Among other manuscript notices of M. Fournel, I possess a vertical geological profile, which gives all the inflexions and inclinations of the strata, representing a section of the surface the whole way from Philippeville on the coast to the Desert of Sahara, at a spot not far from the Oasis of Biscara. The direction of the line on which the barometric measurements were taken is south 20° west; but the elevations determined are projected, as in my Mexican profiles, on a different plane,—a north-south one. Ascending uninterruptedly from Constantine, at an elevation of 332 toises (2122 Eng. feet), the culminating point is found between Batnah and Tizur, at an elevation of only 560 toises (3580 Eng. feet). In the part of the desert situated between Biscara and Tuggurt, Fournel has had a series of Artesian wells dug with success (Comptes Rendus de l’Acad. des Sciences, t. xx. 1845, p. 170, 882, and 1305). We learn from the old accounts of Shaw, that the inhabitants of the country knew of a subterranean supply of water, and relate fabulous tales of a “sea under the earth (bahr tôht el-erd).” Fresh waters flowing between clay and marl strata of the old cretaceous and other sedimentary deposits, under the action of hydrostatic pressure form gushing fountains when the strata are pierced (Shaw, Voyages dans plusieurs parties de la Berbérie, t. i. p. 169; Rennell, Africa, Append. p. lxxxv). That fresh water in this part of the world should often be found near beds of rock salt, need not surprise geologists acquainted with mines, since Europe offers many analogous phenomena.
The riches of the desert in rock-salt, and the fact of rock-salt having been used in building, have been known since the time of Herodotus. The salt zone of the Sahara (zone salifère du désert), is the southernmost of three zones, stretching across Northern Africa from south-west to north-east, and believed to be connected with the beds or deposits of rock-salt of Sicily and Palestine, described by Friedrich Hoffman and by Robinson. (Fournel, sur les Gisements de Muriate de Soude en Algérie, p. 28–41; Karsten über das Vorkommen des Kochsalzes auf der Oberfläche der Erde, 1846, S. 497, 648, and 741.) The trade in salt with Soudan, and the possibility of cultivating dates in the Oases, formed by depressions caused probably by falls or subsidences of the earth in the gypsum beds of the tertiary cretaceous or keuper promotions, have alike contributed to enliven the Desert, at least to some extent, by human intercourse. The high temperature of the air, which makes the day’s march so oppressive, renders the coldness of the nights, (of which Denham complained so often in the African Desert, and Sir Alexander Burnes in the Asiatic), so much the more striking. Melloni, (Memoria sull’ abassamento di temperatura durante le notti placide e serene, 1847, p. 55), ascribes this cold, produced doubtless by the radiation from the ground, less to the great purity and serenity of the sky, (irrigiamento calorifico per la grande serenità di cielo nell’ immensa e deserta pianura dell’ Africa centrale), than to the profound calm, the nightly absence of all movement in the atmosphere. (Consult also, respecting African meteorology, Aimé in the Exploration de l’Algérie, Physique génerale, T. ii., 1846, p. 147.)
The southern declivity of the Atlas of Morocco sends to the Sahara, in lat. 32°, a river, the Quad-Dra (Wady-Dra), which for the greater part of the year is nearly dry, and which Renou (Explor. de l’Alg. Hist. et Geogr., T. viii. p. 65–78) considers to be a sixth longer than the Rhine. It flows at first from north to south, until, in lat. 29° N. and long. 5° W., it turns almost at right angles to its former course, runs to the west, and, after passing through the great fresh water Lake of Debaid, enters the sea at Cape Nun, in lat. 28° 46′ N. and long. 11° 08′ W. This region, which was so celebrated formerly in the history of the Portuguese discoveries of the 15th century, and was afterwards wrapped in profound geographical obscurity, is now called on the coast “the country of the Sheikh Beirouk,” (a chief independent of the Emperor of Morocco.) It was explored in the months of July and August 1840, by Captain Count Bouet-Villaumez of the French Navy, by order of his government. From the official Reports and Surveys which have been communicated to me in manuscript, it appears evident that the mouth of the Quad-Dra is at present very much stopped up with sand, having an open channel of only about 190 English feet wide. A somewhat more easterly channel in the same mouth is that of the still very little known Saguiel el-Hamra, which comes from the south, and is supposed to have a course of at least 600 geographical miles. One is astonished at the length of these deep, but commonly dry river beds. They are ancient furrows, such as I have seen in the Peruvian desert at the foot of the Cordilleras, between those mountains and the coast of the Pacific. In Bouet’s manuscript “Relation de l’Expédition de la Malouine,” the mountains which rise to the north of Cape Nun are estimated at the great elevation of 2800 metres (9185 English feet).
Cape Nun is usually supposed to have been discovered in 1433, by the Knight Gilianez, acting under the command of the celebrated Infante Henry Duke of Viseo, and founder of the Academy of Sagres, which was presided over by the pilot and cosmographer Mestre Jacomè of Majorca; but the Portulano Mediceo, the work of a Genoese Navigator in 1351, already contains the name of Cavo di Non. The passage round this Cape was then as much dreaded as that of Cape Horn has since been, although it is 23′ north of the parallel of Teneriffe, and could be reached in a few days’ voyage from Cadiz. The Portuguese proverb, “quem passa o Cabo di Num, ou tornarà ou não,” could not deter the Infante, whose heraldic French motto, “talent de bien faire,” expressed his noble, enterprising, and vigorous character. The name of the Cape, in which a play of words on the negative particle has long been supposed, does not appear to me to have had a Portuguese origin. Ptolemy placed on the north-west coast of Africa a river Nuius, in the Latin version Nunii Ostia. Edrisi speaks of a town, Nul, or Wadi Nun, somewhat more to the south, and three days’ journey in the interior: Leo Africanus calls it Belad de Non. Long before the Portuguese squadron of Gilianez, other European navigators had advanced much beyond, or to the southward of, this Cape. The Catalan, Don Jayme Ferrer, in 1346, as we learn from the Atlas Catalan published by Buchon at Paris, had advanced as far as the Gold River, (Rio do Ouro), in lat. 23° 56′; and Normans, at the end of the 14th century, as far as Sierra Leone in lat. 8° 30′. The merit of having been the first to cross the equator on the western coast of Africa belongs, however, like that of so many other memorable achievements, to the Portuguese.
[17] p. 8.—“As a grassy plain, resembling many of the Steppes of Central Asia.”
The Llanos of Caraccas and of the Rio Apure and the Meta, over which roam large herds of cattle, are, in the strictest sense of the term, “grassy plains.” Their prevalent vegetation, belonging to the two families of Cyperaceæ and Gramineæ, consists of various species of Paspalum, P. leptostachyum and P. lenticulare; of Kyllingia, K. monocephala (Rottb.), K. odorata; of Panicum, P. granuliferum, P. micranthum; of Antephora; Aristida; Vilfa; and Anthistiria, A. reflexa, and A. foliosa. Only here and there are found, interspersed among the Gramineæ, a few herbaceous dicotyledonous plants, consisting of two very low-growing species of Mimosa, (Sensitive Plant), Mimosa intermedia, and Mimosa dormiens, which are great favourites with the wild horses and cattle. The natives give to this group of plants, which close their delicate feathery leaves on being touched, the expressive name of Dormideras—sleepy plants. For many square miles not a tree is seen; but where solitary trees are found, they are, in moist places, the Mauritia Palm; in arid districts, a Proteacea, described by Bonpland and myself, the Rhopala complicata (Chaparro bobo), which Wildenow regarded as an Embothrium; also the highly useful Palma de Covija, or de Sombrero; and our Corypha inermis, an umbrella palm allied to Chamærops, which is used to cover the roofs of huts. How far more varied is the aspect of the Asiatic plains! Throughout a large portion of the Kirghis and Calmuck Steppes, which I have traversed from the Don, the Caspian, and the Orenburg Ural river the Jaik, to the Obi and the Upper Irtysh near Lake Dsaisang, through a space of 40 degrees of longitude, I have never seen, as in the Llanos, the Pampas, and the Prairies, an horizon like that of the ocean, where the vault of heaven appears to rest on the unbroken plain. At the utmost this appearance presented itself in one direction, or towards one quarter of the heavens. The Asiatic Steppes are often crossed by ranges of hills, or clothed with coniferous woods or forests. Even in the most fruitful pastures the vegetation is by no means limited to grasses; there is a great variety of herbaceous plants and shrubs. In spring-time small snow-white and red-flowering rosaceæ and amygdaleæ (Spiræa, Cratægus, Prunus spinosa, and Amygdalus nana) present a smiling aspect. I have already mentioned the tall and luxuriant Synantheræ (Saussurea amara, S. salsa, Artemisias, and Centaureas), and of leguminous plants, species of Astragalus, Cytisus, and Caragana. Crown Imperials, (Fritillaria ruthenica, and F. meleagroides), Cypripedias, and tulips, rejoice the eye by the bright variety of their colours.
A contrast to the pleasing vegetation of these Asiatic plains is presented by the desolate salt Steppes, particularly by the part of the Barabinski Steppe which is at the foot of the Altai mountains, and by the Steppes between Barnaul and the Serpent Mountain and the country on the east of the Caspian. Here Chenopodias, some species of Salsola and Atriplex, Salicornias and Halimocnemis crassifolia, (each species growing “socially”), form patches of vegetation on the muddy ground. See Göbel’s Journey in the Steppes of the South of Russia (Reise in die Steppe des südlichen Russlands, 1838, Th. ii. S. 244 and 301). Of the 500 phanerogamous species which Claus and Göbel collected in the Steppes, the Syrantheræ, the Chenopodeæ, and the Cruciferæ, were more numerous than the grasses; the latter being only 1⁄11 of the whole, and the former 1⁄7th and 1⁄9th. In Germany, from the mixture of hill and plain districts, the Glumaceæ (i. e. the Gramineæ, Cyperaceæ, and Juncaceæ collectively), form 1⁄7th; the Synantheræ or Compositæ 1⁄8th; and the Cruciferæ 1⁄18th of all our German phanerogamia. In the most northern parts of the flat Siberian lowlands, the fine map of Admiral Wrangell shews that the extreme northern limit of tree and shrub vegetation (Coniferæ and Amentaceæ) is, in the portion towards the Behring’s Straits side, in 67¼° lat.; and more to the west, towards the banks of the Lena, in 71°, which is the parallel of the north cape of Lapland. The plains which border the Icy Sea are the domain of cryptogamous plants. They are called Tundras (Tuntur in Finnish): they are swampy districts extending farther than the eye can reach, partly covered with a thick carpet of Sphagnum palustre and other mosses, and partly with a dry snow-white covering of Cenomyce rangiferina (Rein-deer moss), Stereocaulon paschale, and other lichens. Admiral Wrangell, in describing his perilous expedition to the new Siberian islands so rich in fossil wood, says: “These Tundras accompanied me to the extreme arctic coast. Their soil has been frozen for thousands of years. In the dreary uniformity of landscape, the eye of the traveller, surrounded by rein-deer moss, dwells with pleasure on the smallest patch of green turf showing itself now and then on a moist spot.”
[18] p. 8.—“The causes which lessen both heat and dryness in the New World.”
I have tried to bring together in a brief and compendious manner the various causes which produce greater moisture and a less degree of heat in America; it will of course be understood that the question respects the general hygrometric state of the atmosphere, and the temperature of the New Continent as a whole. Single districts, such as the island of Margarita, the Coasts of Cumana and Coro, are as hot and as dry as any part of Africa. It must also be remarked that the maximum of heat at certain hours of a summer’s day has been found, on a series of years, to be almost equal at very different parts of the earth’s surface, on the Neva, the Senegal, the Ganges, and the Orinoco; being approximately between 27° and 32° Reaumur (93° and 104° Fahrenheit), and generally not higher,—providing the observation be made in the shade, at a distance from all solid bodies which could radiate heat to the thermometer, not in an air filled with hot particles of dust or sand, and not with spirit thermometers, which absorb the light. It is probably to fine grains of sand floating in the air, and forming centres of radiant heat, that we must ascribe the dreadful temperature of 40° to 44°.8 Reaumur (122° to 133° Fah.) in the shade, to which my unhappy friend Ritchie, who perished there, and Captain Lyon, were exposed for weeks in the Oasis of Mourzouk. The most remarkable instance of very high temperature, in an air probably free from dust, has been recorded by an observer who knew well how to place and to correct all his instruments with the greatest degree of accuracy. Rüppell found 37°.6 Reaumur, (110°.6 Fahrenheit,) at Ambukol in Abyssinia, with a clouded sky, strong south-west wind, and an approaching thunderstorm. The mean annual temperature of the tropics, or of the proper climate of palms, is, on land, between 20°.5 and 23°.8 Reaumur (or 78°.2 and 85°.5 Fahrenheit) without any considerable difference between the observations collected in Senegal, Pondichery, and Surinam. (Humboldt, Mémoire sur les lignes isothermes, 1817, p. 54. Asie Centrale, T. iii. Mahlmann, Table iv.)
The great coolness, I might almost say cold, which prevails for a considerable part of the year within the tropics on the coast of Peru, causing the thermometer to sink to 12° Reaumur (59° Fahrenheit), is, as I have noticed elsewhere, by no means to be ascribed to the vicinity of the snow-covered Andes, but rather to the fogs (garua) which veil the solar disk, and to a cold sea current which, commencing in the antarctic regions and coming from the south-west, strikes the coast of Chili near Valdivia and Conçeption, and thence streams rapidly along the coast to the northward, as far as Cape Pariña. On the coast, near Lima, the temperature of the Pacific is 12°.5 Reaumur (60°.2 Fahr.), whilst in the same latitude out of the current it is 21° R. (79°.2 Fahr.) It is singular that so striking a fact should have remained unnoticed until my visit to the shores of the Pacific, in October 1802.