Anniversary Address of the President, H. C. Sorby, F.RS. 131 
I had thus found to be quartz contained the usual fluid cavities, 
with moving bubbles, whilst what I had concluded must be 
felspar contained the well-marked glass cavities of modern volcanic 
rocks. 
There is little chance of confounding felspar and quartz in 
studying British stratified rocks. The only fear is of mistaking 
for one another fragments of decomposed felspar and portions of 
stratified rocks formed of thoroughly decomposed felspar, consoli- 
dated after deposition as separate granules. 
Kaolin, &e.—The principal characters of this substance have 
been already described. It may in general be identified by the more 
or less elongated and flattened shape of the particles, and by its 
strong depolarizing action, which, with crossed Nicols, suffices to 
give a pale bluish white, even when the particles are p3}o5 of an 
inch in diameter. There is no chance of confounding them with 
minute particles of quartz, which to give such a tint must be about 
four times that diameter. There is also no difficulty in distin- 
guishing kaolin from decomposed or comminuted pumice and 
some other analogous modern volcanic products, since the form of 
these particles is very different, and they have little or no action on 
polarized light. There is also no difficulty in distinguishing be- 
tween true kaolin and the minute short needle-like crystals met 
with abundantly in some decomposed volcanic rocks, since their 
shape is so different. 
Application of similar Principles to thin Sections of Rocks. 
As an almost universal rule thin sections of stratified rocks 
should be cut in a plane perpendicular to the stratification. In 
this case the thin flat plates of quartz or mica are almost always 
seen in transverse section, and the fact of their being thin and flat 
is at once apparent. There is also little difficulty in distinguishing 
between the thinnest pieces of quartz and the flakes of mica. 
These latter are usually thinner and of more uniform thickness, 
and with proper illumination and a sufficiently high magnifying 
power the laminar structure of the mica may be easily seen. Its 
colour and dichroism are also important characters, and observed to 
the greatest advantage in transverse sections. The identification 
of the various other minerals may be accomplished in the manner 
already described, and the only point that needs special attention 
is the recognition of the very minute granules of kaolin or micaceous 
substances disseminated amongst the larger fragments, which may, 
however, be accomplished by carefully regulating the aperture of 
the condenser, which must be small. 
