Abstract
A SOMEWHAT interesting consolidated beach exists on the west coast of Ceylon, a few miles to the north of Colombo. The writer had only one opportunity of visiting and examining for a short time this formation: but there are certain features in connection with it that cannot fail to be of interest, however short the examination may be. The beach extends continuously in almost a straight line for about four or five miles, and is manifestly in process of formation at the present time, as some portions of it are so soft that they can be easily crumbled in pieces by the hand, whilst others are much harder than gneiss, and can only with the greatest difficulty be fractured by means of a heavy hammer. Between these extremes are all gradations of hardness, and the ordinary shells of the coast may be found in almost every part of the beach more or less firmly embedded in the rock. The highest part of the formation is just within reach of the waves at high tide; but it is difficult to ascertain with any degree of accuracy how far it extends into the sea, on account of the difference between high and low tide being only about two feet. The beach is seen at a glance to be composed chiefly of a faint brownish-coloured rock, with frequent strata of black material of very varied thickness and irregular shape. An examination of specimens shows that the brown rock is composed almost entirely of quartz fragments, and that it possesses only a low specific gravity (2.91), whilst the darker portions are extremely heavy as well as extremely hard. Several specimens gave a specific gravity of 3.9, 3.93, 3.94, the dried sand, freed from its carbonate of lime by means of dilute hydrochloric acid, possessing a specific gravity of 4.32. A microscopic examination of this sand and also of sections of the rock showed that the chief constituent, and that which gave it its dark appearance, was magnetite, corundum in various forms being also present, with here and there a fragment of quartz. One noticeable point was that the fragments of the harder constituents were in nearly every case hardworn, and rounded, whilst the quartz showed traces of recent fracture in the shape of sharp edges and angles. The size of these fragments varies very considerably, those of magnetite ranging from.005 inch to.02 inch, whilst those of quartz are much larger, frequently reaching.04 inch. The corundum fragments are intermediate in size and rounded in form. It must be remembered that these specimens were taken from only one part of the formation, near the centre of its length and about the limit of high tide. In other positions the fragments will, no doubt, vary very much, the size depending in a great measure on the power of the current to carry them along the coast and up the beach. It was a matter of regret to the writer that he was not able to inspect carefully both extremities of the reef, and examine fragments from many different portions of it. The cementing material of the beach is carbonate of lime, no doubt from Ahe coral reefs along the coast, as there is no limestone rock in the neighbourhood or along the course of the Kelani River, which debouches to the south of the reef. It is not known whence the magnetite and corundum have been derived, except that they have possibly come from the degradation of the gneiss rocks occurring along the coast. Although corundum is comparatively abundant in the interior of Ceylon, it has never been found in its matrix, but always either in pocket holes in streams, or in drift, intermixed with rounded pebbles of quartz. The character of the rivers in the neighbourhood of the coast apparently precludes the possibility of fragments of corundum being carried down to the sea.
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ABBAY, R. Note on a Consolidated Beach in Ceylon . Nature 21, 184–185 (1879). https://doi.org/10.1038/021184a0
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DOI: https://doi.org/10.1038/021184a0