288 PHYSICAL GEOGRAPHY OF 



the small streams between the Tocantins and Xingu, meeting 

 together about Melgdco, 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 Pard, 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 Guamd and Mojii, 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 Purus, Coari, Teffe, 

 Jurua, Jutai, Jabari, led, Japurd, 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 



