120 Transactions of the Royal Microscopical Society. 
this question that perhaps some geologists may be disposed to 
doubt the possibility of determining it in a satisfactory manner ; 
but, though we cannot learn all that we could wish, the form and 
the internal structure of the individual grains of sand or finer par- 
ticles of mud are often sufficiently characteristic to enable us to 
draw several important conclusions. We may thus ascertain 
whether the greater part of the deposit was derived from the 
decomposition of slates, schists, or granitic rocks; and in the latter 
case we may sometimes form a very satisfactory opinion respecting 
the general characters of the parent granite. In order to show how 
it is possible to learn these particulars, it will be necessary to 
describe in detail the various characters of the principal minera. 
constituents of the rocks now under consideration. 
Quartz.—The grains of quartz, which are the chief constituent 
of most sandstones, and occur in greater or less number in nearly 
all shales and slates, have been derived principally from granitic or 
schistose rocks, broken up by the action of currents of water, after 
having been more or less decomposed by the action of the atmo- 
sphere. The form and internal structure of those derived from 
granite may be learned by studying thoroughly decomposed speci- 
mens of that rock. By washing and sieving it is easy to separate 
from the decomposed felspar the sand of coarser or finer grain, 
which in addition to quartz contains mica, and more or less imper- 
fectly decomposed felspar. 
Quarta in Granite.—The form of the quartz grains is extremely 
irregular ; but in most cases they are not much longer in any one 
direction than in another, which is due to the structure of the 
eranite itself being nearly the same in all directions. Usually the 
grains are very angular, but some have such a rounded outline, 
that, if their origin were not known, they might be supposed to 
have been much worn by the action of water. The general 
character will be better understood by referring to Plate CLXXIYV., 
which in Figs. 1 and 3 gives examples of the extreme types, and 
in Fig. 2 a more common shape. 
The irregular angular outline is due to the mutual interference of 
the individual imperfectly grown crystals, which could not develop 
true crystalline planes of any considerable extent. The effect of 
this ‘interference of growth 1s, however, often visible as small sur- 
face ridges, even when the general outline is scarcely at all angular ; 
which thus proves that this exceptional rounded form is due to the 
peculiar conditions of the growth, and not to mechanical wearing ; 
since in that case all such surface irregularities would have been 
removed. That each of these more or less irregular grains is a 
single imperfectly developed crystal can be easily proved by means 
of polarized light. When they are mounted in Canada balsam, 
