In February of last year Professor Bonney read an instructive paper before the Geological Society, in which he enumerated and briefly described a number of the principal breccias found in the geological series. He also gave a short description of some of the best known recent breccias—the stone rivers of the Falkland Islands and the angular detritus that fringes the foot of the hillsides in Persia, Central Asia, and Northern Hindustan. In conclusion, he expressed his opinion that most of the ancient breccias referred to in his paper indicated “a climate, arid, liable to extremes of temperature, with cold winters. The precipitation probably was connected with this season, and took the form of snow.” He thought, 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.