550 Br. John Wm. Evans — Recent Breccias in Bolivia. 



connected with this season, and took the form of snow." He tlionglit^ 

 however, that in some cases " hot summers and occasional torrential 

 rainfalls " might produce the same effects. 



I have recently had an opportunity of examining an extensive 

 breccia at the foot of the Andes and margin of the Amazonian plain, 

 "which is still in process of formation under somewhat different 

 conditions. 



In north-eastern Bolivia the Andes consist of numerous parallel 

 ranges trending north-west and south-east. At the point where 

 the river Beni enters the great forest plain the outer or north-eastern 

 chain is that to which I have referred ^ as the Bala-Susi mountains. 

 The highest summits rise to about 5,000 feet above the sea, but some 

 of the saddles do not exceed 2,000 or 2,500 feet in altitude. 



The lower slopes are clothed with trees, and from their foot, at an 

 altitude of 650 to 1,500 feet, commences a gentle incline. This is 

 covered with thick forest, and extends outwards for two or three 

 miles, till, at a height of from 570 to 700 feet, it reaches the plain 

 which stretches with scarcely a break to the mouth of the Amazon. 



The mountains are composed mainly of sandstone, and the incline 

 to which I have referred is largely made up of a breccia of the 

 same rock, while the lower ground beyond is a tract of alluvial 

 sand and mud. 



The climate is moderately warm, the highest maximum at the foot 

 of the mountains in the months of December and January, 1901-2, 

 being 92-5" F., the average maximum 85°, the lowest minimum 632°, 

 and the average minimum 71-5°. The average pressure of aqueous 

 vapour was -76". The total rainfall in the year is probably about 

 90 inches, of which the greater part falls from October to April, 

 not continuously, but mainly in torrential showers brought by soulh- 

 eastern storms. There is no season completely free from rain. 



The rivers that flow from the mountains to the plain rise in 

 a lofty longitudinal valley, break through the north-eastern hills 

 at intervals in narrow gorges, and after a steep descent reach the 

 inclined surface of which I have spoken. I followed the Eio de 

 Tumupasa through almost all its course ; its length from its origin 

 to the point where it leaves the mountains is rather more than three 

 miles, during which it descends over 1,200 feet. Its bed then forms 

 abroad road through the forest, with an irregular surface of fragments 

 of rock of all sizes, from coarse sand-grains to angular blocks several 

 feet in diameter. There are few rounded pebbles, though the edges 

 of the fragments are in many cases blunted by friction. 



Through this rocky tract flows at ordinary times only a small 

 stream, and even that is found to disappear under the debris as 

 the river is descended, leaving the broad band of rocks to continue 

 its way down the almost imperceptible incline till, at a distance of 

 two miles from the foot of the hills, it reaches the low-lying plain. 

 Here it spreads out and divides into short blunt branches, and 

 thus abruptly comes to an end. 



^ Geographical Journal, xxii, p. 601 (1903). 



