Vol. 62. J CATARACTS OF THE RIVER MADEIRA, ETC. 105 



have taken place to some extent under water, although it is very 

 doubtful whether such is the case. 



From the cataract of Paredao to that of Trez Irmaos (the Three 

 Brothers), crystalline rocks occasionally appear in the river-bed, and 

 hills are usually visible, mainly on the left (north) of the river, which 

 has no doubt been gradually shifting its course to the convex side of 

 the bend, while a tract of alluvial ground has been left on the concave 

 margin. 



The cataract of the Trez Irmaos owes its name to three hills 

 on the left of the river, in continuation of the ridge which is traversed 

 by the Madeira at this point. 1 We passed without landing, and 

 continued our way down the river in a roughly north-east direction 

 to the cataract known as Sal to do Grirao (Leap of the Whirl- 

 pool). Here the river, which had expanded shortly before to a width 

 of 2| miles, is contracted by hills, and passes over a fall 25 feet high 

 in two channels, one 300 yards and the other only some 10 yards in 

 width, when the water is low, as it was when I saw it. 2 



The rocks exposed in the falls were, so far as I was able to examine 

 them, fairly uniform in grain, showing a reddish-brown felspar with 

 a few dark blebs of quartz, apparently without a trace of schistose 

 structure (M 7, PI. V, fig. 3 ; specific gravity = 2*58 to 2-63). 



Under the microscope large crystals of plagioclase-felspar are 

 seen, sometimes measuring as much as from 2 to 3 millimetres in 

 diameter. In some cases they are traversed by a labyrinth of opaque 

 or semi-opaque material, grey or reddish-grey by reflected light. 

 This structure shows everywhere rectilinear outlines, parallel to 

 the two directions of the lamellar twinning which is visible, in the 

 clearer less-decomposed portions, between crossed nicols. The 

 whole structure resembles masonry of stones of various sizes, but 

 with their edges parallel in two directions. The extinctions in the 

 zone of the macrodiagonal approach 20°, while the refractive index 

 is distinctly lower than quartz, so that the felspar appears to be 

 albite. Some felspars show only traces of twin lamellation, and still 

 others, although full of small areas where the relative retardation 

 is somewhat less than elsewhere (presumably on account of incipient 

 decomposition), extinguish quite uniformly ; they are, therefore, 

 probably ortboclase. 



There are a certain number of large porphyritic quartz-grains, 

 which appear to have been attacked by the magma after their 

 formation and rounded or eroded into cavities. 



1 ; On the river Trez Irmaos or Mutum Parana-,' says Col. Church (' The 

 Route to Bolivia via the River Amazon ' 1877, p. 188), ' I found the granite- 

 rock cropping out at several points, but nowhere above the bank of the river 

 to the point we ascended, a league up.' 



- Col. Church (loc. cit. & Diary) states that his party caught sight of five 

 little detached hills, from 80 to 150 feet high, forming a group about a mile 

 east of Girao, but saw nothing farther eastward or north-eastward. One of 

 his men told him that, some years before, he had penetrated beyond these hills 

 2 or 3 miles farther east, and that the country now and then had a gentle 

 rise as high as his head, but was virtually a great plain extending northward 

 and southward. 



