Northern
Province by those interested in Palaeontology. Once, it is recorded,
this Society possessed in its Museum a fossil phalange frorh this
district, but it has been lost.
Of recent formations,
we have on the sea coast between Negombo and Mount Lavinia, and for
some distance beyond these places, a recent breccia formed of particles
of disintegrated rock, more or less compact. At Talpitiya it occurs at
a considerable distance from shore (300 yards), and at a depth of
twenty-live feet. Numerous minute shells and fragments of shells occur
in this. At Pamunugama, on the way to Negombo, the sandstone varies
much. In some cases it is black-banded, with particles of magnetic iron
; in others the particles of iron are evenly distributed throughout the
mass. The nature of the cement which binds these particles together is
carbonate of lime.
Valcareoits Tufa, still
in the course of formation, is a deposit from the hot-water springs of
Jiintenna, the water of which is highly charged with carbonate of lime,
which is deposited as the water cools. It is known as Pennagal by the
.Sinhalese, and is burnt and used by them along with their betel.
The foregoing formations are indicated on the accompanying rough geological sketch map.
I will now consider more particularly the gneiss, which is our most extensive formation. It varies much in texture, colour, composition, hardness, &c.
Its
composition is the same as granite, only the degree of metamorphism has
not been so great as to entitle it fully to that name.
It
is composed of quartz—felspar (both orthoclase and
oligoclase)—mus-covite and biotite (micas), hornblende, chlorite, and
numerous accidental minerals in varying proportions.
In
some localities we find a rock composed of only one of these, as in the
case of hornblende rock; at other times only felspar, but generally the
foregoing components are mingled together in varying proportions,
giving a large number of different kinds of rock.
Orthoclase
forms the main mass. The two felspars are easily distinguished on a
weathered surface. The orthoclase is glassy and somewhat pearly in
lustre, and has a translucent aspect, while the oligoclase is dull and
opaque.
In the gneiss we meet with various beds as limestones, dolomite, magnetite, quartz, hornblende, tremolite, mica, epidote. Some of these occur, also, as veins in the gneiss.
In
the veins we have the minerals, actinolite, tremolite, jade, talc,
mus-covite, biotite, epidote, schoral, and many others of minor
importance.
Actinolite
is found in the Kotagala district. Good specimens of crystalline talc
are obtainable from Mahara quarry and neighbourhood.
The
gneiss in some cases almost passes into syenite. Where the felspar is
flesh-coloured this rock much resembles Peterhead granite. Porphyritic
gneiss occurs on the hill ranges not far from Heneratgoda.
At
Balangoda we meet with a crude jasper, and not far from the same
locality a large mass of graphic granite in which the quartz is
distributed in bands, and when viewed endwise much resembles an
inscription.
When
subject to action of water the felspar of the gneiss soon decomposes,
and so a large number of decomposed forms arc very abundant. As the
island is gradually uprising it is evident that the less elevated
portions have been under the influence of water for a longer period,
and, consequently, the cuttings through such are less difficult than
similar ones' in the more elevated portions. Another peculiarity of
the gneiss is the occurrence of garnets in large quantity.
Dolomile beds. As
far as I have been able to trace during the lime at my disposal, I find
that these beds run through the gneiss in a somewhat pa'rallcd
direction, striking generally NAY. by N. to N., and having various
angles of dip from 10° to 40\
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