whole distance the mountain sides are lined with dense forests; except in the early morning, an almost perpetual mist hangs over the trees, which collect and throw down from their foliage an abundant supply of moisture. On emerging from the dry belt of tropical forest, the ascent commences at once up a dry ridge, covered at first with the same species as grow upon the plain, species of Bombax, Terminalia; Sterculia, Emblica, Duabanga, Alstonia, Gmelina, Bauhinia and others are abundant, with many figs, some species of Artocarpus, and a proportion of bamboos. By degrees a vegetation characteristic of mountain tracts, but still tropical, takes the place of those just mentioned. A Gordonia is extremely abundant, with numerous euphorbiaceous trees allied to Mappa, various species of Garcinia, the toon (Cedrela toona or serrata), a variety of mimoseous trees, arboreous species of Vernonia and Helicia, beautiful Bauhiniæ both érect and scandent, the latter climbing to the tops of the highest trees with a trunk nearly as thick as a man. The road runs along the top, or on one side of the ridge, looking down into deep valleys full of the densest forest. If we leave the road to enter into these dark and moist hollows, we find that there are occasionally small tracts of flat land along the banks of the streams, which however, more frequently run through deep ravines, clothed with dense thickets of shade-loving trees, species of laurel, alder, magnolia, being mixed with the giant figs, which often form a great part of the forest. In these more shady places the plantain and tree fern luxuriate, and a dense brushwood covers the ground. Not unfrequently large tracts are covered with thickets of Calamus, a prickly palm which attaches itself, by long hooked flagelli to the trees, and often presents a formidable barrier to the traveller who tries to penetrate into its recesses. The trunks of the trees are often clothed with a dense mass of Pothos, and of the huge-leaved Scindapsus, completely encircling them all round, and converting them into leafy columns, while the widespreading branches of the higher trees bear a profusion of Orchidea, which overspread them even to the very top, and, when in flower, have a most gorgeous effect.

In shady valleys, as low as 2000 feet, appear the first specimens of oaks and chestnuts, which in the equable temperature of such places, descend much further on the mountain slopes than in the more arid and variable climate of the western Himalaya.

On attaining an elevation of about 6000 feet, the vegetation has become temperate. The purely tropical forms have almost entirely disappeared, and in their place the forest abounds in trees of temperate climes. Species of Oak, Holly, Cherry, Laurel, Rhododendron, Styrax, and Magnolia, of gigantic size, form the forest, densely covered with mosses and orchidea, and with an underwood of species of Berberis, Daphne, Lonicera, many species of Vitis, and smaller, species of bamboo than those of the tropical region. Ferns are at. such elevations extremely abundant.

From the station of Darjiling, the view in every direction overlooks mountain ranges, covered with dense forest, except in a few spots where partial clearances have been made for cultivation. No bare or grassy mountains meet the eye, no rocks or precipices afford any relief from the prevailing uniformity, which, but for the magnificence of the snowy mountains behind, would be undoubtedly monotonous and fatiguing.

The ascent from the plains of north-west India to Simla, is about the same length as that to Darjiling, but presents the most marked contrast in vegetation, being throughout bare and grassy. The road ascends at first in ten miles to an elevation of 6500 feet, then descends to about 1000 feet, and ascends gradually to 5000. The ascent commences from the Pinjore Dhùn, a lateral valley which runs at the foot of the mountains from the Sutlej to the Jumna rivers. There is no forest in this valley, which is open, and to a great extent cultivated. The lower hills are covered with a shrubby vegetation characteristic of a dry climate. Species of Zizyphus, Carissa, Butea, Adhatoda, Bergera, Egle, Flacourtia, and other common shrubs, with one species of bamboo, and only one fig. After the ascent commences these bushes are only scattered at intervals over the hills, the greater part of the surface being bare and grassy. A similar open country extends all the way to Simla, except where a few fir trees (Pinus longifolia) crest the ridges, and in the more shady ravines, which are lined with a few small trees.

The transition from tropical to temperate vegetation begins, in so far as it is indicated by the small amount of shrubby vegetation, at about 5000 feet, but on the more exposed slopes, plants of warm climates extend up 1000 feet higher, and the herbaceous vegetation, principally grasses, is entirely composed of tropical forms.

It is only on approaching Simla, and attaining a height of nearly 7000 feet, that forest vegetation commences; at that elevation, open forests of Oak, Rhododendron, and Andromeda, intermixed with several species of pines, and a great number of temperate shrubs, of such genera as Rosa, Rubus, Viburnum, Berberis, Spiræa, Lonicera, Indigofera, Prinsepia, Salix, Daphne, and others.

The view from Simla presents a very marked contrast with that from Darjiling. The general outline of the mountains is very much the same, but they are more rocky, and very generally bare; the forests, which to the north are dense and abundant, occupying chiefly the north slopes of the mountains, so that in looking from the south the crest of the ridges only are seen to be wooded. The scenery, therefore, is more diversified than in the eastern Himalaya, and abstracting the snowy mountains, more pleasing to the eye.

Between the two extremes which I have described, every intermediate form may of course be met with, the law of alternation being apparently the following, that in advancing westward towards less humid climates, the lower hills from about 6000 to 2000 feet' be

come more and more bare and grassy, while the lower levels and the base of the mountains retain a greater degree of damp and are clothed with forest. It would appear also that above 6000 or 7000 feet, up to 10,000 or 11,000 feet, at which elevation mountain ranges sensibly interrupt the passage of the moist atmosphere, the temperate ranges are more moist than those below them, which do not collect the clouds, and have a higher temperature, and consequently more powerful sun. To the eastward of Sikkim, the same phenomena are very well marked, the lower ranges being extremely dry and arid, while above 7000 feet, dense forest and a humid atmosphere prevail, just as in the mountains of Sikkim.

The valleys of the larger rivers which traverse the Himalaya from north to south, have of course a much lower elevation than the mountains by which they are surrounded; and up them, therefore, tropical vegetation penetrates very far into the interior. In the extreme west, the valleys of the Indus and Chenab, and even of the Sutlej, are up to the height of 5000 feet, which they do not attain till more than 100 miles from their exit into the plains, hot, dry, and tropical. Further east, the tropical forest stretches far up the valleys, and they are only bare for a small portion of their extent, and in the humid atmosphere of Sikkim they are densely wooded throughout. In that province, the valleys of the Teesta and its tributaries carry tropical vegetation far into the interior, almost within a day's journey of the line of perpetual snow, and the luxuriance of the dense and dripping forest requires to be seen to be understood.

The temperate region of the Himalaya may be said to extend from about 5000 feet, or a little above it, to the upper limit of arboreous vegetation; which, to the westward, is about 12,000 feet, to the east about 1000 feet higher. Above 9000 feet, however, the temperate region is characterised by many remarkable forms, which do not extend lower; these are generally, in the west especially, of very European type; but in the eastern flora, it is at such levels that the magnificent Rhododendrons of Sikkim, which form so striking a part of its flora, principally occur. In this zone a great part of the trees are of European genera, alder, oak, birch, hazel, hornbeam, horse-chestnut, and cherry, being characteristic forms. It is also especially the region of coniferous trees, very few of which extend either below or above it. The pine which descends to the lowest level in the Himalaya, is Pinus longifolia, which is a common tree throughout the whole region from the mountains of the Punjab to the east of Bootan. It is confined in a great measure to the outer ranges of the mountains, and commences as low as 1000 feet above the level of the sea, rarely if ever attaining a greater elevation than 7000 feet. This tree appears to have a very great power of enduring varieties of climate, for it seems equally at home in the hot, damp valleys of Sikkim, surrounded by an entirely tropical vegeta

tion; and on the dry stony hills of the Punjab where rain hardly ever falls, and it is at all seasons exposed to a powerful and scorching sun. The only other coniferous tree of low elevations in the Himalaya is Podocarpus, one species of which is a native of the lower ranges of Nepal and Sikkim.

Pinus excelsa, which is allied to P. Strobus, and Pinus Smithiana, which is near Abies, are the more common species of the central zone, which are distributed throughout the whole extent of the Himalaya. In the same zone, the Deodar (Cedrus deodara) is confined to the western mountains, not being, I believe, to be found indigenous in any part of Nepal, while P. Brunoniana, on the other hand, commences in the eastern parts of Kamaon, and extends as far east as Bootan. The most alpine species of the family are P. Gerardiana, P. Webbiana, and several species of juniper, of which all but the first, which is a western tree, seem universally distributed.

It would be needless to dwell at any length on the alpine zone, because luxuriant as is the vegetation, and beautiful as are the plants, the forms at least must be familiar to most of my auditors.

I must be content, in conclusion, with drawing your attention to the change produced in the vegetation, in the temperate and subalpine zones as we advance towards the interior of the mountains, in consequence of the diminution in the amount of rain.

In travelling through the Himalaya we ascend a great river, the -ascent is so very gradual, that the change of climate and of vegetation in ascending is almost imperceptible, and is only detected by careful observation. If, however, on the other hand, we cross a range of considerable elevation, and descend on its northern side into another valley, the transition is often very striking, and if the chain be sufficiently elevated to intercept the greater part of the rain, the contrast between its two sides is perfectly astonishing; when the transition is thus complete, the traveller leaves dense forests and common Himalayan vegetation on the one side, to find on the other a dry, barren, burnt-up soil, with scattered Astragali, Boragineæ, Cruciferæ, of forms quite characteristic of the flora of Siberia. Such is the vegetation of Tibet, which may be reached either suddenly by crossing a lofty pass, or gradually by ascending the Indus, the Chenab, the Sutlej, the Ganges, and many other of the Himalayan rivers. This arid vegetation is met on the Sutlej, as low as 10,000 feet above the level of the sea, and is therefore in no way dependent upon mere altitude.

SIR,Mr Nasmyth having brought forward an hypothesis respecting the origin of light, which I have just met with in your last number, I am emboldened to mention an hypothesis on a kindred subject which occurred to me some .. months past.

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Sir J. Herschel considers that the sun's outer atmosphere is the source of light to our system; and has, I believe, suggested that the sun himself, as well as the rest of the system, has been gradually condensed from a gaseous condition. Now, combining these two ideas, we may conceive of a period when, though both the sun and the earth were in nearly their present state of condensation, the luminous atmosphere of the sun was vastly more extended than it now is, and reached beyond the limits of our orbit. Under these conditions, the earth would receive a nearly uniform amount of external heat, and polar climates would not have begun to exist. We should thus be enabled to account for facts in geology which are more difficult of explanation than the glacial' epoch,* namely, the occurrence of tropical forms of organic life in temperate and even in arctic latitudes, and the approach to a cosmopolitan distribution of species which we meet with in descending the geological series.

This hypothesis-for to no higher title does it aspire-will as well tally with the description in Gen. i. of the prior creation of light, as does that advanced by Mr Nasmyth, though both are open to the objection that day and night are mentioned before the creation of the sun. This, however, is a difficulty which has perhaps not been satisfactorily solved on any supposition.

In favour of the view which I am bringing forward, it may be observed that in Gen. i. 14 the Hebrew for "lights," is rather "luminaries or light-bearers."

* See Hopkins on Changes of Climate, Geol. Quart. Journal, Feb. 1852. VOL. LII. NO. CIV.-APRIL 1852.

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