Изображения страниц
PDF
EPUB
[ocr errors]

as our problem seems to require. What are the objections to it?

Two, principally. First. The rate of increase of temperature, as we go downward, is proportioned to the quantity of heat flowing out and influencing the climate by increase of sensible or thermometric temperature. To augment by 10°, 20°, or 30° the existing temperature of the surface would require an augmentation of heat from the surface downward, in the same proportion as the measure of warmth to be communicated (say 109) to the measure of warmth (say abth of a degree) actually communicated. In the case supposed, 200 times as great as now,

the heat of boiling water would be attained at about 50 feet of depth! How under such circumstances could the Mollusca and other creatures live in the sea, or plants grow on the land?

Secondly. The rate of cooling of the earth by radiation into space is so slow at present, that to reduce the actual effect (say both of a degree) to half that amount would require, according to the calculation of Poisson, the period of one hundred thousand millions of years?. In earlier times, the flow of heat outward being much greater, the rate of cooling would have been much more rapid; still the period of time which must have elapsed in the

1 Hopkins, Address to Geol. Soc. 1851.

reduction of the surface temperature by only 10° is so vast and even inconceivable, that the adoption of the hypothesis to that extent seems to require something more than courage. But even ten degrees of added warmth in lat. 60° would not be sufficient to meet the case of the Corals in the sea, or the Palms, Ferns, or Cycadaceæ on the land.

These objections have been urged as fatal to the opinion that the internal heat of the earth, now hardly sensible among the elements of surface climate, was formerly a real and efficient, if not the principal, cause of the superior mean temperature of the sea and lands in extra-tropical countries. They have even been urged by geologists, who accept the same heat as real and very influential in that general metamorphosis of the lowest rocks, which is observed in every country, and in those disturbances of the strata, which are equally universal, and of various ages. It appears to me, however, that in these ob jections one thing is forgotten—the state of the atmospheric mantle which envelopes the terraqueous globe, mitigates solar heat and stellar radiation, and, like the clothing of a steam cylinder, prevents excessive waste of the warmth treasured within. For nothing can be advanced to justify the supposition that this important element in the economy of nature was always of the same total weight, always identical in chemical composition, and always on the average charged to the same degree with aqueous vapour. On the contrary, to take an example which appears decisive, from the accumulation of the Coal Strata, there is good reason to adopt positively the opinion that the chemical constitution (if that may be termed a chemical constitution which is only a mechanical mixture) of the atmosphere has been greatly altered. For if the Carbon, fixed in the thick and extensive beds of coal since the Palæozoic ages, were again restored to the atmosphere from which it was taken, the weight of Carbonic acid now in the atmosphere (Tooth part) would be more than doubled. Those who think the proportion of the three main constituents of the atmosphere must ever have been as they are now, may if they please double also the Oxygen and Nitrogen, and thus augment the total barometric pressure of the early Palæozoic ages to 60 inches ! But without adopting such an extreme view, there is really no reason to limit our theory of the ancient atmosphere in respect of Oxygen or Nitrogen any more than in regard to Carbonic acid. The whole atmosphere may have weighed more; if so its measured depth must have been greater, and its effect in restraining the waste of heat, and, what is equally important, in reducing the extremes of climatal difference, also greater.

Moreover, a warmer atmosphere, whether of greater total mass or not, would hold more moisture in suspension, and thus the tendency to equalize temperatures might probably be augmented, both by the transport of aqueous vapour to the coolest parts, and by the wider canopy of clouds which are well known to be effective in preventing the wasteful radiation of heat from the surface of the earth. Thus all these inquiries—into the greater diffusion of warmth over the surface by oceanic currents-the greater flow of heat from the interior of the earth, and the greater resistance to the escape and waste of this heat, by the surrounding atmosphere—concur in shewing that causes really founded in nature, and still operating, may be appealed to for solution of the interesting questions regarding ancient climate. Neither perhaps is fully sufficient singly to explain the phenomena, but they are of a nature to be combined without improbability into a general and satisfactory solution of the problem.

PHYSICAL ASPECTS OF THE EARTH.

The rich variety of the earth's surface, as it is now possessed by man, is the legacy of many long ages of busy nature, labouring to upheave the mountains, and depress the seas, and carefully storing up the treasures of those distant years for the enjoyment of the present period. No Coal-fields, to last even a single century, are now growing at the mouths of our rivers ; no metallic veins are spreading through the rocks that we can explore; no great catastrophe breaks down the barriers of seas, or opens picturesque glens through the ridges of the mountains. Yet the forces whose accumulated effects seem to us so mighty are still alive, and still give proof of their power to make further change in the condition of the globe.

If we trace back the physical history of the parts of the earth best known to us, we shall be surprised at the permanence of many of the great features of the land and sea ; where these have changed we can often clearly see the mode and almost the mechanism of change, and not unfrequently discover somewhat of the effect of these variations on the distribution of ancient life, its mutation and discontinuity.

The German Ocean is what remains of a wider Tertiary Sea, which spread over a large part of the country north of the Carpathians, extending eastward across the plains of Southern Russia, and north of the Caucasus, but probably closed to the westward. The Mediterranean of the Tertiary ages stretched northward up the Adriatic gulf, to include the basin of the Po; eastward and southward

« ПредыдущаяПродолжить »