SCllOPE — ON INTERNAL STllUuTURK OF GNlilSSIC ROCKS. 3G3 
these hang back. And thus the stream of liquid moving in an)' 
general direction tends to divide itself, n.'orc or less, into veins or 
threads composed of particles of different degrees of coarseness, and, 
consequently, of different degrees of mobility, and moving at different 
rates. Moreover, by the friction which accompanies, or rather occa- 
sions, this separation of particles of unequal mobility and unequal rate 
of motion, the coarser suspended particles arc turned about until their 
longer axes are brought to coincide generally with the direction of the 
movement ; and they are led to arrange themselves iu strings or trains, 
one behind, or in the wake of the other. 
This is visible to any one who looks down from a bridge or other 
height on a running stream, upon whose surface float sticks, straws, or 
foam. These matters are seen to arrange themselves in streaks or 
linear bands in the direction of the movement, whether of the general 
stream or of its superficial eddies, the streaks being separated by others 
consisting of water comparatively free from foreign matter, and moving 
more rapidly than the former. 
If the floating or suspended substances be susceptible of tension, and 
the moving force is sufficiently powerful, they will be drawn or 
squeezed out by the pressure and friction to which they are subjected 
so as, if circular in outline, to become oval, if globular, ellipsoidal ; and 
of whatever form they be, they will be elongated in the direction of 
the movement and contracted in their other dimensions, until, if the 
motion and pressure and consequent friction be sufficient, they become 
extenuated into long threads, stripes, or planes, having the direction 
of the motion impressed on them; which direction may be occasionally 
varied so as to produce wavy, sinuous, or contorted shapes, under 
varying conditions of friction or lateral pressure. 
This law, which may be called that of "differential motion," in 
particles of different sizes and shapes, and consequently of different 
degrees of mobility, when set in motion within a more or less imperfect 
liquid, may be illustrated by the hand specimens of marbled paper Avhich 
I here exhibit (PI. x.). In them the ribboned appearance is produced 
by merely forcing the colours, as they float on the surface of water in 
Alms of circular or irregular shapes, to move in a lateral direction, by 
whicli motion their outlines are, as is here seen, elongated or stretched 
into parallel lines of more or less regularity, according to the greater or 
less resistance or retarding influence offered by each to the movement 
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