Anniversary Address of the President, H. C. Sorby, F.RS. 127 
In order to assist such determinations, it will be well to express 
the necessary data in the following tabular form : 
Granite. | Schists, 
Larger grains .. .. | Some may show felspar | Some may show quartz 
attached to the quartz. | enclosing plates of 
mica lying parallel to 
the external surfaces. 
Quartz— 
General form .. .. | More or less equiaxed .. | More or less flattened. 
Optical structure .. Simple) 79.8) ) i) 2.0) Oftenicomplex: 
Mica Spr) pdoselas a Hee Brown dichroic .. .. Green dichroic. 
Felspar .. .. .. .. | Unaltered or decom- | Absent. 
posed into kaolin. | 
Hornblende .._ .. .. | Comparatively rare .. Comparatively abun- 
dant. 
Needles of schorl en- | Common.. .. .. .. | Much more rare. 
closed in the quartz. | 
Sorting of the Material. 
In studying stratified rocks it is most essential to understand 
the general laws concerned in the sorting of material suspended in 
water, or drifted along the bottom by currents. When a grain of 
sand or a flake of mica subsides in water, the accelerating force 
of gravitation soon becomes equal to the resistance, and then the 
particle subsides at an uniform rate, called its final velocity. The 
actual amount of this final rate varies directly as the density of 
the material, and inversely as the size of the grains; but the law is 
made somewhat complicated by a variety of circumstances, amongst 
which may be named the shape of the grains. In accordance 
with this law very minute and perfectly detached particles subside 
very slowly, and I find that the smallest granules of kaolin in 
clay subside at the rate of about one foot in five days, whereas 
grains of sand +35 of an inch in diameter subside a foot in about 
ten seconds. If then there were nothing to interfere with this 
law, the very fine particles of clay would seldom occur mixed 
with grains of sand. Such a complete separation is, however, 
very unusual, and as a general rule clays consist of particles of 
extremely various size. ‘This can be easily explained by the very 
peculiar physical characters of kaolin. When suspended in water, 
the particles have a very great tendency to stick together and 
collect into complex granules, somewhat like mist collects into 
drops of rain, and in so doing may enclose grains of sand. The 
rate of subsidence ig therefore not that of the separate indi- 
vidual particles, but that of the complex granules, which is 
tolerably uniform for all of them. It is much more rapid than 
that of the separate minute particles would be, since, compared 
with the surface, the weight of large particles is greater than that 
VOL. XVII. Ti 
