L. Agassiz.
CHAPTER XIII.
PHYSICAL HISTORY OF THE AMAZONS.
Drift about Rio de Janeiro.—Decomposition of underlying Rock.—Different Aspect of Glacial Phenomena in different Continents.—Fertility of the Drift.—Geological Observations of Messrs. Hartt and St. John.—Correspondence of Deposits along the Coast with those of Rio and those of the Valley of the Amazons.—Primitive Formation of the Valley.—First known Chapter of its History.—Cretaceous Fossil Fishes.—Former Extent of the South-American Coast.—Cretaceous Fossils from the Rio Purus.—Comparison between North and South America.—Geological Formations along the Banks of the Amazons.—Fossil Leaves.—Clays and Sandstones.—Hills of Almeyrim.—Monte Alégre.—Situation and Scenery.—Serra Ereré.—Comparison with Swiss Scenery.—Boulders of Ereré.—Ancient Thickness of Amazonian Deposits.—Difference between Drift of the Amazons and that of Rio.—Inferences drawn from the present Condition of the Deposits.—Immense Extent of Sandstone Formation.—Nature and Origin of these Deposits.—Referred to the Ice-Period.—Absence of Glacial Marks.—Glacial Evidence of another Kind.—Changes in the Outline of the South-American Coast.—Souré.—Igarapé Grande.—Vigia.—Bay of Braganza.—Anticipation.
A few days before we left Pará, Senhor Pimenta Bueno invited his friends and acquaintances, who had expressed a wish to hear Mr. Agassiz’s views on the geological character of the Amazonian Valley, to meet at his house in the evening for that purpose. The guests were some two hundred in number, and the whole affair was very unceremonious, assuming rather the character of a meeting for conversation or discussion than that of an audience collected to hear a studied address. The substance of this talk or lecture, as subsequently written out by Mr. Agassiz, afterward appeared in the Atlantic Monthly, and is inserted here, with some few alterations under the head of a separate chapter. The reader will find occasional repetitions of facts already stated in the earlier part of the narrative; but they are retained for the sake of giving a complete and consistent review of the subject at this point of our journey, where it became possible to compare the geological structure of the Amazonian Valley with that of the southern provinces of Brazil and of those bordering on the Atlantic coast.
The existence of a glacial period, however much derided when first announced, is now a recognized fact. The divergence of opinion respecting it is limited to a question of extent; and after my recent journey in the Amazons, I am led to add a new chapter to the strange history of glacial phenomena, taken from the southern hemisphere, and even from the tropics themselves.
I am prepared to find that the statement of this new phase of the glacial period will awaken among my scientific colleagues an opposition even more violent than that by which the first announcement of my views on this subject was met. I am, however, willing to bide my time; feeling sure that, as the theory of the ancient extension of glaciers in Europe has gradually come to be accepted by geologists, so will the existence of like phenomena, both in North and South America, during the same epoch, be recognized sooner or later as part of a great series of physical events extending over the whole globe. Indeed, when the ice-period is fully understood, it will be seen that the absurdity lies in supposing that climatic conditions so different could be limited to a small portion of the world’s surface. If the geological winter existed at all, it must have been cosmic; and it is quite as rational to look for its traces in the Western as in the Eastern hemisphere, to the south of the equator as to the north of it. Impressed by this wider view of the subject, confirmed by a number of unpublished investigations which I have made during the last three or four years in the United States, I came to South America, expecting to find in the tropical regions new evidences of a bygone glacial period, though, of course, under different aspects. Such a result seemed to me the logical sequence of what I had already observed in Europe and in North America.
On my arrival in Rio de Janeiro,—the port at which I first landed in Brazil,—my attention was immediately attracted by a very peculiar formation consisting of an ochraceous, highly ferruginous, sandy clay. During a stay of three months in Rio, whence I made many excursions into the neighboring country, I had opportunities of studying this deposit, both in the province of Rio de Janeiro and in the adjoining province of Minas Geraes. I found that it rested everywhere upon the undulating surfaces of the solid rocks in place, was almost entirely destitute of stratification, and contained a variety of pebbles and boulders. The pebbles were chiefly quartz, sometimes scattered indiscriminately throughout the deposit, sometimes lying in a seam between it and the rock below; while the boulders were either sunk in its mass, or resting loosely on the surface. At Tijuca, a few miles out of the city of Rio, among the picturesque hills lying to the southwest of it, these phenomena may be seen in great perfection. Near Bennett’s Hotel there are a great number of erratic boulders, having no connection whatever with the rock in place; and also a bluff of this superficial deposit studded with boulders, resting above the partially stratified metamorphic rock.[[96]] Other excellent opportunities for observing this formation, also within easy reach from the city, are afforded along the whole line of the Dom Pedro Segundo Railroad, where the cuts expose admirable sections, showing the red, unstratified, homogeneous mass of sandy clay resting above the solid rock, and often divided from it by a thin bed of pebbles. There can be no doubt, in the mind of any one familiar with similar facts observed in other parts of the world, that this is one of the many forms of drift connected with glacial action. I was, however, far from anticipating, when I first met it in the neighborhood of Rio, that I should afterwards find it spreading over the surface of the country from north to south and from east to west, with a continuity which gives legible connection to the whole geological history of the continent.
It is true that the extensive decomposition of the underlying rock, penetrating sometimes to a considerable depth, makes it often difficult to distinguish between it and the drift; and the problem is made still more puzzling by the fact that the surface of the drift, when baked by exposure to the hot sun, often assumes the appearance of decomposed rock, so that great care is required for a correct interpretation of the facts. A little practice, however, trains the eye to read these appearances aright; and I may say that I have learned to recognize everywhere the limit between the two formations. There is indeed one safe guide, namely, the undulating line, reminding one of roches moutonnées,[[97]] and marking the irregular surface of the rock on which the drift was accumulated; whatever modifications the one or the other may have undergone, this line seems never to disappear. Another deceptive feature, arising from the frequent disintegration of the rocks and from the brittle character of some of them, is the presence of loose fragments, which simulate erratic boulders, but are in fact only detached masses of the rock in place. A careful examination of their structure, however, will at once show the geologist whether they belong where they are found, or have been brought from a distance to their present resting-place.
But, while the features to which I have alluded are unquestionably drift phenomena, they present in their wider extension, and especially in the northern part of Brazil, some phases of glacial action hitherto unobserved. Just as the investigation of the ice-period in the United States has shown us that ice-fields may move over open level plains, as well as along the slopes of mountain valleys, so does a study of the same class of facts in South America reveal new and unlooked-for features in the history of the ice-period. Some will say that the fact of the advance of ice-fields over an open country is by no means established, inasmuch as many geologists believe all the so-called glacial traces—viz. striæ, furrows, polish, etc., found in the United States—to have been made by floating icebergs at a time when the continent was submerged. To this I can only answer that, in the State of Maine, I have followed, compass in hand, the same set of furrows, running from north to south in one unvarying line, over a surface of one hundred and thirty miles, from the Katahdin Iron Range to the sea-shore.[[98]] These furrows follow all the inequalities of the country, ascending ranges of hills varying from twelve to fifteen hundred feet in height, and descending into the intervening valleys only two or three hundred feet above the sea, or sometimes even on a level with it. I take it to be impossible that a floating mass of ice should travel onward in one rectilinear direction, turning neither to the right nor to the left, for such a distance. Equally impossible would it be for a detached mass of ice, swimming on the surface of the water, or even with its base sunk considerably below it, to furrow in a straight line the summits and sides of the hills, and the bottoms of the intervening valleys. It would be carried over the inequalities of the country without touching the lowest depressions. Instead of ascending the mountains, it would remain stranded against any elevation which rose greatly above its own base, and, if caught between two parallel ridges, would float up and down between them. Moreover, the action of solid, unbroken ice, moving over the ground in immediate contact with it, is so different from that of floating ice-rafts or icebergs that, though the latter have unquestionably dropped erratic boulders, and made furrows and striæ on the surface where they happened to be grounded, these phenomena will easily be distinguished from the more connected tracks of glaciers, or extensive sheets of ice, resting directly upon the face of the country and advancing over it.