130 Transactions of the Royal Microscopical Society. 
granitic rocks shows with polarized light coloured bands due to 
twin plates, and abite, oligoclase, and labradorite give more or less 
well-marked evidence of their cleavage, the sanidin of modern 
voleanic rocks often occurs as clear transparent fragments, having 
a simple optical structure, and showing no more lines of cleavage 
than a piece of glass. 
On the whole, fragments of unaltered felspar constitute but a 
very small part of our British stratified rocks, and glassy felspar is 
probably almost or quite absent; but when we come to study the 
modern deposits formed at great depths in the Atlantic and Pacific 
oceans, we find that it plays a most important part. For some 
time I feared that no ready means could be discovered to distinguish 
between it and quartz. Both break up into irregular transparent 
fragments, having a vitreous fracture, and, when of the same size, 
give with polarized light the same tints, so that these minerals 
cannot be distinguished by that means. At length, however, it 
occurred to me that perhaps there might be sufficient difference in 
their refractive power to cause them to appear different with 
suitable illumination. As previously named, the refractive power of 
quartz is almost absolutely the same as that of moderately fresh 
Canada balsam, but I find that it is decidedly less than that of very 
hard balsam. On the contrary, the refractive power of glassy 
felspar is equal to that of this very hard balsam, and greater than 
that of new and soft. Hence when both minerals are mounted in 
soft balsam, transmitted light passes so evenly through the quartz 
as to show little or no dark outline, whereas in passing through the 
glassy felspar it is somewhat bent, and if the apertures of both the 
condenser and object-glass are sufficiently small, the fragments 
show a dark outline. The difference is, however, scarcely so well 
marked as is desirable, and it is therefore better to make use of a 
different illumination. By using a condenser with a central stop, 
so as to get a black background, the oblique rays pass through the 
quartz without there being any light reflected, and the outline is 
invisible, or only shown by means of any superficial coating of some 
other material which may be present. There is, however, seldom 
any difficulty in distinguishing this from true surface reflexion. 
On the contrary, the glassy felspar having a somewhat higher 
refractive power than the soft balsam, reflects part of the light, 
and causes the fragment to show a well-marked bright outline. 
As far as I have been able to judge, this method gives satisfactory 
results, which are confirmed by other facts. Thus, for example, on 
examining some of the sandy matter washed from the deep ocean 
clays brought back by the ‘ Challenger,’ I came to the conclusion 
that certain particles were quartz and others glassy felspar, and 
on examining these with a much higher power I saw that what 
