Anniversary Address of the Presodent, H.C. Sorby, F.BR.S. 123 
Though as far as mere form is concerned, the grains of quartz 
in schist may thus sometimes be very similar to those in granite, 
yet, on comparing the more common types of these rocks, there is 
a well-marked difference in other respects. Usually the size of the 
separate crystalline particles of the quartz in granite is much 
larger than in the case of schists. Those in granite are also 
usually bounded on most sides by felspar or mica, and thus, when 
the rock is broken up by decomposition, each of the detached 
quartz grains is, as it were, part of one single crystal, as already 
named. On the contrary, those in schist being much smaller and 
often bounded by similar crystals of quartz, firmly cohering to 
them on all sides, when the rock is broken up there is only an 
imperfect separation of the crystalline particles, and many of the 
resulting grains of sand are of complex character. This dif- 
ference can be detected by using polarized ight with an analyzer. 
Thus, assuming that the general outline was the same in both 
cases, in certain positions of the plane of polarization the grain 
derived from granite would be uniformly dark or coloured over its 
whole area, as shown by Fig. 5; whereas, if derived from schist, it 
would present a tesselated appearance like Fig. 6, the detail varying 
on rotating the plane of polarization. ‘The separate portions may 
be in such perfect optical contact that this complex structure may 
be quite invisible if polarized light be not used. 
I have not been able to recognize any well-marked difference in 
the ultimate microscopical structure of the quartz of granite and 
schists. Minute needles of schorl are on the whole more common 
in the quartz of granite, but they do also occur in that of many 
schists. The quartz of some granites also contain a much greater 
number of fluid cavities than that of most schists, but in the quartz 
of other granites they are quite as rare as in the purest quartz of 
any schists ; and thus the only safe conclusion is that the presence 
of many needle-shaped crystals of schorl, or of many fluid cavities, 
makes it more probable that the quartz was derived from granite 
than from schists. 
Mica, &e.—In comparing the mica from different specimens of 
schists and granites, there is a similar difficulty in drawing any abso- 
lute line as in the case of the quartz. The extremes are distinct 
enough; but there is every connecting link in passing from mode- 
rately altered to very much altered schists, to gneiss and to granite, 
If it were possible to determine the true chemical and mineral cha- 
racter of the mica in every case, even where two or more different 
kinds are mixed together, much more could probably be learned ; 
but in the practical study of the microscopical structure of stratified 
rocks we are almost compelled to content ourselves with what can 
be learned by means of the microscope alone, and to rely on general 
