tact and intersection of small oscillatory movements. Such is the method by which, I hope, it may some day be possible to connect together, by empirical and numerically expressed laws, vast series of apparently isolated facts, and to exhibit the mutual dependence which must necessarily exist among them.

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The lines which I have termed isochimenal and isotheral, (lines of equal winter and equal summer temperature,) are by no means parallel with the isothermal lines, (lines of equal annual temperature.) If, for instance, in countries where myrtles grow wild, and the earth does not remain covered with snow in the winter, the temperature of the summer and autumn is barely sufficient to bring apples to perfect ripeness; and if, again, we observe that the grape rarely attains the ripeness necessary to convert it into wine, either in islands or the vicinity of the sea, even when cultivated on a western coast, the reason must not be sought only in the low degree of summer heat, indicated, in littoral situations, by the thermometer when suspended in the shade, but likewise in another cause that has not hitherto been sufficiently considered, although it exercises an active influence on many other phenomena, (as, for instance, in the inflammation of a mixture of chlorine and hydrogen,) namely, the difference between direct and diffused light, or that which prevails when the sky is clear and when it is overcast by mist.-HUMBOLDT.

THE SNOW LINE.

The law of the decrease of heat with the increase of elevation at different latitudes is one of the most important subjects involved in the study of meteorological processes, of the geography of plants, of the theory of terrestrial refraction, and of the various hypotheses that relate to the determination of the height of the atmosphere.

Since we have acquired a more accurate knowledge of the true relations of the distribution of heat on the surface of the earth-that is to say, of the inflections of isothermal and isotheral lines, and their unequal distance apart in the different eastern and western systems of temperature in Asia, Central Europe, and North America, we can no longer ask the general question, What fraction of the mean annual or summer

temperature corresponds to the difference of one degree of geographical latitude taken in the same meridian? In each system of isothermal lines of equal curvature there reigns a close and necessary connexion between three elements— namely, the decrease of heat in a vertical direction from below upwards; the difference of temperature for every one degree of geographical latitude; and the uniformity in the mean temperature of a mountain station, and the latitude of a point situated at the level of the sea.

The lower limit of perpetual snow, in a given latitude, is the lowest line at which snow continues during summer; or, in other words, it is the maximum of height to which the snow line recedes in the course of the year. But this elevation must be distinguished from three other phenomena: namely, the annual fluctuation of the snow line; the occurrence of sporadic falls of snow; and the existence of glaciers, which appear to be peculiar to the temperate and cold zones.

We know only the lower, and not the upper limit of perpetual snow; for the mountains of the Earth do not attain to these ethereal regions of the rarified and dry strata of air in which we may suppose, with Bouquer, that the vesicles of aqueous vapour are converted into crystals of ice, and thus rendered perceptible to our organs of sight. The lower limit of snow is not, however, a mere function of geographical latitude or of mean annual temperature, nor is it at the equator, or even in the region of the tropics, that this limit attains its greatest elevation above the level of the sea. The phenomenon of which we are treating is extremely complicated, depending on the general relations of temperature and humidity, and on the form of mountains. On submitting these relations to the test of special analysis, as we may be permitted to do from the number of determinations that have recently been made, we shall find that the controlling causes are the differences in the temperature of different seasons of the year; the direction of the prevailing winds, and their relations to the land and sea; the degree of dryness or humidity in the upper strata of the air; the absolute thickness of the accumulated masses of fallen snow; the relation of the snow line to the total height of the mountain; the relative position of the latter in the chain to which it belongs, and the steepness of its declivity; the vicinity of other summits likewise perpetually covered with snow; the expansion, posi

tion, and elevation of the plains from which the snow mountain rises as an elevated peak, or as a portion of a chain; whether this plain be part of the sea-coast or the interior of a continent; whether it be covered with wood or waving grass; and whether, finally, it consists of a dry and rocky soil, or of a wet and marshy bottom.-HUMBOLDT.

GEOGRAPHY OF GEOLOGY.

The vegetable mould is stripped away,1 with all its living inhabitants, animal and vegetable; man himself has disappeared with all that man has built or dug, erected or excavated; and the vast panorama, far as the eye can reach, presents but a dreary wilderness of diluvial clays and gravels, with here a bare rock sticking through, and there a scattered group of boulders. Now, mark a curious fact: the lower clays and gravels in this desert are chiefly of local origin; they are formed mainly of the rock on which they rest. These quartz pebbles, for instance, so extensively used in this part of the country in causewaying footways, were swept out of the magnesian conglomerate of the lower new red; these stiff clays are but re-formations of the saliferous marles of the upper red; these darkened gravels are derived from the neighbouring coal-field; and yonder grey, mud-coloured stratum, mixed up with fragments of limestone, is a deposit from the rather more distant silurians. But not such the character of the widely-spread upper stratum, with its huge granitic boulders. We may see within the range of the landscape whence all the lower beds have come, but no powers of vision could enable us to descry whence the granitic boulders and gravels had come. Strange as the circumstance may seem, they are chiefly Scotch-travellers in the remote past, from the granitic rocks of Dumfries and Kirkcudbright. They lie amid sea-shells of the existing species-the common oyster, the edible cockle and peri

1 The writer looks from the summit of one of the Clent Hills in the vicinity of Hales Owen, in the south of Shropshire. "When Signor Sarti," says Mr. Miller, "exhibits his anatomical models, he takes up one cover after another-first the skin, then the muscles, &c., until at length the skeleton is laid bare." Let us, in the same way, strip the vast landscape here of its upper integuments, coat after coat, beginning first with the vegetable mould-the scarf-skin of the country, wherein its beauty lies, with all its fields and hedge-rows, houses and trees, and proceed downwards, cover after cover, venturing a few remarks on the anatomy of each covering as we go, till we reach those profound depths which carry within their blank folds no record of their origin or history.

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winkle, island-cyprina, rock-whilk, (purpura lapillus,) and a host of others of the kind we may any day pick up on our shores. Now, mark the story which they tell. This region of Central England was once a broad ocean sound, that ran nearly parallel to St. George's Channel: there rose land on both sides of it: Wales had got its head above water; so had the Cotswold Hills in Gloucestershire; and not a particle of the Scotch drift is to be found on either side, where the ancient land lay. But the drift marks the entire course of the central channel, lying thick in Lancashire, Cheshire, Staffordshire, and Worcestershire, in some localities to the depth of a hundred and fifty feet, and in its present elevation, it averages in its course from fifty to five hundred feet over the existing sea. This ancient sound seems to have narrowed towards the south, where it joined on to the Bristol channel; but such was its breadth where we now stand, that the eye would have failed to discover the eastern shore. Its waves beat against the Malverns on the one side, and the Cotswold Hills on the other; it rose high along the flanks of the Wrekin; the secluded dells of Hagley were but the recesses of a submarine rock, shaggy with sea-weed, that occupied its central tide-way; while the Severn, exclusively a river of Wales in those days, emptied its waters into the sea at the Breidden Hills in Montgomeryshire, a full hundred miles from where it now falls into the Bristol Channel. Along this broad sound every spring, when the northern ice began to break up--for its era was that of the British glacier and iceberg-huge ice-floes came drifting in shoals from the Scottish coast, loaded underneath with the granitic blocks which they had enveloped when forming in friths and estuaries; and as they floated along, the loosened boulders dropped on the sea-bottom beneath. Here lie scores in the comparatively still water, and there lie hundreds where the conflicting tides dashed fierce and strong. "In the tract extending from the hamlet of Trescot to the village of Trysall, in the southwestern parts of Staffordshire," says Sir Roderick Murchison, "the quantity, and occasionally gigantic dimensions of these northern boulders, (several tons in weight,) may well excite surprise, seeing that they there occupy one of the most central districts of England. Here the farmer is incessantly labouring to clear the soil, either by burying them, or by piling them up into walls or hedge-banks, and his toil, like that of Sisyphus, seems interminable; for in many spots new

crops of them, as it were, appear as fast as the surface is relieved from its sterilizing burden. So great, indeed, is their abundance, that an observer unacquainted with the region would feel persuaded that he was approaching the foot of some vast granitic range, and yet the source of their origin is one hundred and fifty miles distant."

There are few things that speak more powerfully to the imagination of the geologist than the geography of his science. It seems natural to man to identify the solid globe which he inhabits by its great external features, particularly by its peculiar arrangement of continent and of ocean. We at once recognise it in the prints of our popular astronomical treatises as seen from the moon, or through the telescope from some of the more distant planets, by the well-known disposition of its land and water; and were that disposition made greatly different in the representation, we would at once fail to regard it as the earth on which we ourselves reside. It might be some other of the planets, we would say, but not ours. And yet these great features are exceedingly evanescent, compared with the enduring globe which they diversify and individualize-mere changing mist-wreaths on the surface of an unchanging firmament. The up-piled clouds of one sunset, all gorgeous with their tints of bronze and fire, are not more diverse in place, arrangement, and outline from the streaked and mottled cloudlets of another, radiant in their hues of gold and amber, than the lands and oceans of any one great geologic system from the lands and oceans of the system that has preceded or come after it. Every geologic era has a geography of its own. The earth, like a child's toy, that exhibits a dozen different countenances peeping out in succession from under the same hood, has presented with every revolution a new face. The highest lands of Asia and continental Europe formed ocean-beds in the times of the Oolite: the highest lands of our own country were swum over by the fish of the Old Red Sandstone.

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In the great Pacific we find trace of a sinking continent -a continent once of greater area than all Europe-in the act of foundering, with but merely its mast-heads above the water. Great coral reefs, that whiten the green depths league after league, and degree after degree, for hundreds and thousands of miles, with here and there a tall mountainpeak, existing as a surf-engirdled island, are all that remain