372 _ DYNAMICAL GEOLOGY. —  [Boox HI. 
‘water, and rotating on its axis with a mean velocity of 0:80 to 1 
metre in a second. He found that after the first 25 kilometres 
(about 154 English miles) the angular fragments of granite had lost 
75 of their weight, while in the same distance fragments already 
well rounded had not lost more than +35 to zij. The fragments 
‘rounded by this journey of 25 kilometres in a cylinder could not be 
distinguished either in form or in general aspect from the natural 
detritus of a river-bed. A second product of these experiments was 
an extremely fine impalpable mud which remained suspended in the 
water several days after the cessation of the movement. During the 
production of this fine sediment, the water, even though cold, was 
found after a day or two to have acted chemically upon the granite 
fragments. After a journey of 160 kilometres, 3 kilogrammes 
(about 63 lb. avoirdupois) yielded 3°3 grammes (about 50 grains) of 
soluble salts consisting chiefly of silicate of potash. A third product 
was an extremely fine angular sand consisting almost wholly of 
quartz, with scarcely any felspar, almost the whole of the latter 
mineral having passed into the state of clay. The sand grains, as they 
are continually pushed onward over each other upon the bottom of 
a river, become rounded as the larger pebbles do. But a limit is 
placed to this attrition by the size and specific gravity of the grains. 
As a rule the smaller particles suffer proportionately less loss than 
the larger, since the friction on the bottom varies directly as the 
weight and therefore as the cube of the diameter, while the surface 
exposed to attrition varies as the square of the diameter. Mr. Sorby, 
in recently calling attention to this relation, remarks that a grain 75 
of an inch in diameter would be worn ten times as much as one 
of an inch in diameter, and a pebble 1 inch in diameter would be 
worn relatively more by being drifted a few hundred yards than a 
sand grain +455 of an inch in diameter would be by being drifted for 
a hundred miles.? So long as the particles are borne along in 
suspension they will not abrade each other, but remain angular. 
Daubrée found that the milky tint of the Rhine at Strasburg in the 
months of July and August was due, not to mud, but to a fine 
angular sand (with grains about =, millimetre in diameter) which 
constitutes +5759 of the total weight of water. Yet this sand had 
travelled in a rapidly flowing tumultuous river from the Swiss 
mountains, and had been tossed over waterfalls and rapids in its 
journey. He ascertained also that sand with a mean diameter of 
grain of +4; mm. will float in feebly agitated water ; so that all sand of 
finer grain must remain angular. ‘The same observer has noticed 
that sand composed of grains with a mean diameter of 4 mm., and 
carried along by water moving at a rate of 1 metre per second, gets 
rounded, and loses about yo} of its weight in every kilometre 
travelled.’ 
' “ Géologie Expérimentale,”’ p. 250, et seq. 
2 Q. J. Geol. Soc. xxxvi. p. 59. 
* “ Géologie Expérimentale,” pp. 256, 258, 

