92G REPORT— 1898. 



of sucli erosion, at a point called Matucare, on the lower Madeira liiver. All the 

 other barriers which form the falls are varieties of granitic and metamorphic 

 rocks. 



About eighteen miles above the mouth of the Beni branch of the Madeira, and 

 below that of the Mayii-tata, is the fall of Esperanza, having a drop of 20 feet in a 

 length of 1,000. According to M. V. Ballivian, the rock is of canga, the same as 

 that of Guajaramirim. 



Pinkas found that, on the right bank of the upper Madeira River, several places 

 are still visible where the erosive action of the waters has stripped the primitive 

 rocks, anciently covered by a bed of ferruginous sandstone. 1 also saw evidences 

 of the erosive action of the river, near the upper falls, and am disposed to believe 

 that, in the lapse of centuries, one or two rapids, higher up the river, have 

 disappeared ; not, however, entirely, for a reef of ferruginous conglomerate still 

 partly crosses the Mamor^ River, between the mouth of the Guapor^ and 

 the town of Exaltacion. It is probable that the western Matto Grosso hills 

 once extended westward to the Beni fall of Esperanza, where they met the 

 Andean foot-hills, and added to the height of the barrier which prevented the 

 river system of Bolivia from breaking through to the north-east. How high that 

 barrier may have been it is difficult to determine, owing to the country being 

 densely forested ; but in the fork of the Beni and Mamor6 I found hills perhaps 

 150 feet elevation above the river ; and Keller mentions some on the left bank of 

 the Beni near its mouth. The rocks of Matto Grosso are so soft as to offer but 

 slight obstacle to the erosive power of the mighty flood of the Madeira ; and, if we 

 admit that the lip of its upper fall, when the river began to flow over it, was only 

 100 feet higher than now, it will be but a small allowance in comparison to the 

 depths which even insignificant streams have excavated over the immense area of 

 Matto Grosso. The falls of the Madeira appear to have completely cleared off and 

 exposed the granitic and metamorphic rocks upon which the Matto Grosso shales 

 and sandstones once rested. 



Immense quantities of detritus have found their way down the Andes. The 

 volume carried by the little river La Paz, the remotest branch of the Beni, is 

 astounding. I once descended it to ride round the base of Mount Illimani. The 

 river is so hemmed in between the material of the Titicaca basin and that monarch 

 of the Andes that 1 had to ford it 108 times in one day. It has cut a profound 

 gorge thi'ough the inland range perhaps 50 feet wide and 600 feet deep. The over- 

 hanging precipices looked not over 40 feet wide at the top. Through this dark 

 rent, which I had to penetrate or turn back, the river swept me, horse and all, as 

 if I had been launched from a catapult. 



The bed of detritus and alluvium which the river skirts for about 50 miles 

 is one of the most remarkable in the world. Forbes gives it a total thickness of 

 2,000 to 2,500 feet ' of alternating beds of grey, bluish and fawn-coloured clays, 

 gravel and shingle beds, with boulders of clay slate, greywacko and granite, ire- 

 quently of enormous size and well rounded, as if b}' the action of water ' In my 

 ride down the valley I saw Nature at work tearing into tliis depo.sit, and sending 

 it on its way to the great basin of the Beni and Slojos. During certain months, 

 generally from November to March, a prolonged and violent local storm may 

 arise in some lateral valley of the river. The waters then sweep impetuously 

 down, taking with them hundreds of thousands of cubic yards of material, which 

 they pile across the La Paz River. The mass of clay and boulders rapidly becomes 

 cemented and compacted, and holds its place until the La Paz in turn, swollen by 

 some storm from Illimani, bursts the huge dam and hurls its contents down the 

 valley. I noticed boulders of many tons weight, at least 300 feet above the bed of 

 the river, sticking, like half-exposed marbles, in the side of the perpendicular wall 

 of detritus which towered even high above them. 



Similar denudation is going on along the entire Andean slope. The great Rio 

 Grande carries a prodigious mass of alluvium into the gap which lies between the 

 extreme eastern counterfort of the Andes and the Chiquitos sierras. Even its little 

 branch, the Piray, which I descended, desolates the country far and wide, in periods 

 of flood, with trees, sand, and mud. The Rio Grande and the Parapiti must have 



