On the heights of the Cordillera the process is already at work. The waste of the mountain-slopes, broken off by rills and torrents, and carried by them into the main river, slowly drifts down-stream in the form of gravel-banks, until, scattered and rent asunder in a thousand ways, it finally takes permanent form as light-green islands, which are soon covered and protected with a dense coat of vegetation. As every zone of geologic formation in the extensive valley adds its tribute, these banks are a kind of mineralogical collection, which shows samples of all the rocks on the river-banks, with the exception, perhaps, of light pumice-stone, the produce of the volcanoes of the Andes, which drifts down-stream in large pieces, and is highly prized by the Tapuia population (on the lower course) for sharpening and cleaning their weapons and tools. Even when not picked up by hunter or fisher, it is not lost. It will be arrested by some snag or projection of the shore, it will so get embedded in the newly-forming sediment, and thousands of years hence its silicic acid will afford the necessary material for the hard glassy bark of a bambusacea, or the sharp edge of a reed. When the currents are not strong enough to move the larger banks, they at least carry sand and earth with them, and deposit them as shoals or new alluvion at less exposed spots.... The undermined concave shores are sometimes a serious danger to the passing barque, as even the slight ripple of a canoe is sufficient to bring down the loosely overhanging earth, often covered with gigantic trunks. These concave sides, with their fallen trees and their clusters of sinking javary-palms, supported sometimes by only a tangled net-work of tough lianas, give to the scenery that peculiar character of primeval wildness which is so charming to foreigners. When one has climbed up the steep shore, often forming huge terrace-like elevations, and has safely passed through a labyrinth of interwoven roots and creepers into the interior of the forest, which is getting freer from underwood at some distance from the river, he is oppressed with the sensation of awe and wonder felt by man on entering one of the venerable edifices of antiquity.