Vol. 62.] CATARACTS OF THE RIVER MADEIRA, ETC. 99 



independently, and as inclusions, which in size and shape recall the 

 minerals of a granulite. 



I did not ascend the Mamore any farther. A. d'Orbigny, 1 who spent 

 a few days at Beira on the right bank of the Guapore or Itenes, some 

 distance above its junction with the Mamore, found, on the south- 

 western flank of the Serra dos Parecis or Cordilheira Geral, hills formed 

 of friable sandstones which were very ferruginous, and generally 

 red in colour. They were of great thickness, and dipped south- 

 eastward 2 at an angle of 12° or 15°. These sandstones, which 

 appeared to extend far to the north, were covered in the neigh- 

 bourhood of the river by ferruginous conglomerates containing much 

 oxide of iron, and forming perfectly-horizontal beds. 



Dr. Fonseca states 3 that, at the cataract of Guajara-merim, a 

 chain of rocks some 150 metres broad traverses the river from 

 side to side, expanding on its margins into two enormous pave- 

 ments of dioritic appearance. In some places there is, he says, a 

 porous formation of a kind of varnished can ga (ferruginous quartz- 

 conglomerate) ' resembling phonolite ; ' while on the bank, amid the 

 vegetation, ' argillotalcose schists ' without visible stratification 

 are exposed. He also states that, above the cataract of Lages, at 

 the mouth of the river of the same name, the rocks are covered 

 by sands resting upon dark- grey clay with ' nuclei of silex' (op. cit. 

 p. 270). 



According to Col. Church (' The Route to Bolivia via the Eiver 

 Amazon' 1877, p. 187) the upper rapids are composed of ferru- 

 ginous conglomerate, the surface of which is as black as ink, and 

 he quotes the following from a report by the Kellers : — 



' The ferruginous conglomerate which is found on the surface of the earth, 

 only covered with a bed of clay of from 5 to 6 metres of thickness, is a con- 

 glomerate of gritstone, little pieces of dolerite cemented with oxide of iron, 

 full of openings and cavities which give it the appearance of a sponge or scoria. 

 Its beds are generally horizontal, and are from 4 to 5 metres thick. In the 

 inferior beds the seams are smaller, at some points disappearing entirely, and 

 forming then a more homogeneous mass of red gritstone, very argillaceous.' 



They declare that this formation extends over more than 12 degrees 

 of latitude. Col. Church (loc. cit.) further states that the rock of the 

 Bananeira rapid is 



'distinctly granitic, but with much iron disseminated. The surface of the 

 rocks, wherever the water has been flowing over them, is blacker than ink .... 

 the hornblende, feldspar, and quartz are well disseminated in the rock.' 



(4) The Cataracts of the Madeira. 



Immediately below the confluence of the Beni and Mamore is the 

 cataract of Madeira, where the river of the same name flows 

 rapidly for 2 miles between rocky islands, the total fall being over 

 8 feet (2| metres). 



1 'Voyage dans l'Amerique meridionale, execute pendant les annees 1826-33, 

 vol. iii, pt. 3 (Geologie) Paris, 1842, p. 203. 



2 This is not the dip that would have been expected. Possibly it may be a 

 verbal mistake for south-westward. 



3 'Yiagem ao Eedor do Brasil' vol. ii (1881) p. 263. 



i2 



