صور الصفحة
النشر الإلكتروني

not concentrated at one or two points, and of comparatively great power, so as to form regular craters of eruption or to elevate rocks to a great height, but that they extended over a considerable area, and that their intensity and mode of action varied greatly at different places.

Amongst the most common volcanic products is one, small in size, and varying in its character from common indurated argillaceous and lithomargic, to porcellanous and jaspidious, which occurs in very singular forms, vermicular, pseudo corraloidal, columnar,* and frequently resembling pieces of ginger root, externally smooth, granulated, corrugated, reticularly fibrous, &c. These are the compact forms, but there often occur vesicular, or rather rudely ramose cavities descending between the short thick irregular branches towards the centre, the branches being themselves also sometimes perforated.

Another product is a small smooth faintly shining black stone like a fine gravel.

At other places a gravel similar in shape but with a brownish or chestnut-coloured coat or enamel occurs. These latter products may readily be mistaken for water worn gravel, especially as they often occur in broad thin beds, but on closer examination it is clear they are of volcanic origin.

All the various forms of ejected substances met with are due, I conceive, in some degree to differences in the original mineral ingredients of the rocks, but chiefly to the inequality of torrefaction, and the circumstance of the heated, fused or semi-fused substances cooling in the air or in mud or loose sand or clay.

At an early stage in my inquiries I was led to think that the causes of the eruptions were in part what have been called pseudo-volcanic, and if coal shall be discovered it will then become a question whether many of the geological phenomena of Singapore are not due to volcanic action giving rise to and accompanying the contlagration of coal beds. This would account for the paucity of proper volcanic products at the surface, and the abundance of merely altered fragments agreeing in

Amongst the common large slags which are generally of irregular rounded shapes, I have occasionally seen one agreeing in form with those small columnar stones and externally rugose and roughly fibrous. In fact one may say it is the same as one magnified in bulk from a few cubic inches to 10 or 15 cubic feet, and with all its characters rendered Coarse in proportion.

character with the existing superficial strata, and of slagsy and scoreous rocks of which the materials, with the exception of the oxides of iron, might have been derived from similar strata at no great depth. The iron might, on this supposition, have been supplied by beds of ore occurring amongst the carboniferous rocks.

At present this view is inadmissable; and it would still remain so even if no other hypothesis derived from analogy were probable. there have been many volcanoes without streams of lava, from which earth and altered rocks, gases, steam, water, or mud have been ejected, and there are abundant marks of igneous action throughout the series of stratified rocks, proving how frequently volcanic forces have operated from beneath, often without reaching the surface at all, and at other times producing mechanical, igneous, or electrical changes in the superficial rocks, unaccompanied by the more marked phenomena of proper volcanoes.

But the absence of such products in Singapore is not universal, nor are there wanting proofs of the direct connection of the superficial igneous action with a great nether fountain of volcanic power. It is clear that the action reached below the stratified rocks, for in some of the hills near town I have discovered fragments of unaltered sienite, and on one, a large block of sienite passing into basalt, which may either be an ejected fragment, or the protruded summit of a continuous mass, is now being quarried by Chinese. In the Bukit Temah group solid masses of sienite are exposed, and appear to compose a large part of one of the hills. At some places I found it passing into basalt. That the elevation of the sienite and basalt was contemporaneous with the production of the ordinary volcanic or igneous phenomena of Singapore (if the basalt itself was not also then formed) is, to say the least, highly probable. Not only the sides in general, but the summits of the hill, consist of a thick mass of soft ferruginous clay or mould, holding large quantities of the common igneous rocks found elsewhere, but often bearing marks of a more intense igneous action. Thus on the same side of the hill where the sienite and basalt are laid bare I found, in contact with soft sandstone, a piece of compact, dull, igneous rock of a light yellowish brown colour, with veins of a violet colour and vesicles whose sides were similar. At the plane of contact, the rock changed into a dark green translucent-glass, which included some

small opaque white specks. Within the glass, the igneous rock, for a narrow space, was finely vesicular, and violet-coloured like veins and some grains of the sandstone were scattered through this band. The opaque spots in the glass were evidently included grains of sand semifused at their edges. This specimen is identical in character with some products of proper volcanoes. In the slopes to the west of Bukit Temah, which are covered with thick beds of clays and sands, included layers, composed of fragments of torrified granite, occur.

Many of the islands and rocks near Singapore exhibit most decisive proofs of volcanic convulsions. Thus in a reef of sandstone rocks lying between the Island of Blakan Mati and Pulo Sikijang, a black ferruginous rock has been obtruded as a lava through seams and fissures in the sandstone, and at some places has spread over that rock and boiled up above it, assuming fantastic shapes, the sandstone is altered by heat in the same manner as the rock is often seen to be in Singapore.* Basalt and greenstone are found on Pulo Ooban, which lies close to the north-east coast of Singapore. Similar rocks of various structure and character, compact, vesicular, &c. with claystone, porphyries and other volcanic minerals, are brought from Islands in the neighbourhood to Singapore to be used for the foundations of houses. The original production of the latter rocks must of course be referred to an epoch long anterior to that of the former, which undoubtedly corresponds with that of the Singapore semi-volcanic rocks.

We are therefore, I think, justified in considering Singapore and the neighbouring Islands to have been the seat of volcanic convulsions spread over a considerable area, if nowhere of great intensity. There are many reasons, but not strictly local, to believe that their date was in a late era of geological time. The subject however is a difficult one, and there is not room for its full discussion in this


here only mention amongst the local facts tending to the above conclusion, the softness of some of the rocks which have not been altered by volcanic action, but have been elevated and greatly stretched or drawn out, contorted or compressed in the process; the absence of any superficial changes not due to atmospherical causes since the time of their eleva

I may

* Mr. Thomson describes to me an analogous injection of a reddish-black substance, lateritic in its appearance, into the fissures of a block of granite on the north coast of Buitang. This I shall describe on procuring a specimen, if I do not visit the locality,

tion, and the very moderate effects of these causes ; the apparent continuity of some of the hill beds of sand and clay in adjacent hollows, having a ferruginous and torrified appearance in the former, while in the latter they are not distinguishable from soft modern alluvinim ; and lastly, some remarkable cases of the elevation of soft alluvial and vegetable deposits agreeing in their character with beds now forming in the Island or along its shores.* Unfortunately the non-observation hitherto of any organic remains, while it is perhaps a reason for assigning a higher antiquity to the soft rocks above mentioned than their general appearance seems to claim, renders it very difficult to compare them with the observations of European Geologists, or to ascertain whether they can be made to occupy any determinate place in their systems. This last enquiry is however of the least importance for the present, and if entered upon before the phenomena of this locality, (so far removed from any of wbich the geology is, in any considerable measure, understood,) have been minutely and faithfully studied by themselves, is more likely to mislead than to aid research. I may state however that, in the present state of our knowledge, the only European system with which the rocks of Singapore, notwithstanding the apparently recent origin of some of them, can he mineralogically compared, is the New Red standstone. The sandstones, clays, marls, (noncalcareous) and shales, in many respects resemble the same rocks of that system. The rareness, if not the absence, of fossils, is a striking circumstance, and even if the two formations be remote in time from each other (for no chronological conclusion can be drawn from merely lithological characters), points to the existence of anologous conditions during the periods of their respective accumulation.

If we now recur to the present superficial igneous and ferruginous deposits of Singapore, the only remaining question under our hypothesis would be, whether their superposition on the hills (to which they are confined) took place before or after the emergence of the latter from

In other words, was the present configuration of the Island

the sea.

* It is to be remarked however, that in a climate like that of Singapore, clay rocks and aluminous sandstones at or near the surface, unless highly indurated, are liable to become soft. The age of the elevation of the Island will be more fully considered in the paper on the straits, in connection with several instances of recent elevation occurring along its borders where the evidence is of a more satisfactory nature, being derived from organic remains.

If the agency

assumed under the level of the sea, and then the whole tract of land from which the hills spring, elevated by one movement, or is it more probable that before the hills were upraised the general level of the land was the same or nearly the same as it now is, and the hills consequently obtruded from that level in whole or in part in the air? The action of the waters of the sea in spreading out the materials brought to the surface by volcanic forces might seem an obvious explanation of some of the facts formerly noticed. But if this cause be admitted at all, its operation must have been transient and limited, otherwise the surface accumulations on the different hills and parts of the same hill would not have retained their striking local characters.* of the sea is to be admitted, the most probable hypothesis, with our present information, would be, that when the process, which dislocated and pushed up the strata in different places into hills, began to operate, the general level of the sea bed was much lower than it now is, and that the same action caused its general elevation. In this way the surfaces of the hills may have emerged so gradually from beneath the sea as to admit of a partial action of its waters on their summits and sides during and subsequent to the eruptions of matter, and yet not so slowly as to give time for such extensive denudation as to obliterate the local peculiarities of the ejected substances. My own opinion at present is, that all the phenomena may be accounted for by purely volcanic, succeeded by ordinary meteoric causes. At one time rock fragments and semi-fused matter would be voided, heaped up at particular places, or ejected into the air and showered over the surface. At another time, when the heat was less intense or when steam or gases, not ignifluous or melted matter, burst out, masses of soft clays and sandstone might be disembowelled and spread over the bed of fragments. At other places the rocks might be broken and pulverized in situ, and receive a considerable vertical pulsion so as transiently to form an incoherent and agitated mass, especially towards the surface, but without the fragments or sand being freely projected into the air.t

See ante page 527, Diluvial hypothesis. + Whether the mechanical action by which the hills were upraised long preceded, or was accomp or soon followed by, semi-volcanic action in the most intense degree which it here attained, or rather whether the semi-volcanic emissions and eruptions continued during a long period to find vent through the fissures formed when the hills were elevated, is a question that must lie over for the present. It is probable that they

« السابقةمتابعة »