Anniversary Address of the President, H.C. Sorby, F.RS. 121 
since its index of refraction is almost exactly the same as that of 
quartz, light passes through them almost as though they were thin 
flat sections, polished on both sides. Each grain then shows a 
simple optical structure, and the colours round the circumference 
are usually of low orders, which gradually or more suddenly rise to 
higher orders towards the centre ; which proves that the grains are 
not flat, but thickest in the centre, and more or less angular in 
their outline, even in a plane parallel to the line of vision. 
The internal microscopical structure is also easily seen when the 
grains are mounted in Canada balsam, and there is no difficulty in 
using sufficiently high powers. Amongst the more striking cha- 
racters are the larger or smaller fluid cavities ; the hair-like crystals 
of schorl ; and the extremely minute granules or crystals which can 
scarcely be defined individually, but give rise to a general white 
milkiness, when the illumination is of such a kind that they appear 
white on a black background, and is to a great extent due to a vast 
number of very minute bright specks, many of which are out of 
focus. The chief variations in the character of different specimens 
depend upon the size and amount of these various inclosures in 
relation to one another and to the general mass of clear quartz; 
but, as in the case of the granites themselves, these variations may 
be sufficient to distinguish more or less completely those of par- 
ticular districts, as, for example, the granites of Cornwall from 
those of Norway or the Highlands of Scotland. 
Quartz in Schists—I have not been able to detect any micro- 
scopical character which enables us to distinguish between massive, 
thick foliated gneiss and granite. There is a most perfect and 
gradual passage from true and characteristic metamorphic schists 
to typical granite, in the external form, internal structure, and 
optical properties of all the constituent minerals. In certain dis- 
tricts where the schistose rocks have undergone extreme meta- 
morphism it thus appears probable that the granite has to some 
extent been formed im situ, by the still further metamorphism 
of the sedimentary rocks. There is, however, no difficulty 
whatever in distinguishing between the less highly metamorphic 
schists and granite, and more especially when the latter rock is 
undoubtedly intrusive and no longer in the place where it was 
formed. There is also a gradual passage from schistose to slaty 
rocks which have undergone very little change since deposition. 
It is therefore impossible to draw any absolute line of division 
between rocks which on the whole have a very different struc- 
ture, and have been formed under very different conditions, and 
thus the only course open to us is to deduce general laws from 
the study of characteristic specimens, fairly representing the 
main masses of each particular class of rocks. This is more 
