File:The Quarterly journal of the Geological Society of London (12960682974).jpg

86
PKOCEEDINGS OF THE GEOLOGICAL SOCIETY.
many small interfering crystals. Occasionally some of the sur-
rounding clear quartz is in crystalline continuity with these grains ;
but usually it has crystallized independently, probably because the
grains were originally completely surrounded by deposit, and not
by the empty spaces which existed in the case of the consolidated
sandstones with deposited quartz, already described. As an illus-
tration of one of these grains, I give fig. 9, magnified 30 linear.
In other cases the quartz of the grain is clearer, and the contrast
less, and in others so slight that it is impossible to be quite sure
whether they were originally grains of sand, or only detached parts
of the quartz which has crystallized in situ. It is, however, almost
certain that many such grains must be present, since quartz in
general is much more transparent than those grains which are
well marked in the manner described. The great difficulty of re-
cognizing clear grains must be borne in mind, since the apparent
absence of sand from some specimens of schist may be due to this
cause.
Fig. 9. — Grain of Quartz-sand in tJie Mica-scMst of ArroquTiar.
Along with the grains of quartz just described occur some of
what appear to have been felspar, much decomposed before the
deposit was altered into schist; and there are also some grains
very much as if they had been derived from a felsite. In some
respects these various felspathic grains are more distinct than those
of quartz, because they are granular and quite unlike the sur-
rounding rock. Possibly this is why they are sometimes seen in
specimens of schist in which no grains of quartz can be recognized ;
but still the evidence is not so rehable as in the case of milk-white
quartz, since one cannot but feel that it is always possible, even
when it is not probable, that they are portions of altered felspar
which crystallized in situ when the rock was metamorphosed. In
any case, taking all the facts into consideration, the proof of the
original presence of true grains of sand in some thoroughly crystal-
line and typical mica-schists is as perfect as in the case of slates.
Leaving doubtful cases out of consideration, about i of all my
sections of schist afford good evidence of the presence of such
grains ; and I find that also ^ of my sections of slates show similar