Vol. 62.] CATARACTS OF THE RIVER MADEIRA, ETC. 89 



Cordilheira Geral. At first it constitutes the watershed between 

 the tributaries of the Paraguay and those of the Tapajos, while 

 farther to the north-west it forms the north-eastern boundary of 

 the basin of the River Guapore or Itenes, which pursues its way 

 in a parallel direction for nearly 400 miles on the north-eastern 

 margin of the alluvial plain of South-Eastern Bolivia, until it unites 

 with the Mamore. The combined stream turns almost due north 

 across the line of elevation, and, at a height of nearly 500 feet 

 above the sea, its waters commence their descent in a long succession 

 of cataracts to the Amazonian plain. 



After passing the falls of Guajara-merim, Guajara-guacu, Bana- 

 neira, Pao Grande, and Lages (in Spanish i Layes '), the Mamore 

 arrives at its confluence with the Beni, which has already passed 

 the cataract of Esperanza. The joint stream now takes the name 

 of the Madeira, and flows first northward and then north-eastward 

 across the line of elevation, descending cataract after cataract, until 

 it reaches the settlement of Santo Antonio, at a height of only about 

 200 feet above the sea. Thence, after a winding course of some 

 500 miles through swampy forest-clad alluvium, it empties itself 

 into the Amazon. 



At the cataracts the rivers make their way through low ridges of 

 crystalline rocks, which form the subject of the present paper. 

 Where these are foliated their strike is, as a rule, north-west and 

 south-east, in the same direction as the ridges. The total width of 

 this hilly region is about 170 miles, although the rivers in their 

 windings cover a much longer distance in traversing it. 



The hills continue to the north-westward beyond the cataracts, 

 but how far they extend is not known. We are told of falls on the 

 Eiver Ituxy, a tributary of the Purus, which flows on the north- 

 west of, and more or less parallel to, the Madeira, so that the line 

 of crystalline rocks probably reaches at least thus far. There is also 

 a rapid, known as the 'Cachoeira,' on the Purus, still farther in the 

 same direction, which may perhaps be referred to the same cause. 

 It is noteworthy, too, that the Purus, the Tarahuaca-Jurua, 1 the 

 Yacarana or Yavary, and the Ucayali-Maranon, all have a change of 

 direction corresponding to that of the Mamore-Madeira at the 

 cataracts, and that the points at which these changes of direction 

 occur are in continuation of the line of strike of the crystalline 

 rocks of the cataracts of the Madeira. 



This may be explained by the fact that the general inclination of 

 the country on the borders of Brazil, Bolivia, and Peru is slightly 

 to the east of north, while the elevation that forms the cataracts 

 runs obliquely across it: the result being that, when the rivers reach 

 the ridge, they are diverted down the slope north-westward until a 

 point of weakness or inferior elevation is reached, by which a passage 

 is effected. 



It is true that there is no record of hard rocks having been 

 observed on most of the rivers that I have mentioned, but they 



1 That is, the stream formed by those rivers successively. 



