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rienced in part of Sweden; and as the shores of the Gulph of Bothnia are extended not only by the influx of sediment brought down by rivers, but also by the elevation and consequent drying up of the bed of the sea, so a similiar combination of causes may have extended the low tract of land where marine shells and fossil bones now occur in Siberia. It has been suggested, that as, in our own times, the northern animals migrate, so the Siberian elephant and rhinoceros may have wandered towards the north in summer. The musk oxen annually desert their winter-quarters in the south, and cross the sea upon the ice, to graze for four months, from May to September, on the rich pasturage of Melville Island, in lat 75°. The mammoths, without passing so far beyond the arctic circle, may nevertheless have made excursions, during the heat of a brief northern summer, from the central or temperate parts of Asia to the sixtieth parallel of latitude; in which case the carcasses of such as were drowned, or overwhelmed by drift snow, may have been hurried down into the polar sea, and imbedded in the deposits there accumulating. I have been informed by Dr. Richardson, that in the northern parts of America, comprising regions now inhabited by many herbivorous quadrupeds, the drist snow is often converted into permanent glaciers. It is commonly blown over the edges of steep cliffs, so as to form an inclined talus hundreds of feet high; and when a thaw commences, torrents rush from the land, and throw down from the top of the cliff alluvial soil and gravel. This new soil soon becomes covered with vegetation, and protects the soundation of snow from the rays of the sun. Water occasionally penetrates into the crevices and pores of the snow; but, as it soon freezes again, it serves the more rapidly to consolidate the mass into a compact iceberg. It may sometimes happen that cattle grazing in a valley at the base of such cliffs, on the borders of a sea or river, may be overwhelmed, and at length inclosed in solid ice, and then transported towards the polar regions. The result of these investigations, therefore, may lead us to conclude that the mammoth, and some other extinct quadrupeds fitted to live in high latitudes, were inhabitants of northern Asia at a time when the climate was milder, and more uniform, than at present. Their extermination was probably connected with changes in the physical geography of the arctic regions, of which I shall consider the effects in the next chapter. Change of climate proved by fossils in older strata.-If we pass from the consideration of these more modern deposits, whether of marine or continental origin, in which existing species are abundantly intermixed with the extinct, to the older tertiary strata, we can only reason from analogy; since none of the species of vertebrated animals, and scarcely any of the testacea of those formations, are identifiable with species now in being. In the deposits of that more remote period, we find the
remains of many animals analogous to those of hot climates, such as the crocodile, turtle, and tortoise, together with many large shells of the genus nautilus, and plants indicating such a temperature as is now found along the southern borders of the Mediterranean. A great interval of time appears to have elapsed between the formation of the secondary strata, which constitute the principal portion of the elevated land in Europe, and the origin of the last-mentioned Eocene deposits. In that great series of secondary rocks, many distinct assemblages of organized fossils are entombed, all of unknown species, and many of them referable to genera and families now most abundant between the tropics. Among the most remarkable are many gigantic reptiles, some of them herbivorous, others carnivorous, and far exceeding in size any now known even in the torrid zone. The genera are for the most part extinct, but some of them, as the crocodile and monitor, have still representatives in the warmer parts of the earth. Coral reefs also were evidently numerous in the seas of the same periods, and composed of species belonging to genera now characteristic of a tropical climate. The number of very large chambered shells also leads us to inser an elevated temperature; and the associated fossil plants, although imperfectly known, tend to the same conclusion, the Cycadeae constituting the most numerous family. But it is from the more ancient coal deposits that the most extraordinary evidence has been supplied in proof of the former existence of an extremely hot climate in those latitudes which are now the temperate and colder regions of the globe. It appears from the fossils of the carboniserous period, that the flora consisted almost exclusively of large vascular cryptogamic plants. We learn, from the labours of M. Ad. Brongniart, that there existed at that epoch Equiseta upwards of ten feet high, and from five to six inches in diameter; tree ferns, or plants allied to them, from forty to fifty feet in height; and arborescent Lycopodiaceae, from sixty to seventy feet high." Of the above classes of vegetables, the species are all small at present in cold climates; while in tropical regions there occur, together with small species, many of a much greater size ; but their development, even in the hottest parts of the globe, is now inferior to that indicated by the petrified forms of the coal formation. An elevated and uniform temperature, and great humidity in the air, are the causes most favourable to the numerical predominance and the great size of these plants within the torrid zone at present. It is true that, as the fossil flora consists of such plants as may accidentally have been floated into seas, lakes, or estuaries, it may very commonly give a false representation of the numerical relations of families then living on the
* Consid. Générales sur la Nature de la Végétation, &c. Ann. des Sci. Nat., Nov. 1828.
land. Yet, after allowing for liability to error on these grounds, the argument founded on the comparative numbers of the fossil plants of the carboniserous strata is very strong.
“In regard to the geographical extent of the ancient vegetation, it was not confined," says M. Brongniart, " to a small space, as to Europe, for example ; for the same forms are met with again at great distances. Thus, the coal plants of North America are, for the most part, identical with those of Europe, and all belong to the same genera. Some specimens, also, from Greenland, are referable to ferns, analogous to those of our European coal mines."*
The fossil plants brought from Melville Island, although in a very imperfect state, have been supposed to warrant similar conclusions ;t and assuming that they agree with those of Baffin's Bay, mentioned by M. Brongniart, how shall we explain the manner in which such a vegetation lived through an arctic night of several months' duration ?’
It may seem premature to discuss this question, until the true nature of the fossil flora of the arctic regions has been more accurately determined; yet, as the question has attracted some attention, let us assume for a moment, that the coal plants of Melville Island are strictly analogous to those of the strata of Northumberland—would such a fact present an inexplicable enigma to the vegetable physiologist ?
Plants, it is affirnied, cannot remain in darkness, even for a week, without serious injury, unless in a torpid state ; and if exposed to heat and moisture they cannot remain torpid, but will grow, and must therefore perish. If, then, in the latitude of Melville Island, 75° N., a high temperature, and consequent humidity, prevailed at that period when we know the arctic seas were filled with corals and large multilocular shells, how could plants of tropical forms have flourished ? Is not the bright light of equatorial regions as indispensable a condition of their well-being as the sultry heat of the same countries and how could they annually endure a night prolonged for three months ?
Now, in reply to this objection, we must bear in mind, in the first place, that, so far as experiments have been made, there is
every reason to conclude, that the range of intensity of light to which living plants can accommodate themselves is far wider than that of heat. No palmıs or tree ferns can live in our temperate latitudes without protection from the
* Prodrome d'une Hist. des Végét. Foss. p.
| König, Journ. of Sci. vol. xv. p. 20. Mr. König informs me, that he no longer believes
any of these fossils to be tree ferns, as he at first stated, but that they agree with tropical forms of plants in our English coal-beds. The Melville Island specimens, now in the British Museum, are very obscure impressions.
# Fossil Flora of Great Britain, by John Lindley and William Hutton, Esqs. No. IV.
Ş Fossil Flora, No. IV.
cold; but when placed in hot-houses they grow luxuriantly, even under
* Fossil Flora, No. X.
same land, the destruction of which provided materials for the sandstones and conglomerates of the group of strata in which they are imbedded ; especially as the coarseness of the particles of many of these rocks attests that they were not borne from very remote localities.
Before we are entitled to enlarge farther on this question of transportation, we must obtain more precise information respecting the state of the various fossils which have been found principally in the coal sandstones of high latitudes, and we must learn whether they bear the marks of friction and decay previous to their fossilization.
To return, therefore, from this digression, the uninjured corals and chambered univalves of Igloolik (lat. 69° N.), Melville Island, and other high latitudes, sufficiently prove that, during the carboniserous period, there was an elevated temperature even in northern regions bordering on the arctic circle. The heat and humidity of the air, and the uniformity of climate, appear to have been most remarkable when the oldest strata hitherto discovered were formed. The approximation to a climate similar to that now enjoyed in these latitudes does not commence till the era of the formations termed tertiary; and while the different tertiary rocks were deposited in succession, the temperature seems to have been still further lowered, and to have continued to diminish gradually, even after the appearance upon the earth of a great portion of the existing species,
FARTHER EXAMINATION OF THE QUESTION AS TO THE DISCORDANCE OF THE
ANCIENT AND MODERN CAUSES OF CHANGE.
On the causes of vicissitudes in climate-Remarks on the present diffusion of heat
over the globe-On the dependence of the mean temperature on the relative position of land and sea-Isothermal lines—Currents from equatorial regions (p. 116.)Drifting of icebergs-Different temperature of Northern and Southern hemispheres -Combination of causes which might produce the extreme cold of which the earth's surface is susceptible (p. 125.)-Conditions necessary for the production of the extreme of heat, and its probable effects on organic life (p. 130.)
Causes of vicissitudes in Climate.- As the proofs enumerated in the last chapter indicate that the earth's surface has experienced great changes of climate since the deposition of the older sedimentary strata, we have next