Of the creation of this valley we have already spoken. No region on the face of the globe of equal extent has such a monotonous geology. Around the rim of the basin are the outcroppings of a cretaceous deposit; this rests on the hidden mezozoic and palæozoic strata which form the ribs of the Andes. Above it, covering the whole basin from New Granada to the Argentine Republic,[160] are the following formations: first, a stratified accumulation of sand; second, a series of laminated clays, of divers colors, without a pebble; third, a fine, compact sandstone; fourth, a coarse, porous sandstone, so ferruginous as to resemble bog iron-ore. This last was, originally, a thousand feet in thickness, but was worn down, perhaps, in some sudden escape of the pent-up waters of the valley. The table-topped hills of Almeyrim are almost the sole relics.[161] Finally, over the undulating surface of the denuded sandstone an ochraceous, unstratified sandy clay was deposited.
It is a question to what period this great accumulation is to be assigned. Humboldt called it "Old Red Sandstone;" Martius pronounced it "New Red;" Agassiz says "Drift"—the glacial deposit brought down from the Andes and worked over by the melting of the ice which transported it.[162] The Professor farther declares that "these deposits are fresh-water deposits; they show no sign of a marine origin; no sea-shells nor remains of any marine animal have as yet been found throughout their whole extent; tertiary deposits have never been observed in any part of the Amazonian basin." This was true up to 1867. Neither Bates, Wallace, nor Agassiz found any marine fossil on the banks of the great river. But there is danger in building a theory on negative evidence. These explorers ascended no farther than Tabatinga. Two hundred miles west of that fort is the little Peruvian village of Pebas, at the confluence of the Ambiyacu. We came down the Napo and Marañon, and stopped at this place. Here we discovered a fossiliferous bed intercalated between the variegated clays so peculiar to the Amazon. It was crowded with marine tertiary shells! This was Pebas vs. Cambridge. It was unmistakable proof that the formation was not drift, but tertiary; not of fresh, but salt water origin. The species, as determined by W.M. Gabb, Esq., of Philadelphia, are: Neritina pupa, Turbonilla minuscula, Mesalia Ortoni, Tellina Amazonenis, Pachydon obliqua, and P. tenua.[163] All of these are new forms excepting the first, and the last is a new genus. It is a singular fact that the Neritina is now living in the West India waters, and the species found at Pebas retains its peculiar markings. So that we have some ground for the supposition that not many years ago there was a connection between the Caribbean Sea and the Upper Amazon; in other words, that Guiana has only very lately ceased to be an island. There is no mountain range on the water-shed between the Orinoco and the Negro and Japurá, but the three rivers are linked by natural canals.[164] Interstratified with the clay deposit are seams of a highly bituminous lignite; we traced it from near the mouth of the Curaray on the Rio Napo to Loreto on the Marañon, a distance of about four hundred miles. It occurs also at Iquitos. This is farther testimony against the glacial theory of the formation of the Amazonian Valley. The paucity of shells in such a vast deposit is not astonishing. It is as remarkable in the similar accumulation of reddish argillaceous earth, called "Pampean mud," which overspreads the Rio Plata region.[165] Some of the Pampa shells, like those at Pebas, are proper to brackish water, and occur only on the highest banks. The Pampean formation is believed by Mr. Darwin to be an estuary or delta deposit. We will mention, in this connection, that silicified wood is found at the head waters of the Napo; the Indians use it instead of flint (which does not occur there) in striking a light. Darwin found silicified trees on the same slope of the Andes as the Uspallata Pass.
The climatology of the Valley of the Amazon is as simple as its physical geography. There is no circle of the seasons as with us—nature moves in a straight line. The daily order of the weather is uniform for months. There is very little difference between the dry and hot seasons; the former, lasting from July to December, is varied with showers, and the latter, from January to June, with sunny days, while the daily temperature is the same within two or three degrees throughout the year. On the water-shed between the Orinoco and Negro it rains throughout the year, but most water falls between May and November, the coolest season in that region. On the Middle Negro the wet season extends from June 1st to December 1st, and is the most sultry time.
Comparatively few insects, birds, or beasts are to be seen in summer; but it is the harvest-time of the inhabitants, who spend the glorious weather rambling over the plaias and beaches, fishing and turtle-hunting. The middle of September is the midsummer of the valley. The rainy season, or winter, is ushered in by violent thunder-storms from the west. It is then that the woods are eloquent with buzzing insects, shrill cicadas, screaming parrots, chattering monkeys, and roaring jaguars. The greatest activity of animal and vegetable life is in June and July. The heaviest rains fall in April, May, and June. Scarcely ever is there a continuous rain for twenty-four hours. Castelnau witnessed at Pebas a fall of not less than thirty inches in a single storm. The greatest amount noticed in New York during the whole month of September was 12.2 inches. The humidity of the atmosphere, as likewise the luxuriance of vegetation and the abundance and beauty of animal forms, increases from the Atlantic to the Andes. At the foot of the Andes, Pœppig found that the most refined sugar in a few days dissolved into sirup, and the best gunpowder became liquid even when inclosed in canisters. So we found the Napo steaming with vapor. Fogs, however, are rarely seen on the Amazon.
The animals and plants are not all simultaneously affected by the change of seasons. The trees retain their verdure through the dry veraō, and have no set time for renewing their foliage. There are a few trees, like Mongruba, which drop their leaves at particular seasons; but they are so few in number they create the impression of a few dead leaves in a thick-growing forest. Leaves are falling and flowers drooping all the year round. Each species, and, in some cases, each individual, has its own particular autumn and spring. There is no hibernation nor æstivation (except by land shells); birds have not one uniform time for nidification; and moulting extends from February to May.
Amazonia, though equatorially situated, has a temperate climate. It is cooler than Guinea or Guiana. This is owing to the constant evaporation from so much submerged land, and the ceaseless trade winds. The mean annual temperature of the air is about 81°.[166] The nights are always cool. There are no sudden changes, and no fiery "dog days." Venereal and cutaneous affections are found among the people; but they spring from an irregular life. A traveler on the slow black tributaries may take the tertiana, but only after weeks of exposure. Yellow fever and cholera seldom ascend the river above Pará; and on the Middle Amazon there are neither endemics nor epidemics, though the trades are feebly felt there, and the air is stagnant and sultry. According to Bates, swampy and weedy places on the Amazon are generally more healthy than dry ones. Whatever exceptions be taken to the branches, the main river is certainly as healthy as the Mississippi: the rapid current of the water and the continual movement of the air maintaining its salubrity. The few English residents (Messrs. Hislop, Jeffreys, and Hauxwell), who have lived here thirty or forty years, are as fresh and florid as if they had never left their native country. The native women preserve their beauty until late in life. Great is the contrast between the gloomy winters and dusty summers, the chilly springs and frosty autumns of the temperate zone, and the perennial beauty of the equator! No traveler on the Amazon would exchange what Wallace calls "the magic half-hour after sunset" for the long gray twilight of the north. "The man accustomed to this climate (wrote Herndon) is ever unwilling to give it up for a more bracing one."
The mineral kingdom is represented only by sand, clay, and loam. The solid rock (except the sandstone already mentioned) begins above the falls on the tributaries. The precious gems and metals are confined to the still higher lands of Goyaz, Matto Grosso, and the slopes of the Andes. The soil on the Lower Amazon is sandy; on the Solimoens and Marañon it is a stiff loam or vegetable mould, in many places twenty feet deep.
Both in botany and zoology, South America is a natural and strongly-marked division, quite as distinct from North America as from the Old World; and as there are no transverse barriers, there is a remarkable unity in the character of the vegetation. No spot on the globe contains so much vegetable matter as the Valley of the Amazon. From the grassy steppes of Venezuela to the treeless Pampas of Buenos Ayres, expands a sea of verdure, in which we may draw a circle of eleven hundred miles in diameter, which shall include an ever green, unbroken forest. There is a most bewildering diversity of grand and beautiful trees—a wild, unconquered race of vegetable giants, draped, festooned, corded, matted, and ribboned with climbing and creeping plants, woody and succulent, in endless variety. The exuberance of nature displayed in these million square acres of tangled, impenetrable forest offers a bar to civilization nearly as great as its sterility in the African deserts. A macheta is a necessary predecessor: the moment you land (and it is often difficult to get a footing on the bank), you are confronted by a wall of vegetation. Lithe lianas, starred with flowers, coil up the stately trees, and then hang down like strung jewels; they can be counted only by myriads, yet they are mere superfluities. The dense dome of green overhead is supported by crowded columns, often branchless for eighty feet. The reckless competition among both small and great adds to the solemnity and gloom of a tropical forest. Individual struggles with individual, and species with species, to monopolize the air, light, and soil. In the effort to spread their roots, some of the weaker sort, unable to find a footing, climb a powerful neighbor, and let their roots dangle in the air; while many a full-grown tree has been lifted up, as it were, in the strife, and now stands on the ends of its stilt-like roots, so that a man may walk upright between the roots and under the trunk.[167]
The mass of the forest on the banks of the great river is composed of palms (about thirty species[168]), leguminous or pod-bearing trees, colossal nut-trees, broad-leaved Musaceæ or bananas, and giant grasses. The most prominent palms are the architectural Pupunha, or "peach-palm," with spiny stems, drooping, deep green leaves, and bunches of mealy, nutritious fruit; the slender Assaï, with a graceful head of delicate green plumes; the Ubussú, with mammoth, undivided fronds; the stiff, serrated-leaved Bussú, and gigantic Mirití. One of the noblest trees of the forest is the Massaranduba, or "cow-tree" (Brosimum galactodendron), often rising one hundred and fifty feet. It is a hard, fine-grained, durable timber, and has a red bark, and leathery, fig-like foliage. The milk has the consistency of cream, and may be used for tea, coffee, or custards. It hardens by exposure, so as to resemble gutta-percha. Another interesting tree, and one which yields the chief article of export, is the Caucho, or India-rubber tree[169] (Siphonia Brasiliensis), growing in the lowlands of the Amazon for eighteen hundred miles above Pará. It has an erect, tall trunk, from forty to eighty feet high, a smooth, gray bark, and thick, glossy leaves. The milk resembles thick, yellow cream, and is colored by a dense smoke obtained by burning palm-nuts. It is gathered between August and December. A man can collect six pounds a day, though this is rarely done. It is frequently adulterated with sand. The tree belongs to the same apetalous family as our castor-oil and the mandioca; while the tree which furnishes the caoutchouc of the East Indies and Africa is a species of Ficus, and yields an inferior article to the rubber of America. Other characteristic trees are the Mongruba, one of the few which shed their foliage before the new leaf-buds expand; the giant Samaüma, or silk-cotton tree (called huimba in Peru); the Calabash, or cuieira, whose gourd-like fruit furnishes the cups used throughout the Amazon; the Itauba, or stone-wood, furnishing ship-timber as durable as teak; the red and white Cedar, used for canoes (not coniferous like the northern evergreen, but allied to the mahogany); the Jacarandá, or rose-wood, resembling our locust; Palo de sangre, one of the most valuable woods on the river; Huacapú, a very common timber; Capirona, used as fuel on the steamers; and Tauarí, a heavy, close-grained wood, the bark of which splits into thin leaves, much used in making cigarettes. The Piassaba, a palm yielding a fibre extensively manufactured into cables and ropes, and exported to foreign countries for brushes and brooms, being singularly elastic, strong, and more durable than hemp; and the Moira-pinima, or "tortoise-shell wood," the most beautiful wood in all Amazonia, if not in the world, grow on the Upper Rio Negro. A small willow represents the great catkin family.
The valley is as remarkable for the abundance, variety, and value of its timber as for any thing else. Within an area of half a mile square, Agassiz counted one hundred and seventeen different kinds of woods, many of them eminently fitted, by their hardness, tints, and beautiful grain, for the finest cabinet-work. Enough palo de sangre or moira-pinima is doubtless wasted annually to veneer all the palaces of Europe.