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Page:Popular Science Monthly Volume 38.djvu/223

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away; but all this is the work of a rapid current, for the surf of the pororóca does not reach Macapá. Moreover, there is a marked difference in character between the washing done by the pororóca and that done by the ordinary river or tide current. The latter works from below, and, by undermining and softening the bank, causes what is known through the Amazon Valley as terras cahidas, or fallen banks. The land falls into the stream in sections of various widths, and not infrequently these form temporary terraces miles in length. These terras cahidas are most common and most extensive on the upper Amazon during high water; but they may be seen on a small scale at various places through the valley.[1] From this it is clear that the work of destruction goes on entirely below the surface. With the pororóca, on the contrary, the water is dashed fairly against the banks, the earth is washed away from above as well as from below, and the shore is left clear of loose débris. The depth to which the banks are cut shows that this disturbance is also a profound one; so much so, indeed, that on the northwest side of Porquinhos the deepest place in the channel of the river was, in 1881, close to this island, where the action of the pororóca was most violent.

Throughout this region of the Araguarý the pororóca is largely instrumental in the rapid and marked changes that are constantly going on. The water of the Amazon is notoriously muddy, and, as would naturally be expected, these disturbances in comparatively shallow places make it much more so, and fill it with all the sediment it can possibly carry. Even when I entered the Araguarý, a time when there was the least possible tidal disturbance, the water near the mouth of this stream was so muddy that a thick sediment would settle in the bottom of a vessel of it left standing a single minute; though the water of the Araguarý proper, as far down as the Veados, is of a clear, dark color. But the work of tearing down and that of building up is equally rapid, and the vegetable world takes quick possession of what the sea offers it; and, while some islands are being torn away, others are being built up, old channels being filled, islands joined to the mainland, and promontories built out. To the northwest of Faustinho is an island known as the Ilha Nova (New Island), about ten miles long by about three wide, when I saw it, and which, I was assured by several trustworthy persons, did not exist six years before. In 1881 it was covered by a dense forest. The young plants were sprouting at the water's edge, those behind were a little taller, and so on; so that the vegetation sloped upward and backward to a forest from twenty to thirty metres high in the

  1. For a good description of the terras cahidas, see The Naturalist on the Amazon, by Bates, fifth edition, p. 249.