THE ROCKS AND MINERALS OF CEYLON.
By A. C. Dixon, B. Sc, (Honors) London.
(From the Ceylon Branch A'. A. S. Journal, 1880.)
The science of geology divides itself naturally into three departments;—
(a.)—The study of rocks, or Petrology.
(b.)—The study of the minerals of which rocks are composed, or Mineralogy.
(c.)—The study of the remains of animal and vegetable life contained in the rocks, or Palaeontology.
To
the one who makes this last division his object of research there is
but a poor field before him in Ceylon, save in the north of the island;
but for the one interested in rocks and their component minerals, there
is plenty of scope for research. Geological time is divisible into
three great periods separated by great breaks in lime, but this cannot
really be the case, for, as nations have sprung up and passed away
gradually, so also have formations. These have always been and will be
continuous. Although in England we have great gaps separating one
formation from another, yet wc have beds of passage in several parts of
the world, which bridge over these gaps and so form a connecting link.
The three great epochs of geological time arc the Palaeozoic or old life period, the Mesouric or middle life, and the Cainozoic or recent life.
Each
of these has numerous divisions. The bulk of this island consists of
ancient sedimentary beds; whether deposited in sea or lake, wc are
unable to say, for the metamorphism which these beds have undergone due
to internal heat, pressure, time and various other causes) has
obliterated all traces of fossil remains. Over this gneiss around Colombo and in many other parts of the island, we have the well-known laterite or
cabook, so largely used for building purposes. This formation has given
rise to much discussion. It is essentially a derivative from the
gneiss; and, beyond doubt, in many cases in situ, as is evident
in several cuttings which have been made, a notable one, which I
visited some time ago, occurring in a cutting made while searching for
plumbago between Polgahawela and Ambepussa. In many ravines in the hill
districts of the island, especially in Dimbula and Dikoya, we have an
iron conglomerate at present in course of formation, composed of the
debris of surrounding rocks, firmly held together by ferruginous
matter, which rock when subjected to decay, would furnish a formation
exactly akin to our laterite. I have dealt with this subject (Laterite)
at greater length in a paper to the Royal Academy of Sciences, Sweden.
In the north of the island we have a formation of the Mesozoic, or secondary period, viz., the Cretaceous. These
beds are no doubt contemporaneous with the Pondicherry beds, which have
yielded numerous fossils, by which their age has been determined. I
have no doubt that many fossils might be gathered in our
