23. _Quartz-rock_, or _quartzite_, is an altered quartzose sandstone or
grit; it is generally a white or grayish-yellow rock, very hard and
compact. The original gritty character of the rock is distinct, but the
granules appear as if they had been fused so far as to become mutually
adherent. When the altered sandstone has been composed of grains of
quartz, felspar, or mica, set in a siliceous, felspathic, or
argillaceous base, we get a rock called _greywacke_, which is usually
gray or grayish blue in colour.
24. _Clay-slate_ is a grayish blue, or green, fine-grained hard rock,
which splits into numerous more or less thin laminae, which may or may
not coincide with the original bedding. Most usually the 'cleavage,' as
this fissile structure is termed, crosses the bedding at all angles.
25. _Crystalline limestone_ is an altered condition of common limestone.
_Saccharoid marble_ is one of the fine varieties: it frequently contains
flakes of mica. _Dolomite_, or magnesian limestone, already described,
is probably in many cases an altered limestone; the carbonate of lime
having been partially dissolved out and replaced by carbonate of
magnesia. _Serpentine_ is also believed by some geologists to be a
highly metamorphosed magnesian limestone.
26. _Schists_.--Under this term comes a great variety of crystalline
rocks which all agree in having a foliated texture--that is to say, the
constituent minerals are arranged in layers which usually, but not
invariably, coincide with the original bedding. Amongst the schists come
_mica-schist_ (quartz and mica in alternate layers); _chlorite-schist_
(chlorite with a little quartz, and sometimes with felspar or mica);
_talc-schist_ (talc with quartz or felspar); _hornblende schist_
(hornblende with a variable quantity of felspar, and sometimes a little
quartz); _gneiss_ (quartz, felspar, and mica).
27. _General Character of Metamorphic Rocks._--All these rocks betray
their aqueous origin by the presence of more or less distinct lines of
bedding. They consist of various kinds of arenaceous and argillaceous
deposits, which, under the influence of certain metamorphic actions, to
be described in the sequel, have lost their original granular texture,
and become more or less distinctly crystallised. And not only so, but
their chemical ingredients have in many cases entered into new
relations, so as to give rise to minerals which existed either sparingly
or not at all in the original rocks. Frequently, it is quite impossible
to say what was the original condition of some metamorphic rocks; often,
however, this is sufficiently obvious. Thus, highly micaceous
sandstones, as they are traced into a metamorphic region, are seen to
pass gradually into mica-schist. When the bedding of gneiss becomes
entirely obliterated, it is often difficult to distinguish that rock
from granite, and in many cases it appears to pass into a true granite.
28. _Granite_ is a crystalline compound of quartz, felspar (usually
potash-felspar), and mica. Some geologists consider it to be invariably
an igneous rock; but, as just stated, it sometimes passes into gneiss in
such a way as to lead us to infer its metamorphic origin. There are
certain areas of sandstone in the south of Scotland which are partially
metamorphosed, and in these we may trace a gradual passage from highly
baked felspathic sandstones with a sub-crystalline texture into a more
crystalline rock which in places graduates into true granite. Granite,
however, also occurs as an igneous rock.
29. _Syenite_ is a crystalline compound of a potash-felspar and
hornblende, and quartz is frequently present. _Diorite_ is a crystalline
aggregate of a soda-felspar and hornblende. Both syenite and diorite
also occur as igneous rocks.
There are a number of other metamorphic rocks, but those mentioned are
the most commonly occurring species.