SCllOPE—OX INTERNAL STRUCTURE OF GNEISSIC ROCKS. 363 



these hang back. And thus the stream of liquid moving in any 

 general direction tends to divide itself, more 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 are turned about until their 

 longer axes are brought to coincide generally with the direction of the 

 movement ; and they are led to arrange themselves in 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 

 li^iid, may be illustrated by the hand specimens of marbled paper which 

 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 

 films of circular or irregular shapes, to move in a lateral direction, by 

 which 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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