« السابقةمتابعة »
myself of the natural vertical sections afforded by the shores of Singapore, and the smaller Islands, into which the southern extremity of the Peninsular range is broken, and was thus soon in possession of a body of facts which gave a certainty and consistency to the above veiws. I minutely examined the Islands of Púlo Brání, Blákan Mátí, Sikúkúr, and Sikíjáng on the one side, and Púlo U’bin, Púlo Tikong, Bæsár, Pálo Tikong Kechil, Sejáhát Bæsár and Kechil, &c. on the other side. I also explored the neighbouring coasts of the Peninsula, and the banks of the Johore river. The result was that I found the foregoing hypothesis, so far as it had been developed, to be substantially an expression of the facts. It had however given too much prominence to some modes of the volcanic or semi-volcanic action, and too little to others. Thus, although there has been a certain degree of eruption in some cases where the gases in forcing their way to the surface have excited an unusual mechanical force, their action has, in general, been limited to a partial reduction and metamorphosis of the rock in the zones or dykes through which they have passed up (or in those larger tracts beneath which the surface of the plutonic sea has risen to such high subterraneous levels that the whole superincumbent matter has been saturated by its exhalations. I have also noticed several facts which appear to require us to believe that some portions at least of Singapore, were under water at the time when the gaseous action first reached the surface. The vast abundance of hydrated peroxide of iron and the mode in which ancient ferruginated breccias and conglomerates sometimes occur, would be most simply explained by this hypothesis. The circumstances adverted to in the paper on this subject must be borne in mind. In some places considerable quantity of matter derived from the hills has been deposited in the intervening valleys, probably at or soon after the time of elevation, and been subsequently covered up by modern sea mud on which mangroves have rooted and spread.]
The most difficult branch of the enquiry has been the relation between the volcanic action to which the sedimentary rocks have been subjected, and the crystalline rocks which are associated with them. But, disregarding this for the present, and considering the volcanic action apart from any hypothesis of its origin or its relations, and reasoning from its visible effects, we may lay down this position absolutely, that the whole region in question and a much wider one, as it
will be found, extending to the lower ranges of the Ilimalayas, a large part of Australia, a part of Africa, &c.) has been exposed to a well marked and peculiar, perhaps a unique, * igneous action. It has varied in its intensity and mode of operation, but every where certain prevailing characters demonstrate its unity. These are both chemical and mechanical, the first depending principally on the never failing presence of iron, and the latter evinced by the extraordinary uniformity in the shapes, ramifications and even sizes of the ranges in which the rocks affected have been raised. Whatever be the nature of the original sedimentary strata, this mighty agent has impressed them with the same marks, and the more powerful its grasp has been the more have their native peculiarities been confounded. But between the effects of this intensest force and that so weak that we barely detect its touch, the degrees are almost infinite. Still the only way in which I can render this slight immethodical sketch at all intelligible, will be to note
This I had been very slow to believe, because although there may be places where a fossil fauna or flora altogether peculiar is found, it is scarcely conceivable that any plutonic action should have an entirely locul character, or that one repeated over so many parts of an extensive region in Asia, should not hitherto have been observed by Geologists in Europe or America. I have however, read nearly every English work on Geology without meeting a description of any considerable development of rocks in those quarters of the globe resembling our laterites, and have consequently been obliged to work out their true theory with little help from books, and by dint of patient and minute observation. A few months ago I was led to think that English writers were too much occupied in establishing their own opinions to present a full view of those of continental Geologists, and that the latter were leaving them behind in the science of rocks of injection, reduction and eruption. It appeared necessary therefore to gather their views from their own explanations of them. In the first work which I ordered, and which I received two days ago by the Overland mail, I found an allusion to a district in Europe, which has been described by an eminent French Geologist, and which, if I may judge from the few lines in which it is referred to, must be in many respects analogous to the lateritic tracts of the Malay Peninsula, and consequently of India, &c. also. In a few months I hope to have the means of ascertaining whether this is the fact, and also whether in the writings of other continental Geologists any similar tracts are noticed. A few days ago Mr. Balestier, put into my hands a letter which he had received from one of the gentlemen attached to the recent French Embassy to China, a pupil of the celebrated chemist Dumas, in which he explains the views of himself and another member of the embassy on the Geology of Singapore. His theory of the origin of the laterite had occurred to me when I first began to suspect its real nature. As my observations extended and became more minute, I found that such a theory only explained a small part of the phenomena, and that which I have now held for about 2 years, gradually developed itself, growing clearer and simpler in proportion as it embraced wider ranges of facts.-J. R. L. 16th March, 1817.
a few of the better marked disguises which the rocks assume under this potent influence. I say disguises, because the geology of the Malay Peninsula almost wholly resolves itself into the identification of the original rock under its multiplex transformations. Without a key to this, derived from a minute examination and comparison of the modes of alteration, the whole is a dark riddle, or our geology becomes a congeries of bewildered gropings and sheer mistakes.
The first or lowest degree of alteration, let us say in a clay, is the formation of isolated blotches of a reddish colour in the rock, but unaccompanied by any other apparent change.
2d. A slight comparative hardness in the blotches.
3rd. In addition a grittiness,—they may now be termed nodules or concretions, and we may include in this catalogue all degrees from an incipient grittiness to a hard compact character, which gives the nodule the appearance of an imbedded pebble ; [the nodules are sometimes hard and compact without being gritty or quartzose, and they are gritty in their nascent state where the rock is originally arenaceous in any degree.]
4th. The nodules bulge out at different points, and the preceding three degrees may be repeated in nodules of this shape.
5th. The arms or branches unite so that the rock is pervaded by a complete congeries or ramification of red, rounded, but irregularly shaped, branches. The form of these branches varies very much, but is generally uniform for a considerable space. Very frequently it is as if ginger roots were continued in all directions. At other times the spaces between the ramifications are narrow sinuous perforations or isolated vesicles or deep straight tubes or chambers in tubes. This structure is sometimes the result of an allied or predisposing structure in the rock affected, and at other times, it appears to be wholly superinduced by the altering agency. In this last form the red portion is found of various degrees of hardness, but not so soft as the first degree. In general it possesses a medium degree of hardness so as to be cut with an axe.
6th. In this class we may include the products all degrees of heat that has been suddenly applied in sufficient force to produce calcination, and this distinguishes it from all the preceding, in which the
* It hardens on free exposure to the atmosphere and is used in building.
rock has been merely impregnated with hot ferruginous gases or vapours -where the calcination has not been great the original structure of the rock is better preserved than in the merely impregnated rock, because, in the latter, the indurating action of the iron, the different degrees of its oxidation when it comes within the influence of water and air, and the washing out of the softer portions in the hollows, often give it an amygdaloidal or vesicular structure totally different from that of the original rock-a slight roasting on the other hand preserves the latter and saves it from meteoric destruction. The limit of this preserving power is soon reached, and every higher degree of heat and larger infusion of iron exerts in each rock, a corresponding destructive or altering power, and approaches nearer that point where the original differences in the rocks cease to be distinguishable. The extreme limits of this class appear to be where the rock is merely scorched on the surface, preserving its original character beneath, and where it is thoroughly reduced to a cinder. This class of rocks very frequently presents mamillated and botryoidal surfaces.
It occurs in dykes, and on the sides of fissures through which hot blasts appear to have rushed. It also occurs in an outer layer or thick crust over rocks of the 5th class, in which case it would appear that the different effects producerl by the same gas arose from the upper crust being exposed to the air and consequently burnt. In the same way the calcination to some depth on the sides of fissures may have arisen in certain cases, not from the gas that rushed through them being hotter than that in the body of the rock (though this was most likely the fact in general) but from the presence of air producing combustion. Between dykes of this last class rocks altered in the above 5th degree are common-but dykes of the 5th degree also occur. The difference in every case will depend on the relative intensity of the heat and degree of ferrugination of the gas, and the fact whether there was air to support combustion or not.
The preceding remarks are applicable chiefly to rocks either composed of clay or in which there is a basis of clay. But a very small proportion of clay suffices for the exhibition of the above modes of action. When the rock is wholly arenaceous, nodules are not formed. The rock is reduced to a dry incoherent or friable mass where the action has been slight. Where it has been greater, a net work of cracks
pervades the rocks, and the seams have either a thin plate of blackish ferruginous crust included between them, or their sides have a similar thin coating which is often covered with an exceedingly minute mamillation. In some cases the matter between the seams or ferruginous walls, has been dissipated, and the rock appears as a black honeycomb. In all instances of high calcination the sandstone is greatly indurated. It is sometimes converted into a crystalline rock.
Friable shales, again, are sometimes changed into a dry powdery matter resembling volcanic ash.
Where the bodies of the strata are not altered their planes of junction are sometimes slightly indurated and mamillated. The gas in every case has taken the readiest channels to the surface,--and where fissures have not assisted its emission, it has forced itself through the planes of least cohesion, such as the junction planes of different beds, cleavage planes, &c. It thus often exposes the internal structure of the rock where it would otherwise appear compact. The composition of the rock has often had a great influence in determining the channel of emission, so that its action sometimes is chiefly confined to one or more strata, the adjoining beds appearing to be little if at all affected.
Quartz frequently accompanies the ferruginous change, but rarely to a considerable extent.
The above are the most common modes of alteration, but there are others approaching nearer to true metamorphism. Clay is converted into a porcellainous or jaspideous substance,-sandstone into a hard siliceous flinty substance. Conglomerates and breccias have frequently a base of this nature.*
The mechanical force accompanying the evolution of the hot ferruginous gases or vapours has been great, but it has been exerted within, narrow limits. Thus the strata are often vertical, and generally rise at high angles, but the dip varies much, and even in adjacent hills of the same connected range is sometimes reversed. Yet they are never raised more than a few hundred feet above the common basal level, and the majority of the almost innumerable hillocks which compose the ranges of Singapore, are probably rather under than above 100 feet.
* I have since found on the eastern coast of Púlo Krímún Kíchí (the Little Carimon) great masses of clays and conglomerates transformed into a perfect crystalline chert as hard as fint.-J. R. L.