being
seldom the same, and from the commixture of other minerals. The
varieties most frequent are mixtures of dolomite with felspar and mica,
and even quartz. It is from the purer kinds of dolomite rock that all
the lime employed in building in the interior is procured. The
presence of magnesia injures its qualities as a cement; but though
inferior in this respect to the lime from shell and coral, it answers
sufficiently well for ordinary uses.
In
external character and general structure, the varieties of primitive
rock exhibit fewer marked differences than might have been expected.
The masses that are exposed, are generally rounded, seldom rising to
craggy points or appearing in grotesque shapes. The nature of the rock
may often be surmised, from its external appearance, but generally
cannot be precisely determined but by an examination of a recently
fractured surface. In structure the granitic varieties most commonly
exhibit an appearance of stratification, but is not easy to decide
positively whether this appearance is to be attributed to the mass
being composed of strata or of large laminx or layers. Some great
masses of insulated rock, several hundred feet in height, exhibit
incontrovertible proofs of this structure. In these the same layer may
be seen extending over the rock, like the coat of an onion, and which
if but partially exposed, might be adduced as a strong proof of
stratification, and if examined in different places on the top and at
each side, might be deemed an extraordinary instance of the dip of the
strata in opposite directions. With this hypothesis of the structure of
the rocks, the appearance of stratification in all the granitic
varieties may be easily reconciled.
Rocks
of recent formation are of two kinds, limestone and sandstone. The
first is said to be confined to the province of Jaffna, the most
productive and populous district of Ceylon, which is an extended level
plain without a single hill or valley, and contains numerous decomposed
shells, and other marine productions; it is generally grey or light
brown, very fine grained and compact, and breaks with a conchoidal
fracture. It is generally nearly a pure carbonate •f lime, affording
but slight traces of the presence of vegetable or animal matter, and
containing a little water. Where it occurs, the whole of the country is
similar, and elevated but a few feet above the surface of the sea, by
which it was once probably covered. The recession of the sea from this
district is even now going on, many natives recollecting the waves
covering spots now far above high-water mark. It is proved also from
the fact of coral rock being found mixed with the limestone rock
several miles from the sea. Minute inquiry on the spot might elicit
some valuable information on the formation of this rock, which is still
probably extending in the shallows of the adjoining seas, and along the
coasts of Jaffnapatam. Its formation may possibly be connected with
coral, which is so abundant in the narrow seas between Ceylon and the
Indian Peninsula, that most, if not" all, of the islets in the strait
are composed of it, and the gradual increase of coralline in the waters
near these shores proves the natural and steady encroachment of the
land. The only difficulty is, to find the cause of the solution of
calcareous matter in some places, and its precipitation in others
adjoining.
Sandstone,
the other rock belonging to the recent formation, may be considered to
surround the island with an almost uninterrupted chain. It exhibits in
every part the same general character, and is found under the same
circumstances, in horizontal beds along the shore, chiefly between
high and low water mark, which in Ceylon, where the tide rises only
about three feet in perpendicular height, is a very limited extent. In
shallow water, it may extend perhaps farther into the sea. Towards the
land, it does not extend beyond the beach. A remarkable instance of
this is found on the north side of the Kelaniganga. In width the bed
varies from a few to fifty or even a hundred feet. Towards the sea, it
presents a bold face, above twelve feet deep, perpendicular like a
wall, over which the waves break, and which, when the sea runs high, as
it does on this shore, a great part of the year is completely under
water, On
