Vol. 6 2.1 CATARACTS OF THE RIVER MADEIRA, ETC. 103 



of the Amazon farther to the north-west, in the manner that I have 

 described. 



At the next cataract, that of Pederneira, the rock (M6) is 

 a strongly-foliated gneiss, consisting of pink felspar with a 

 dark-green constituent. Where the latter is predominant, the 

 felspar occurs in small porphyritic eyes. The strike is usually about 

 north 60° west, but in places it is due east and west. At one point 

 there is a narrow vein of chalcedonic quartz, from which flints for 

 use as knives and for striking fire were formerly obtained. The 

 name of the cataract refers to these flints. 



Just below the cataract, the Madeira receives on the left the Bio 

 Arapongas or Perreiros, which appears to flow in a longitudinal 

 valley. The river then turns to the east. I was told that on the 

 Arapongas were slaty rocks with quartz-reefs. Col. Church (' The 

 Boute to Bolivia via the Biver Amazon ' 1877, p. 188) also states 

 that he recognized slates on the main stream. He has kindly 

 furnished me with the following extract from his journal : — 



' On the western extremity of the great bend I find a 30-foot bluff of slate. It 

 is probably this slate-formation which catches the river after it shoots so far 

 to the westward, and turns it at a right angle to the north-north-east.' 



I did not see these slates ; but the river is wide, and the course of 

 my canoe was near the left bank. 



A microscopical examination of the rock of the cataract shows a 

 highly-foliated structure. There is much plagioclase that has 

 a crushed appearance ; it is usually untwinned. Microcline occurs 

 in large comparatively-unaltered crystals, which sometimes contain 

 microperthitic inclusions. Orthoclase is also present. One large 

 crystal, apparently a Carlsbad-twin of microcline, but with the 

 characteristic twin - striations very faintly developed, includes 

 numerous crystals of andalusite, which have dark centres comparable 

 to those of chiastolite ; thin plates of a hydrous mica, and rod-like 

 forms which may perhaps be referred to sillimannite, also occur. 

 There is abundant quartz, especially along certain planes, with 

 irregular interlocked boundaries. Hornblende and biotite occur in 

 lenticles, as well as occasional crystals of sphene with irregular 

 boundaries. The formation of the andalusite in this rock is, no 

 doubt, due to metamorphism similar to that which causes the 

 development of chiastolite in a slate. 



From Pederneira we could see to the south-east the hills on the 

 left of the river, near the cataract of Paredao, so called from the 

 wall-like outcrop traversed at that point. l 



The rock is a fairly-coarse gneiss, the felspars in some places 



1 Dr. Fonseca describes (' Viagem ao Eedor do Brasil ' vol. ii, 1881, p. 289) 

 the mass or platform on the right bank as having a width of 126 metres. ' This 

 mass,' he says, ' is a most magnificent specimen of rock, with its beds one above 

 the other revealing the state of liquefaction in which they were deposited, 

 like a great outpouring of honey thick and ready to crystallize, which moves 

 slowly, sliding on in broad sheets above beds already solidified. This shows 

 that either the crystallization was very rapid, or the streams of the material 

 in fusion were very sluggish in their flow. But, closer to the river, the rock 



