56 Prof. T. G. Bonney—Rounding of Alpine Pebbles. 
books which may be thus extended theoretically for pebbles formed 
of an average rock. 
Diameter of pebbles Miclocitrotistream 
just moved. 
meh yess ce eeraae 2 feet per second, 1°3638 miles an hour. 
2oinches seer 2-82 feet ,, 
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Ei any andtiodssdacasat COONS ees 4:0914 ,, 30 
Hence we may infer that a deposit in which fairly well-rounded 
pebbles of about 4 inches diameter are so common as to be 
characteristic, is the result of a stream which flowed pretty steadily 
at a rate of about 2% miles an hour, and one in which they are about 
8 inches or 9 inches diameter, is the result of a stream flowing about 
4 miles an hour. It must, however, be remembered that this 
would be the velocity at the bottom of the stream, which is estimated 
at about half that of the surface in the middle. Great variability in 
the size of the pebbles, and especially the presence of frequent 
boulders or subangular blocks much exceeding the average size,—as, 
for example, a mixture of blocks something like two feet in diameter, 
with pebbles generally not more than one foot in diameter, and 
commonly less,—is indicative of torrential discharge. 
For brevity I use the term ‘ Alpine rock’ to mean gneisses, more 
or less granitoid, and mica-schists of various kinds, together with 
hornblendie or chloritic schists, and possibly serpentines—that is to 
say rocks, which are in all cases crystalline, and in some at least of 
lgneous origin. 
Grovp I. 
Bed of Romanche above Villard d’ Aréne (Dauphiné).—This stony 
plain lies at a height of about 5000 feet above the sea—at the foot 
of a steep descent of 1500 feet. The river is fed by streams from 
snow-beds and glaciers, and its sources may be roughly estimated 
as from 7000 to 8000 feet above the sea, and at distances of three to 
five miles’ from the place of observation. The stones are chiefly 
granitoid rocks, but some are Jurassic mudstones (these are from 
near at hand). Leaving the. latter out of consideration, the stones, 
which vary from boulders downwards, are generally subangular, 
even the smaller pebbles not being well rounded. 
Bed of torrent through village at Windisch Matrei (Tyrol).—This 
comes down from the valley running up to the Kalser Thorl, and 
probably has descended some 2000 feet. The ‘torrential’ character 
is indicated not only by the variability in the size of the stones and 
boulders, but also by the high walls which protect the village from 
its ravages. Stones from Sins. to 6ins. diameter were common; others 
up to about a foot diameter were fairly common, and larger occurred. 
1 Distances in this paper, unless otherwise stated, are measured on a map, and so 
are obviously less than the actual course of the stream, 
