It is composed of clays and of beds of conchiferous terrestrial limestone, which have supplied the lime-kilns for more than a century.

Photograph by Boote.

Plate XXI.

[Click to view larger image.]

But the different scenes of the river region are most of all due to different conditions of erosion and formation. Above Rosario the configuration is due to floods. Each succeeding flood alters it and leaves some trace of itself in the topography. The beds of sand that it lays down are fixed by rushes and floating weeds, then by willows (Salix humboldtiana). This screen of vegetation encourages accretion, and the edges tend to rise higher. In the middle of the island are low, marshy lands. The irregularity of the alluvial deposits causes marked undulations in the whole region of the river, and everywhere gives rise to alternate beds of clay and sand. Below Rosario the river gradually loses its power. The islands become more stable and flatter. Clumps of willow and spiny ceibos (Erythrina cristagalli) still cover the edges of them, and sometimes spread over the interior. But as the climate is now less humid, the vegetation fixes the soil less firmly, and the wind becomes the chief sculptor of the landscape. It heaps up the sand during the low-water season, and makes dunes which rise above the level of the greatest floods. These dunes form an unbroken line along the land in the southern part of Entre Rios, in the north of the main arm, with ridges at right angles, advancing toward the south, which rest upon the river clay; like the one which the Ibicuy railway follows across the floodable area. The cattle of the district take refuge on the dunes during floods. During periods of drought, on the other hand, they retain a quantity of water, and this is drawn from surface-wells at their base.

The limits of the zone of the river are clearly marked on the whole of the lower Paraná. It is enclosed on both sides by high barrancas (cliffs), vertical in places where the main current washes their feet, but sloping slightly where there is only a secondary arm with little erosive power. The cliff is broken only at the confluences of small valleys, the flat, filled-up bottoms of which are on the level of the alluvial plain of the Paraná. The cliffs are at their highest in the district of Villa Paraná, where they rise in places to a height of 300 feet. On the right bank the cliffs show a section of the upper layers of the Pampean clays. On the left bank there are æolian clays only at the top of them. Below these are Tertiary marine strata (marls and sandstones with beds of shells). The cliffs of the left bank stretch northwards, with a few breaks, as far as Corrientes, and even into Misiones. Their height gradually diminishes, and the Tertiary marine strata are replaced by granitic red sandstone.[125] On the right bank the height of the cliffs gradually diminishes up river. They are still conspicuous at the confluence of the Carcaraña, but at Santa Fé they rise only about thirty-four feet. North of 31° S. lat., and for some distance beyond Pilcomayo, the plain of the Chaco is very low, and it is impossible to define exactly the limit of the alluvial zone of the Paraná. The fine clays, grey and white, which form the soil of the Chaco, reach the left bank north of Corrientes, in the esteros of Neembucu. The red sandstone hills of the Asunción district rise like an archipelago out of this level bed of lacustrine deposits.

There is no obstacle to navigation in the entire stretch from Posadas to the falls of the Guayra on the Paraná and the Salto Grande on the Yguassa. Sixteen miles below Posadas the Paraná passes through a series of graduated rapids for about sixty miles (1,467 kil. to 1,558 kil. from Buenos Aires) wrongly called the Salto de Apipé. The current then rises to a speed of eight knots, and the depth is three feet at low water. These rapids are due to beds of melaphyre, which emerge amongst the granitic sandstone, and the water makes its way between large rocky islands. At Ituzaingo (1,455 kil.) the current loses force. There is, however, still a rocky bottom lower down, for ninety miles, at a depth of five feet. Below this the rock only appears on the left bank, and in a few ridges near the bank, or in isolated reefs which it has been easy to mark with buoys.

From Corrientes to La Paz the river flows from north to south at the feet of the Corrientes cliffs. These line the main stream between Corrientes and Empedrado, and for thirty-five miles south of Bella vista. In the latitude of Riachucho, especially about Bellavista, the cliffs form a series of creeks and capes, in which the west winds create a heavy sea that was dreaded by ships of light draught coming down the river. North of Bellavista, and for more than a hundred miles south of Goya, the main stream is separated from the cliff by a series of alluvial islands; behind these are lateral arms (riachos) into which pour the rivers of Corrientes. These arms were much used by the early navigators.

Between Esquina and La Paz the main bed, which is not in touch with the land on either of its banks, flows in a meandering path for some seven miles, the scale of the bends being double that of the meandering of the Paraguay north of the confluence. The islands are very small, and are strung in a rosary at the top of each bend. The depth is sixty feet at the top of the bend. The shallows are in a line with the islands at the point where the current runs evenly again before the next curve. The depth here is seven, and sometimes even five feet.[126] These shallows change their places quickly, and it is not always the same bad spot that determines the maximum draught for ships that are to be used in this section. This migration of the shallows is very different from the permanence of the rocky bottom of the stretch between Corrientes and Posadas.