Page:Science vol. 5.djvu/322

 [Vol. v.. No. 1M

��lulter Uowi m mi ekvatloci of only 123 inetn-s at Cujftbit. neur its source), afford uninterrupted naviga- tion for almost tliclr entire course. The triliutariss of these ivii rivers, aud the otiier Brazilian rivers in general, are, however, plateau streams, inni liave two navigable portions, — one on the upper sireaiu on Ibe plateau; and the oilier in the ilepresslon, or coast border region. The difference of level between tliete two portions is one or more hundred mntrea, anil the descent hits to be accomplislied by a. series of cata- racts situated nt a relatively short distance abovi- the mouth of the river. Of these, the most notable are the Paulo Affonso cataract on the SSo Francisco, and tbe Sete Quedns ('seven falls') on the PuraniL The upper tributaries of the Amaionas, between tiif foot of the Andes and the Kio Negro on the north, and the Sladeira on Llie south, are exceptions to tbis rule; since they descend from the plateau on which they rise in their upper courses, and afford long lines of navigation. Thpy thus reveal Ibe interesting fact that a vast area of the almost unknown upper Ama- zonian region is at a much lower level than the adjs^ cent plateaus. Of the rivers that flow directly m the Atlantic, those of the province of Maranhoo and the Pamahyba, in Piauhy, otter the greatest facililles for navigation ; because they rise at a lower level than the rivers to the southward, and effect their descent to the sea-level fay a gradual slope distributed along the whole course, instead of being concentrated in one or morp grand series of rapids.

The .\iDa2unas and Paraguay present peculiar fea- tures in the very extensive alluvial plains that border the main river and tlia lower courses of their Irihu- tarles. and in the great number of anastomosing lat- eral channels that cut these plains and put the main river in communication with the tributaries, often al long distances above the junction, and these last with each other. These canals are particularly notable in the case of the Amazonas, where they are called [larn- nnmlrina, or .furo* ; ' and it is said that a boat may traverse almost the entire length ol the Amaionas valley without entering the main stream. The for- mation of these canals is to be attributed in part to the formation of alluvial islands that are cnnstanUy being created by the sediment- loaded waters of the great river. The number and character of many of them, anil especially of the I'uron, seem, however, to Indicate a more general ca'ise, and suggest the idea, that, since a relatively slight <lcpression of the sur- face would transform the river-valley Into an estuary. It may reasonably be supposed that at some time a correspondingly slight elevation has transformed an estuary Into a river-valley. The long distance to which the Influence of the tide (It Is sensible at Oby- dos five hundred miles above the mouth of the Ama- ( felt, gives an air of probability to this hypolhesis. In this ca<n cnnali Umi r.'tum lo Ihc <mmt river TiMm

��a IriplA _ iilfltalnii^ ParaniJH I'arui^H Llthouah ^

��The paranamiriHi would tri this liypotbesia sent the marhie channels of the muddy bottom the estuary. It is certain that the lower port! of the Amaznnas valley still presents so much the character ot an estuary, that a question In arisen among geographers as to whether the TocJU' tins should be considered as a tributary, or an Inde- pendent river. The fact that It receives a coaslderabte iiuantity of water from the Amaitoiiai throogb vari- ous fiU'oa decliies the question ot lis being a tribu- tary, since the Amazonas contributes much more wnler than the Tocantins to the so-called ParS Biver, which is only the southern branch of the great Ama- zonian estuary.

Attention has often been called to the curious fact, that, unlike most large sediioent-loaded riven, the Amazonas lias no delta. The reason appears to be, that Its lower course la still in a transition slate be- tween the estuary and fluvial conditions; and the delta Is not to be looked for at what is generally con- sidered as the mouth of the river, but higher np at the head of the estuary. In this case the network of canals between the mouth of the Xingii and th* western end of the island of Marajo may be consid- ered as the true delta.

In a certain sense, the La Plata basin is a Iriplo, one, since a slight change of level, which shouM the head ot the estuary to the month of Iho Pj would effect the separation nf the Paraguay. 1' and Uruguay ns three distinct basins. Although smaller than the Paranii, the Paraguay should be considered as the main stream on account ot il« rela- tions with the elevateil portions of the continent Iv the east and west. The Parana, as already statodf Is essentially a hlt:hland river. Its tributaries lit into it before it enters the depression by the gi caUract of Sete Quedas. The only excepUon ~ Iguassii, which has its great cataract of Santa Marik close to the junction. A peculiarity of the Paruui is that the eastern mai^ln of its basin is so close lo (he Atlantic, that one of Us tributaries, the Tiet^, aimj be said lo rise in sight of the sea. Another peculiar feature Is the tendency of its eastern tributaries, especially marked in the case of the Tiet£, to flow In a north-westerly direction, as if they were seeking the source, rather than the mouth, of the main river. This indicates a general norlh-we.itcrly slope tfl this part of the plateau.

A few peculiarities in the principal rivers of Ihe direct Atlantic drainage system, which indicate Inter- esting points in the lopusraphlcal sti country, may tie mentioned here. Such a point is I general parallelism ot the Sao Francisco line in the greater part of its course, due to Ihe rl' lieing confined behind theSorrado i^^plnhavo, whickil terminating to the north wanl, finally permits the ri*«c^ to escape, and direct its courretowai ~ a right angle to lis former course. The same pbeooni' enon Is presented In n still more interesting manner by the Pamhyba, which exhibltsadouble parallelism, the river making a [T-curve iu (he upper part of its course, and, after a course of about two huudietf miles, pa^sin^- clu*e by [is ^oui'i'i-. This Is due to a

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