few,
and seldom well defined. The most prevailing species are granite or
gneiss; the less frequent are quartz-rock, hornblende rock, and
dolomite rock, which may be classed under the head of imbedded minerals.
The
varieties of granite and gneiss are endless, passing often from one
into another and at times losing their character by the transition, and
assuming appearances for which, in small masses, there would be a
difficulty in finding appropriate names. These mutations and remarkable
variations are traceable chiefly to composition, the proportions of the
elements, excess or deficiency of one or more, or on the addition of
new ingredients. Nor should mechanical •tructure, variation in which,
though hardly palpable in reference to causes, has an evident effect in
regulating appearances, be omitted. Regular granite is rare; where
found it is generally of a grey colour and fine grained. Graphic
granite is still rarer. The quartz, where it is found, is black or grey
rock crystal, and the felspar highly crystalline and of a bright flesh
colour. The quartz envelopes the felspar in very thin hexagonal or
triagonal cases, so that nothing can more vary in appearance than the
longitudinal and transverse fracture of the rock. Petrifactions of
wood, combining quartz and felspar, have been occasionally found in the
interior. This is a mineralo-gical novelty, the latter substance never
having been found in petrifactions of a similar nature.
Moonstone
has also been found embodied in porphyric rocks in large masses, and is
more beautiful than moonstone hitherto dug from rocks of decomposed
white clay. Sienite is uncommon. It occurs in the interior, rather
forming a part of rocks of a different kind than in great mountain
masses.
Well
formed gneiss is more abundant than granite. Its peculiar structure may
be seen in many places, but no where so clearly as at Amana-poora in
the Central Province, where it consists of white felspar and quartz in
a finely crystalline state, with layers of black mica, containing,
disseminated through it, numerous crystals of a light-coloured garnet.
Both the granite and gniess are very much qualified by an excess or
deficiency of one or other of the ingredients. When quartz abounds in a
fine granular state, the rock often looks very like sandstone; of this
there is an instance in the vicinity of Kandy. When felspar or adularia
abound, the rock acquires a new external character : this variety is
.common. In a few places the rock contains so much of these minerals
that it might be correctly called adularia, or felspar rock. When mica
prevails in gneiss, which in Ceylon is very rare, it acquires not only,
the appearance, but very much the structure of mica slate. The
instances of change of appearance in the granitic varieties from the
presence of unusual ingredients, are neither few in number nor
unfrequent in occurrence.
The
more limited varieties of primitive rock, as quartz, hornblende, and
dolomite rock, seldom occur in the form of mountain masses. Quartz is
found in some places so abundantly in granite rocks as even to mal
mountain masses. It is generally quite bare, and stands erect like
denuded veins. From its pre-cipitousness it often exhibits the
appearance of buildings in ruins. The quartz is in general milk-white,
translucent, full of rents, and so very friable as to resemble
unaimealed glass. Pure hornblende rock and primitive greenstone are not
uncommon, and though they constitute no entire mountain, form a part of
many, particularly of Samanala and the Kandyan mountains.
Dolomite
rock is almost entirely confined to the interior, where it is found in
veins and imbedded, and sometimes constitutes low hills. The varieties
of dolomite rock are almost as numerous as those of granite. When
purest, it is snow-white, generally crystalline, composed of rhombs
that are easily separated by a blow, but rarely finely granular. When
highly crystalline it is composed of about 56 of carbonate of magnesia,
36*9 carbonate of lime, 4-1 alumina, 1 silica, 2 water. A very fine
granular kind is found, but it is so uncommon, that it was appropriated
under the Kandyan dynasty to the sole use of the king. The great
variety of this rock arises both from the proportion of carbonate of
lime and of magnesia
