The Atlantic Monthly, Volume 18, No. 106, August, 1866. Various
in cathedrals cold and vain,
Built on loose sands of "It is said."
My search is for the living gold,
Him I desire who dwells recluse,
And not his image, worn and old,
Day-servant of our sordid use.
If Him I find not, yet I find
The ancient joy of cell and church,
The glimpse, the surety undefined,
The unquenched ardor of the search.
Happier to chase a flying goal,
Than to sit counting laurelled gains,
To guess the Soul within the soul,
Than to be lord of what remains.
PHYSICAL HISTORY OF THE VALLEY OF THE AMAZONS
II
Major Coutinho and myself passed three days in the investigation of the Serra of Erreré. We found it to consist wholly of the sandstone deposits described in my previous article, and to have exactly the same geological constitution. In short, the Serra of Monte Alegre, and of course all those connected with it on the northern side of the river, lie in the prolongation of the lower beds forming the banks of the river, their greater height being due simply to the fact that they have not been worn to the same low level. The opposite range of Santarem, which has the same general outline and character, shares, no doubt, the same geological structure. In one word, all these hills were formerly part of a continuous formation, and owe their present outline and their isolated position to a colossal denudation. The surface of the once unbroken strata, which in their original condition must have formed an immense plain covered by water, has been cut into ravines or carried away over large tracts, to a greater or less depth, leaving only such portions standing as from their hardness could resist the floods which swept over it. The longitudinal trend of these hills is to be ascribed to the direction of the current which caused the denudation, while their level summits are due to the regularity of the stratification. They are not all table-topped, however; among them are many of smaller size, in which the sides have been gradually worn down, producing a gently rounded surface. Of course, under the heavy tropical rains this denudation is still going on, though in a greatly modified form.
I cannot leave this Serra without alluding to the great beauty and extraordinary extent of the view to be obtained from it. Indeed, it was here that for the first time the geography of the country presented itself to my mind as a living reality, in all its completeness. Insignificant as is its actual height, the Serra of Erreré commands a wider prospect than is to be had from many a more imposing mountain; for the surrounding plain, covered with forests, and ploughed by countless rivers, stretches away for hundreds of leagues in every direction, without any object to obstruct the view. Standing on the brow of the Serra, with the numerous lakes intersecting the low lands at its base, you look across the Valley of the Amazons, as far as the eye can reach, and through its midst you follow for miles on either side the broad flood of the great river, carrying its yellow waters to the sea. As I stood there, panoramas from the Swiss mountains came up to my memory, and I fancied myself standing on the Alps, looking across the plain of Switzerland, instead of the bed of the Amazons, the distant line of the Santarem hills on the southern bank of the river, and lower than the northern chain, representing the Jura range. As if to complete the comparison, I found Alpine lichens growing among cactus and palms, and a crust of Arctic cryptogamous growth covered rocks, between which sprang tropical flowers. On the northern flank of this Serra I found the only genuine erratic boulders I have seen in the whole length of the Amazonian Valley, from Pará to the frontier of Peru, though there are many detached masses of rock, as, for instance, at Pedreira, near the junction of the Rio Negro and Rio Branco, which might be mistaken for them, but are due to the decomposition of the rocks in place. The boulders of Erreré are entirely distinct from the rock of the Serra, and consist of masses of compact hornblende.
It would seem that these two ranges skirting a part of the northern and southern banks of the Lower Amazons are not the only remnants of this arenaceous formation in its primitive altitude. On the banks of the Japura, in the Serra of Cupati, Major Coutinho has found the same beds rising to the same height. It thus appears, by positive evidence, that over an extent of a thousand miles these deposits had a very considerable thickness in the present direction of the valley. How far they extended in width has not been ascertained by direct observation, for we have not seen how they sink away to the northward, and towards the south the denudation has been so complete that, except in the very low range of hills in the neighborhood of Santarem, they do not rise above the plain. But the fact that this formation once had a thickness of more than eight hundred feet within the limits where we have had an opportunity of observing it, leaves no doubt that it must have extended to the edge of the basin, filling it to the same height throughout its whole extent. The thickness of the deposits gives a measure for the colossal scale of the denudations by which this immense accumulation was reduced to its present level. Here then is a system of high hills, having the prominence of mountains in the landscape, produced by causes to whose agency inequalities on the earth's surface of this magnitude have never yet been ascribed. We may fairly call them denudation mountains.
At this stage of the inquiry we have to account for two remarkable phenomena. First, the filling of the Amazonian bottom with coarse arenaceous materials and finely laminated clays, immediately followed by sandstones rising to a height of more than eight hundred feet above the sea; the basin meanwhile having no rocky barrier towards the ocean on its eastern side. Second, the wearing away and reduction of these formations to their present level, by a denudation, more extensive than any thus far recorded in the annals of geology, which has given rise to all the most prominent hills and mountain chains along the northern bank of the river. Before seeking an explanation of these facts, let us look at the third and uppermost deposit.
This deposit, essentially the same as the Rio drift, has been minutely described in my former article; but in the north, it presents itself under a somewhat different aspect. As in Rio, it is a clayey deposit, containing more or less sand, and reddish in color, though varying from deep ochre to a brownish tint. It is not so absolutely destitute of stratification here as in its more southern range, though the traces of stratification are rare, and, when they do occur, are faint and indistinct. The materials are also more completely comminuted, and, as I have said above, contain hardly any large masses, though quartz pebbles are sometimes scattered throughout the deposit, and occasionally a thin seam of pebbles, exactly as in the Rio drift, is seen resting between it and the underlying sandstone. In some places this bed of pebbles even intersects the mass of the clay, giving it in such instances an unquestionably stratified character. There can be no question that this more recent formation rests unconformably upon the sandstone beds beneath it; for it fills all the inequalities of their denudated surfaces, whether they be more or less limited furrows, or wide, undulating depressions. It may be seen everywhere along the banks of the river, above the stratified sandstone, sometimes with the river mud accumulated against it; at the season of the enchente, or high water, it is the only formation left exposed above the water level. Its thickness is not great; it varies from twenty or thirty to fifty feet, and may occasionally rise nearly to a hundred feet in height, though this is rarely the case. It is evident that this formation also was once continuous, stretching over the whole basin at one level. Though it is now worn down in many places, and has wholly disappeared in others, its connection may be readily traced; since it is everywhere visible, not only on opposite banks of the Amazons, but also on those of all its tributaries, as far as their shores have been examined. I have said that it rests always above the sandstone beds. This is true, with one exception. Wherever the sandstone deposits retain their original thickness, as in the hills of Monte Alegre and Almeyrim, the red clay is not found on their summits, but occurs only in their ravines and hollows, or resting against their sides. This shows that it is not only posterior to the sandstone, but was accumulated in a shallower basin, and consequently never reached so high a level. The boulders of Erreré do not rest on the stratified sandstone of the Serra, but are sunk in the unstratified mass of the clay. This should be remembered, as it will presently be seen that their position associates them with a later period than that of the mountain itself.