Vol. 62.] THE ROCKS OF THE MADEIRA. CATARACTS, ETC. 95 



After a short stay at Riveralta, literally ' high bank,' which 

 stands on a cliff such as I have described, I ascended the Manutata, 

 and thence travelled overland to the Orton. Between the two 

 rivers is a narrow plateau, extending for many miles in the 

 direction of its length, and raised some 40 feet above the level of 

 the lower forest. Though it is possible that these tracts of com- 

 paratively-high ground may in some places be formed of hard rocks 

 decomposed by lateritization, there can be little doubt that, in the 

 great majority of cases, the materials of which they are composed 

 are of fluviatile origin, and were laid down by rivers under condi- 

 tions similar to those which now exist. They are, therefore, in all 

 probability the remains of the former surface of an ancient alluvium, 

 the rest of which was removed by the present rivers, whose level 

 has been lowered by the wearing-away of the barrier at the 

 cataracts. I was unable to see a^' definite evidence of local changes 

 of relative level in these deposits, which may be provisionally 

 referred to the ' Neogene ' of Dr. Eriedrich Katzer. 1 



I followed the Orton to its confluence with the Beni, and subse- 

 quently descended the latter river. 



(2) The Cataract of Esperanza on the Beni. 



A short distance below the mouth of the Orton the Beni passes at 

 the ' Correnteza/ or rapid, a line of rocks, said to be thirteen in 

 number, projecting here and there above the water, and having a 

 general north-westerly and south-easterly direction. Although there 

 is no perceptible fall at this point, the current runs with considerable 

 force, 2 and it was impossible to stop the boat to examine the rocks. 

 This rapid forms the cabecera or head of the cataract of Esperanza. s 



A mile or two farther down stream the cataract itself is reached, 

 where the river passes a number of ridges of crystalline rocks which 

 form low elevations in the forest on either side. The principal fall 

 has a height of 9 or 10 feet, and below it are rapids which con- 

 stitute the coda or tail of the cataract, where the river passes yet 

 another ridge of rock. 



Megascopically the rock (M l) 4 of the Esperanza Cataract shows a 

 granular mixture of pinkish felspar, with a little greyish quartz and 

 specks of a black mica, the latter, however, being mostly aggregated 

 into parallel lenticles or irregular masses. The specific gravity is 

 2*633. The strike of the foliation is fairly constant, in a west- 

 north-westerly and east-south-easterly direction. 



1 'Grundziige der Geologie des TJnteren Aruazonasgebietes ' 1903, p. 108. 



2 In 1897 an island at this point was swept away by a flood. 



3 So called from the words of the devoted Indian who accompanied Dr. Heath, 

 the first European to descend the Beni from the Andes to its mouth : ' Then 

 let us call this cataract Esperanza, for now that we have passed it we have 

 hope of life.' \ 



4 Numbers in parentheses preceded by the letter M refer to the rock-specimens 

 and microscope-sections now in the Mineialogical Department of the British 

 Museum (Natural History). 



