Vol. 62.] CATARACTS OF THE RIVER MADEIRA, ETC. 93 



compact eurite rise to form other crags : either by breaking through the 

 metamorphic crust, or because they are enveloped by decomposed gneiss the 

 surface of which, now eroded by lapse of time, was formerly at the same level 

 as that of the dykes.' 



1 In the large cauldrons, which are dry, there are not infrequently con- 

 glomerates of small fragments of dioritic rocks, principally black diorite, which 

 appeared to me to be cemented with hydrate of iron.' 



' I brought back with me some specimens of the most notable of these rocks, 

 as well as of the pebbles inserted in the cracks of the pavements, where a 

 secondary process of agglutination with the sand of the river and the clay 



which it carries in suspension, forms a puddingstone These sedimentary 



rocks are rare in the localities where the stream runs strongly, but very frequent 

 in the backwaters.' 



In one of the cataracts, he believes that of Bananeira, he found a 

 piece of petrified charcoal formed of bright shining lamellae, which 

 is now in the Museum of the Archaeological Institute of Alagoa. 



It will be seen from the following pages that, although I have 

 been able to confirm some of Dr. Ponseca's statements, I never saw 

 any rocks that could be classed as trachytes, or that showed any 

 evidence of having flowed at the surface. 



III. Geological and Petrolooical Observations. 



(1) Between the Andes and the Cataracts. 



In the year 1902, when returning from Bolivia by way of the 

 Beni and Madeira, I had an opportunity of examining the rock- 

 exposures on those rivers. 



Leaving the Bala-Susi Mountains, the last outworks of the 

 Andes, behind me at Rurenabaque, I descended the Beni until I 

 reached, twenty days later, the ' rubber-metropolis ' Biveralta, 

 at the mouth of the Manutata (Madre de Dios). Throughout 

 this distance (some 220 miles in a direct line, and 470 following 

 the stream) the Beni flows in innumerable meanders through a wide 

 forest-plain, between 500 and 600 feet above the sea. For the 

 greater portion of the distance the ground is scarcely raised above 

 the water, although the margin of the river is usually marked by a 

 low bank or levee forming a natural breakwater, which, however, is 

 often flooded when the river is high. Beyond are lagoons repre- 

 senting abandoned reaches of the river. At a greater distance from 

 the Andes there are, however, tracts of rising ground, occasionally 

 as much as 50 or 60 feet above the level of the stream, which some- 

 times cuts into them, forming well-marked cliffs. These elevations 

 are built up of argillaceous or fine sandy materials with varying 

 amounts of iron, which is sometimes present in such quantity as to 

 constitute a hard stone. With this exception there is no solid rock, 

 either in situ or in fragments larger than fine sand-grains, 1 below 

 the rapid of Altamarani, some 10 miles below Burenabaque, where 

 the river, slackening its pace after leaving the mountains, has 

 thrown down a thick bank of more or less rounded fragments. 



7 Even this sand is remarkably poor in heavy minerals. 



