60 Prof. T. G. Bonney—Rounding of Alpine Pebbles. 
The Isel (including a tributary) : Tauern Haus to Windisch 
Matrei (about 10} miles) about 164 feet per mile; Windisch Matrei 
to Lienz (18 miles) 58 feet per mile. 
It is very difficult to estimate the fall in the Po and its tributaries. 
Turin is 602 feet above the sea, probably the edge of the plain is about 
800 feet. That would give a fall of about 4 feet per mile. In the 
tributary valleys I should think that the fall would be not less than 
100 feet per mile. As an example of a true torrent, the Reuss between 
Andermatt and Amsteg falls about 212 feet per mile; from the latter 
place to the Lake of Lucerne only 26°38 feet per mile. 
The facts thus collected appear to me to warrant the following 
conclusions :— 
1. The rapidity with which a pebble is formed depends ceteris 
paribus on the nature of the rock. 
2. Pebbles are rounded with comparative rapidity when the 
descent is rapid; that is, when they are dashed down rock slopes by 
a roaring torrent, capable of sweeping along blocks of far greater size. 
3. Pebbles are rounded with comparative slowness when the 
descent is gentle, and the average pace of the river is just about 
able to push them along its bed. 
4, As indicated by Daubrée’s interesting experiments, the process 
of rounding ceteris paribus always goes on more rapidly at first. 
In the above observations the rocks (with the exception of some 
vein-quartz which generally proved a rather intractable material) 
may be taken as ranging in hardness from rather above 8 to about 6. 
The limestone pebbles would probably range from 8 to 4, and there 
would be chemical action in addition. The Alpine rocks, as a rule, 
would probably range from 5 to 6, for though quartz is present, there 
is always a good amount of felspar or mica. The easy rounding 
of the more micaceous schists, due to their softness, is to some extent 
counteracted by their fissility. 
If, then, we find in any conglomerate, a large number of well- 
rounded pebbles of a rock not less hard than felspar, we are justified 
in concluding that they are the deposit of a river which, in any case, 
has had a course of several miles, and has either descended as a very 
rapid stream from snow-capped mountains of considerable elevation, 
the detritus, in short, of a strong, full, torrent in a valley running down 
from a great mountain-chain, or of a river which, rising at a more 
moderate elevation and swollen by many tributaries, has flowed for 
a very much longer distance. Perhaps we might venture to say, as 
a rough standard of comparison, that the effect of a thousand feet of 
rapid descent is equivalent to that of the more leisurely traverse of 
at least twenty miles, so that fairly well rounded pebbles of a rock 
with hardness not exceeding 6 signify at least either a rapid descent 
of three thousand feet or a journey at a less speed of sixty miles.! 
' Prof. Daubrée (Géol. Experim. vol. i. sec. ii. ch. v.) obtained experimentally a 
distance of a little less than 16 miles for the manufacture of a rounded pebble of 
granite, but this must be regarded as the least possible distance, for in his experiments 
the fragments would be knocked one against another much more than in transport by 
a stream, except perhaps in a ‘‘ pothole.”’ 
