants and animals. Ozone-how its presence is detected-its probable nature-its existence in certain states of the atmosphere-and the inferences to be deduced from its presence.

IN my former Lecture I considered the general causes which influence climate, namely, the proximity to the sea, the existence of high mountains, the radiation from a large surface of sandy desert, and the like.

I shall now point out to you certain causes of a more local kind which affect the temperature of particular parts of the globe, and shall begin with one which seems to be chiefly instrumental in bringing about that mild and equable character of the seasons for which this spot, in common with most parts of the western coasts of Great Britain and Ireland, is celebrated.

D