2S8 PHYSICAL GEOGRAPHY OF
the small streams between the Tocantins and Xingu, meeting together about Melgaco, and flowing through a low swampy country in two directions, towards the Amazon, and towards the Para - river.
At high tides the water becomes brackish, even up to the city of Para, and a few miles down is quite salt. The tide flows very rapidly past Para, up all the adjacent streams, and as far as the middle of the Tagipuru channel ; another proof that a very small portion, if any, of the Amazon water is there to oppose it.
The curious phenomenon of the bore, or " piroroco," in the rivers Guama and Moju, I have described and endeavoured to explain in my Journal, and need not now repeat the account of it. (See page 89.)
Our knowledge of the courses of most of the tributaries of the Amazon is very imperfect. The main stream is tolerably well laid down in the maps as far as regards its general course and the most important bends ; the details, however, are very incorrect. The numerous islands and parallel channels, — the great lakes and offsets, — the deep bays, — and the varying widths of the stream, are quite unknown. Even the French survey from Para to Obidos, the only one which can lay claim to detailed accuracy, gives no idea of the river, because only one channel is laid down. I obtained at Santarem a manu- script map of the lower part of the river, much more correct than any other I have seen. It was, with most of my other papers, lost on my voyage home ; but I hope to be able to obtain another copy from the same party. The Madeira and the Rio Negro are the only other branches of the Amazon whose courses are at all accurately known, and the maps of them are very deficient in anything like detail. The other great rivers, the Xingu, the Tapajoz, the Puriis, Coari, Teffe, jurua, Jutaf, Jabari, lea, Japura, etc., though all inserted in our maps, are put in quite by guess, or from the vaguest information of the general direction of their course. Between the Tocantins and the Madeira, and between the Madeira and the Uaycali, there are two tracts of country of five hundred thousand square miles each, or each twice as large as France, and as completely unexplored as the interior of Africa.
The Rio Negro is one of the most unknown in its charac- teristic features ; although, as before stated, its general course
