We arrived at Galle on the 15th December, having during our passage from Singapore had a pretty steady and favourable monsoon. While sailing through the Straits of Malacca strong ball-lightning was often seen a little after sunset. The electrical discharges appeared to go on principally from the mountain heights on both sides of the Straits.
I allowed the Vega to remain in the harbour of Point de Galle, partly to wait for the mail, partly to give Dr. Almquist an opportunity of collecting lichens on some of the high mountain summits in the interior of the island, and Dr. Kjellman of examining its algæ, while I myself would have time to visit the famous gem-diggings of Ceylon. The return was as good as could have been expected considering our short stay at the place. Dr. Almquist's collection of lichens from the highest mountain of Ceylon, Pedrotalagalla, 2,500 metres high, was very large, Kjellman, by the help of a diver, made a not inconsiderable collection of algæ from the neighbourhood of the harbour, and from an exclusion which I undertook in company with Mr. ALEXANDER C. DIXON, of Colombo, to Ratnapoora, the town of gems, where we were received with special kindness by Mr. COLIN MURRAY, assistant government agent, I brought home a fine collection of the minerals of Ceylon.
Precious stones occur in Ceylon mainly in sand beds, especially at places where streams of water have flowed which have rolled, crumbled down, and washed away a large part of the softer constituents of the sand, so that a gravel has been left remaining which contains considerably more of the harder precious stone layer than the original sandy strata, or the rock from which they originated. Where this natural washing ends, the gem collector begins. He searches for a suitable valley, digs down a greater or less depth from the surface to the layer of clay mixed with coarse sand resting on the rock, which experience has taught him to contain gems[388]. At the washings which I saw, the clayey gravel was taken out of this layer and laid by the side of the hole until three or four cubic metres of it were collected. It was then carried, in shallow, bowl-formed baskets from half a metre to a metre in diameter, to a neighbouring river, where it was washed until all the clay was carried away from the sand. The gems were then picked out, a person with a glance of the eye examining the wet surface of the sand and collecting whatever had more or less appearance of a precious stone. He then skimmed away with the palm of the hand the upper stratum of sand, and went on in the same way with that below it until the whole mass was examined. The certainty with which he judged in a moment whether there was anything of value among the many thousand grains of sand was wonderful. I endeavoured in a very considerable heap of the gravel thus hastily examined, to find a single small piece of precious stone which had escaped the glance of the examiner, but without success.
The yield is very variable, sometimes abundant, sometimes very small, and though precious stones found in Ceylon are yearly sold for large sums, the industry on the whole is unprofitable, although now and then a favourite of fortune has been enriched by it. The English authorities, therefore, with full justification, consider it demoralising and unfavourable to the development of the otherwise abundant natural resources of the region. For the numerous loose population devotes itself rather to the easy search for precious stones, which is as exciting as play, than to the severer but surer labours of agriculture, and when at any time a rich find is made, it is speedily squandered, without a thought of saving for the times when the yield is little or nothing. A large number of the precious stones are polished at special polishing places at Ratnapoora, but the work is very bad, so that the stones which come into the market are often irregular, and have uneven, curved, ill-polished surfaces. Most of them perhaps are sold in the Eastern and Western Indian peninsulas and other parts of Asia, but many are also exported to Europe. The precious stones which are principally found at Ratnapoora, consist of sapphires, commonly blue, but sometimes yellow or violet, sometimes even completely colourless. In the last case they have a lustre resembling that of the diamond[389]. Rubies I saw here only in limited numbers.
The precious stones occur in nearly every river valley which runs from the mountain heights in the interior of the island down to the low land. According to a statement by Mr. Tennent (i. p. 33), the river-sand at many places contains so much of the harder minerals that it may be used directly for the polishing of other stones. The same writer, or more correctly Dr. GYGAX, who appears to have written the rather scanty mineralogical contributions to Tennent's famous work, states that a more abundant yield ought to be obtained by working in the solid rock than by the usual method. This idea is completely opposed to the experience of mineralogy. The finest gems, the largest gold nuggets, as is well known, are never, or almost never, found in solid rock, but in loose earthy layers. In such layers in Ceylon the abundance of precious stones, that is to say, of minerals which are hard, translucent, and strongly lustrous, is very great, and enormous sums would be obtained if we could add up the value of the mass of precious stones which have been found here for thousands of years back. Already Marco Polo says of Ceylon: "In ista insula nascuntur boni et nobiles rubini et non nascuntur in aliquo loco plus. Et hic nascuntur zafiri et topazii, ametisti, et aliquæ aliæ petræ pretiosæ, et rex istius insulæ habet pulcriorem rubinum de mundo".
But some one perhaps will ask, where is the mother-rock of all these treasures in the soil of Ceylon? The question is easily answered. All these minerals have once been imbedded in the granitic gneiss, which is the principal rock of the region.
In speaking of granite or gneiss in southern lands, or at least in the southern lands we now visited, I must, in the first place, point out that these rocks next the surface of the earth in the south have a much greater resemblance to strata of sand, gravel, and clay than to our granite or gneiss rocks, the type of what is lasting, hard, and unchangeable. The high coast hills, which surround the Inland Sea of Japan, resemble, when seen from the sea, ridges of sand (osar) with sides partly clothed with wood, partly sandy slopes of a light yellow colour, covered by no vegetation. On a closer examination, however, we find that the supposed sandy ridges consist of weathered granitic rocks, in which all possible intermediate stages may be seen between the solid rock and the loose sand. The sand is not stratified, and contains large, loose, rounded blocks in situ, completely resembling the erratic blocks in Sweden, although with a more rugged surface. The boundary between the unweathered granite and that which has been converted into sand is often so sharp that a stroke of the hammer separates the crust of granitic sand from the granite blocks. They have an almost fresh surface, and a couple of millimetres within the boundary the rock is quite unaltered. No formation of clay takes place, and the alteration to which the rocks are subjected therefore consists in a crumbling or formation of sand, and not, or at least only to a very small extent, in a chemical change. Even at Hong Kong the principal rock consisted of granite. Here too the surface of the granite rock was quite altered to a very considerable depth, not however to sand, but to a fine, often reddish, clay, thus in quite a different way from that on the coast of the Inland Sea of Japan. Here too one could at many places follow completely the change of the hard granite mass to a clay which still lay in situ, but without its being possible to draw so sharp a boundary between the primitive rock and the newly-formed loose earthy layers as at the first-named place. We had opportunities of observing a similar crumbling down of the hard granite at every road-section between Galle, Colombo, and Ratnapoora, with the difference that the granite and gneiss here crumbled down to a coarse sand, which was again bound together by newly-formed hydrated peroxide of iron to a peculiar porous sandstone, called by the natives cabook. This sandstone forms the layer lying next the rock in nearly all the hills on that part of the island which we visited. It evidently belongs to an earlier geological period than the Quaternary, for it is older than the recent formation of valleys and rivers. The cabook often contains large, rounded, unweathered granite blocks, quite resembling the rolled-stone blocks in Sweden. In this way there arise at places where the cabook stratum has again been broken up and washed away by currents of water, formations which are so bewilderingly like the ridges (osar) and hills with erratic blocks in Sweden and Finland that I was astonished when I saw them. I was compelled to resort to the evidence of the palms to convince myself that it was not an illusion which unrolled before me the well-known contours from the downs of my native land. An accurate study of the sandy hills on the Inland Sea of Japan, of the clay cliffs of Hong Kong, and the cabook of Ceylon would certainly yield very unexpected contributions to an explanation of the way in which the sand and rolled-stone osar of Scandinavia have first arisen. It would show that much which the Swedish geologists still consider to be glacial gravel transported by water and ice, is only the product of a process of weathering or, more correctly, falling asunder, which has gone on in Sweden also on an enormous scale. Even a portion of our Quaternary clays have perhaps had a similar origin, and we find here a simple explanation of the important circumstance, which is not sufficiently attended to by our geologists, that often all the erratic blocks at a place are of the same kind, and resemble in their nature the underlying or neighbouring rocks
It is this weathering process which has originated the gem sand of Ceylon. Precious stones have been found disseminated in limited numbers in the granite converted into cabook. In weathering, the difficultly decomposable precious stones have not been attacked, or attacked only to a limited extent. They have therefore retained their original form and hardness. When in the course of thousands of years streams of water have flowed over the layers of cabook, their soft, already half-weathered constituents have been for the most part changed into a fine mud, and as such washed away, while the hard gems have only been inconsiderably rounded and little diminished in size. The current of water therefore has not been able to wash them far away from the place where they were originally imbedded in the rock, and we now find them collected in the gravel-bed, resting for the most part on the fundamental rock which the stream has left behind, and which afterwards, when the water has changed its course, has been again covered by new layers of mud, clay, and sand. It is this gravel-bed which the natives call nellan, and from which they chiefly get their treasures of precious stones.