the mexican plateaus, and the slopes of sikkim.
geological formation of mexico—the tierra caliente—the tierra templada—the tierra fria.
the sylvan wonders of sikkim—changes of the forest on ascending—the torrid zone of vegetation—the temperate zone the coniferous belt—limits of arboreal vegetation—animal life.
the prodigious height attained in the torrid zone, not only by single mountains, but by vast tracts of land, and the diminution of temperature which is the necessary consequence of their elevation above the level of the sea, enable the inhabitants of many tropical countries, without leaving their native land, to view the vegetable forms of every zone, and to pluck nearly every fruit that is found between the equator and the arctic circle. in asia, africa, and america, in the islands of the indian archipelago, and in the hawaiian group, where the mauna loa towers to the height of mont blanc, and girdles his foot with palms, while snow rests for a great part of the year upon his summit, we find numerous examples of a rapid transition from the torrid to the temperate or frigid zone, often within the range of a single day’s journey.
it would far exceed my limits were i to attempt to follow all these gradations of climate throughout the wide extent of the tropics; but a short description of the mexican plateaus, and of the slopes of sikkim, which i have selected as remarkable instances of the wonderful change of vegetation resulting from the progressive elevation of the land, will, i hope, prove not uninteresting to the reader.
after traversing south america and the isthmus of darien,80 the giant chain of the andes spreads out, as it enters mexico, into a vast sheet of table-land, which maintains an elevation of from 6,000 to 9,000 feet for the distance of 200 leagues, until it gradually declines in the higher latitudes of the north, or descends in successive stages to the sea-board of the atlantic. to this remarkable geological formation the land, though warmed during a part of the year by the rays of a vertical sun, owes that astonishing variety of climate and productions which would make it the envy of the earth, if peopled by a race that knew better how to utilize the gifts of nature.
all along the mexican gulf stretches a broad zone of lowlands, called the tierra caliente, or hot region, which has the usual high temperature of the tropics. parched and sandy plains, dotted with mimosas and prickly opuntias, are intermingled with savannahs, and woodlands of exuberant fertility.
the branches of the stately forest trees are festooned with clustering vines of the dark purple grape, convolvuli, and other flowering parasites of the most brilliant dyes. the undergrowth of prickly aloe, matted with wild rose and honeysuckle, makes in many places an almost impervious thicket. in this wilderness of sweet-smelling buds and blossoms flutter birds of the parrot tribe, and clouds of butterflies, whose colour, nowhere so gorgeous as here, rival those of the vegetable world; while birds of exquisite song,—the scarlet cardinal, and the mocking-bird that comprehends in his own notes the whole music of a forest,—fill the air with melody.
cardinal.
mocking-bird.
but, like the genius of evil, the malaria engendered by the decomposition of rank vegetable substances in the hot and humid soil, poisons these enchanting retreats, and from the spring to the autumnal equinox renders them dangerous or fatal to man.
hastening to escape from its influence, the traveller, after passing some twenty leagues across the dreaded region of the yellow-fever, finds himself rising into a purer atmosphere. his81 limbs recover their elasticity. he breathes more freely, for his senses are not now oppressed by the sultry heats and intoxicating perfumes of the lowlands. the aspect of nature, too, has changed, and his eye no longer rests on the gay variety of colours with which the landscape was painted there. the vanilla, the indigo, the chocolate-tree disappear as he advances, but the sugar-cane and the glossy-leaved banana still remain; and when he has ascended about four thousand feet, he sees, in the unchanging green and the rich foliage of the liquidambar-tree, that he has reached the height where clouds and mists settle in their passage from the mexican gulf, and keep up a perpetual moisture.
he is now beyond the influence of the deadly vomito on the confines of the tierra templada, or temperate region, where evergreen oaks begin to remind him of the forests of central europe. the features of the scenery become grand, and even terrible. his road sweeps along the base of mighty mountains, once gleaming with volcanic fires, and still glistening in their mantles of snow, which serve as beacons to the mariner for many a league at sea. all along he beholds traces of their ancient combustion as his road passes over vast tracts of lava, bristling in the fantastic forms into which the fiery torrent has been thrown by obstacles in its career. perhaps at the same moment, as he casts his eyes down one of those unfathomable ravines or barrancas, which often, to a depth of more than 1,200 feet, rend the mountain-side, he sees its sheltered and sultry recesses glowing with the rich vegetation of the tropics: as if these wonderful regions were anxious to exhibit, at one glance, the boundless variety of their flora. cactuses, euphorbias, and dracænæ, with a multitude of minor plants, cling to the rocky walls; while in the depth of the gorge stand huge laurels, fig-trees, and bombaceæ, whose blossoms exhale almost overpowering odours, and whose trunks are covered with magnificent creepers, expanding their gay petals in the torpid air. still pressing upwards, he mounts into regions favourable to other kinds of cultivation. he has traced the yellow maize growing from the lowest level; but he now first sees fields of wheat and the other european cereals, brought into the country by the spanish conquerors, and with these, plantations of the american agave, which, among other uses,82 provides the mexican with his favourite beverage. the oaks acquire a sturdier growth; and at an elevation of about eight thousand feet, the dark forests of pine announce that he has entered the tierra fria, or cold region,—the third and last of the great natural terraces into which the country is divided.
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loaded with vapours, the prevailing southerly sea-winds, after crossing the dead level occupied by the delta of the ganges and burrampooter, strike against the mountain-spurs of sikkim, the dampest region of that stupendous chain, and expending their moisture on their flanks, clothe them with a thick mantle of verdure to an enormous height. the giant peaks of donkiah, kinchinghow, and kinchinginga, the third great mountain of the world (28,178 feet), form the culminating points of this magnificently wooded region, and look down upon the dense forests which, varying as they rise, extend between the plains of bengal and their own perpetual snows.
dark green woods, of an exclusively tropical character, cover the valleys and declivities to a height of from 4,000 to 5,000 feet. mighty palms rise above the mass of the forest, while innumerable shrubs cover the ground. the prevalent timber is gigantic, and scaled by climbing leguminosæ, bauhinias, and robinias, which sometimes sheathe the trunks, or span the forest with huge cables, joining tree to tree. large bamboos rather crest the hills than court the deeper shade, of which there is abundance, for the torrents cut a straight and steep course down the hill-flanks. the gulleys which they traverse are choked by vegetation, and bridged by fallen trees, whose trunks are clothed with epiphytical orchids, pendulous lycopodia, ferns, pothos, peppers, vines, bignonias, and similar types of the hottest and dampest climates. the beauty of the drapery of the pothos leaves is pre-eminent, whether for the graceful folds of the foliage or for the liveliness of its colour. of the more conspicuous smaller trees the wild banana is the most abundant, its broad crown contrasting with the smaller-leaved plants amongst which it nestles; next comes a screw-pine, with a straight stem and a tuft of leaves, each eight or ten feet long, waving on all sides.
at an elevation of about four thousand feet many plants of the temperate zone, increasing in numbers as the traveller83 ascends, begin to mingle with the tropical vegetation, and to impart new charms to the forest; oaks and walnuts are here seen thriving near palms and arborescent ferns; mighty rhododendrons expand over thickets of tropical herbage; parasitical orchids adorn the trunks of the oaks, while thalictrons and geraniums blossom underneath.
at a height of about 7,000 feet the forest, assuming a decidedly temperate physiognomy, is chiefly composed of oaks, magnolias, chestnuts, laurels, and walnuts. in many parts arborescent rhododendrons prevail, and ferns are generally very abundant.
about 10,000 feet above the level of the sea begins a zone or belt of coniferæ, chiefly characterised by the silver fir (abies webbiana) and the abies brunoniana, a beautiful species, forming a stately pyramid, with branches spreading like the cedar, but not so stiff, and drooping gracefully on all sides. only at intervals other trees, such as willows, magnolias, ashes, birches, poplars, apple and cherry trees, appear among the thick pine-woods. the shrubbery and herbaceous plants of this zone are representatives of the whole temperate flora of europe and america, intermixed with many chinese, japanese, and malayan plants in the richest variety. several epiphytic orchids grow to an elevation of 10,000 feet, and large spaces are frequently occupied by rhododendrons, which either ascend from the temperate zone into the coniferous belt, or first appear in the latter. but very few trees, such as the willows, birches, maples, and ashes, rise above the coniferous forest, which reaches an upper limit of about 13,000 feet. most arboreal plants now appear only in a dwarfed condition; but the willows still rise in powerful growth over the many alpine shrubs—juniperus, rosa, lonicera, potentilla, rhododendron—which cover the ground; and single specimens, though low and stunted, are even found at a height of 16,000 feet.
the whole zone between the extreme limits of arboreal vegetation and the upper boundary of shrubs, generally occupies an elevation of from 13,500 to 16,000 feet, and may justly be called the region of the alpine rhododendrons: these plants are here by far the most numerous, and frequently belt the mountains with a girdle of richly coloured blossoms, even to the verge of the perennial snows.
84 a large number of herbs, cruciferæ, compositæ, ranunculaceæ, grasses, sedges, grow and bloom beyond the limits of the shrubs, frequently forming luxuriant pastures, on which numerous herds of yacks or grunting-oxen, graze during the summer. many plants are even exclusively confined to these enormous heights; such as the rhododendron nivale, the most alpine of woody plants, which dr. hooker found at 17,000 feet elevation, the delphinium glaciale, and the arenaria rupifraga, a curious species forming great hemispherical balls, and altogether resembling in habit the curious balsam-bog of the falkland islands, which thrives in similar scenes. while on the summits of the swiss alps, lichens but sparely cover the rocks, wherever they are denuded of snow, the wanderer in sikkim enjoys the sight of many a gay-coloured flower in regions 3,000 or 4,000 feet higher than the summit of mont blanc.
musk-deer.
while thus in sikkim a wonderful variety of vegetation rises in successive zones from the foot of the mountains to heights unparalleled in any other part of the world, animal life abounds only in its lower classes; for the higher orders appear only in few species, and in very scanty numbers. on ascending from the foot of the himalaya, one is astonished at the silence of the woods, broken at intervals only by the voice of a bird, or the chirping of a cicada. the solitude increases on penetrating into the interior of sikkim, and is but rarely enlivened by a few monkeys in the valleys, some musk deer on the spare grass of the mountains, in heights of from 8,000 to 13,000 feet, or a few larks, sparrows, finches, pigeons, swallows, falcons, and other birds, some of which ascend to a surprising height.
the insects, however, and other invertebrata, make up, by their numbers, for the scarcity of warm-blooded animals, and are often insupportable plagues to the wanderer. beautiful butterflies sometimes ascend to heights of 10,000 feet, along with the less agreeable mosquitoes and ticks, and in all the streams up to an elevation of 7,000 feet, hill leeches infest the waters in such multitudes that bathing is almost impossible.
