122 Transactions of the Royal Microscopical Society. 
especially desirable in connection with the subject now before us, 
since in general the more doubtful passage beds would not have 
a very great influence on the nature of the material derived from 
the denudation of a large tract of country. 
If, then, we compare the microscopical structure of granite with 
that of a thin foliated mica-schist, cut perpendicular to the lamine, 
we find that the difference is very great, independent of the more 
or less complete absence of felspar. The schist is made up of flat 
plates of mica, not as in granite lying in every possible plane, but 
on the whole more or less closely parallel to one particular plane. 
In the case of rocks having stratification-foliation this plane cor- 
responds to the layer of different mineral character, and thus the 
thin layers of quartz are often bounded on both sides by layers of 
flat plates of mica. Hyen when the amount of this latter mineral 
is not sufficient to constitute a continuous layer, the detached flakes 
lie in one plane. The result of this is that the presence of so many 
parallel flat plates of mica has to such a great extent interfered with 
the growth of the crystals of quartz, that a very large proportion 
of them are bounded by flat parallel planes of interference, and thus, 
instead of having the extremely irregular form of those in granite, 
they are on the whole characterized by having two surfaces 
more or less flat and parallel. Fig. 4 will show the general cha- 
racter of the transverse section of the forms thus produced, which 
thus differ much from those derived from granite shown in Figs. 1, 
2, and 3. 
When detached and lying flat on glass, their characteristic 
shape might easily be overlooked, but their flattened form is 
revealed by using polarized light and an analyzer; since we 
obtain comparatively uniform tints over their whole surface, instead 
of tints rising rapidly from the margin towards the centre. 
If the foliation be thin, much of the quartz may occur in such 
flat plates ; but if the foliation of the rock be thick, more than one 
crystalline particle may be and often is developed between the 
layers of mica, and thus the grains of quartz may more or less 
completely cease to be of a flattened shape. This is especially the 
case if the schist possess cleavage-foliation, since then those grains 
of quartz alone are flat which lie in the layers of mica. Those 
that lie between the layers may be of an altogether irregular form, 
and therefore, if the crystallization be coarse-grained, it might be 
impossible to distinguish such quartz from that of some granites. 
It is also impossible in the case of the quartz of the coarse-grained 
quartzose layers of schists possessing stratification foliation. The 
result of this is that even in sand derived exclusively from schists 
a few grains of quartz may be expected to occur which could not 
be distinguished from grains derived from granite. 
