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this letter I made the following remarks with reference to laterite :
Many of the clayey hills here [in Singapore] appear to me to be decomposed sienite, sometimes unaltered by supervening volcanic action, but generally partaking in the metamorphism which the matter of most of the elevated land has suffered from that cause.”
May I venture to suggest that the hypothesis which is developed in this paper for Singapore, might, if applied to the laterite of India, perhaps explain its origin, and, in doing so, to a certain extent also reconcile the conflicting opinions that have been maintained regarding it. All that I have read of the great laterite formations of the south of India, and which extend to the heart of Bengal, where they are described by Dr. Buchanan Hamilton, leads to the conclusion that they are not purely volcanic, sedimentary, or decomposed matter, but what I have termed semi-volcanic. The same formation is found at Malacca, and analogous deposits occur at Singapore, and both are inseparably associated, and evidently contemporaneous, with altered rocks of the kind previously noticed. If we conceive an area with trap, granite, sandstone, shale, &c. exposed at the surface, (in the atmosphere or in the sea,) and partly decomposed or disintegrated, to be subjected to a peculiar species of minor volcanic action like that which is described in this paper* (the distinctive phenomenon, probably, of one and the same geological epoch), the results would be, that with the occasional exception of matter ejected from no great depth, and some dykes and reins, the previous soft surface rocks would be merely altered and metamor. phosed by heat and impregnated with iron, derived perhaps from the basaltic and other ferriferous rocks through which the discharged steam, gases and water had passed in their ascent. Whether the action took place under or above the sea would be determined by the presence or absence of the ordinary marks of oceanic denudation. When clays strongly ferruginous and soft from saturation with water, are dried, the iron previously held in solution by the water is deposited between the particles and cements them into a hard compact rock. Hence the
* Whether the upper plutonic rocks were the direct sources of the igneous action, or were themselves, together with the sedimentary rocks acted on by a lower plutonic sea, does not affect my explanation of the formation of laterites ; for whether I adopt the one or the other view of the source of the injections and impregnations which produced the laterites, or remain in doubt on the subject, the fact, deduced from the actual examination of these rocks, that they have been so produced, is not at all rendered doubtful.
induration of laterite clays on exposure to the atmosphere." My opinion therefore was that though proper laterite was nothing more than one of the forms of alteration produced by plutonic ferruginous gases,that which, in the arbitrary scale formerly given, I have called the 5th degree,—and that any rock in which a sufficient quantity of clay was present, whether it were purely sedimentary or a decomposed crystalline or compact rock, or whatever its origin or character in other respects was,--would, on being exposed to certain degrees of impregnation by such gases, and under the conditions before adverted to, become laterised. This opinion was abundantly confirmed by later observa. tions, but these also proved that iron alone was capable of producing rocks of a lateritic form. The result therefore was that although proper laterite is produced in the mode which I have mentioned, yet that mode is not essential to the formation of a lateritic structure. The only essential thing is the diffusion of iron in ramifications throughout a clayey rock. Get the iron so diffused, and it is of little consequence by what door it was introduced. The only distinctive quality of proper laterite is that it has not merely got the iron, but has been, in various degrees, baked in the process of impregnation, and close examination can always discover traces of this. On the other hand, iron may be introduced by aqneous saturation, and if the soft rocks so saturated have planes of inferior cohesion, as many rocks have, the iron will there accumulate. If the iron solution pervaile a homogeneous clayey rock as water does a sponge the segregating or concretionary quality of iron so diffused may gradually draw it into connected nodules or ramifications; and indeed it is probable that in all cases of volcanic gascous impregnation of the compact parts of rocks the ferruginous matter remained for a time diffused throughout the rock, and that this segregating tendency subsequently superinduced its contraction into ramifications and blotches. Where the gaseous impregnation was weak, it would speedily draw into isolated blotches, - where stronger into isolated concretions,—where strongest, and the heat not too great, into ramifications. Again the iron may be laid up in the heart of a crystalline rock solidified from a plutonic fluid holding iron, and the essential condition for the production of the laterite structure may be found in decomposed hornblendic, or even black micaceous granites that have not been subjected to any supervening volcanic action. The oxidation
of iron solutions in clays on exposure to the air, and the combustion of
little attention to these subjects and had not formed the preceding views. When an opportunity occurred at the beginning of last month of revisiting the place, I eagerly seized the occasion of testing these views in a new locality, and one which had been described by Geologists, such as Captain Newbold, and Dr. Ward, familiar with the much vexed laterites of southern India. Captain Newbold, in his work on the Straits describes the Malacca hills “as being generally of granite with the exception of a few near the sea coast, which are of laterite overlying the granite. Specimens of hornblende rock have been brought to me, he continues, from a hill a little south of Malacca—the islets on the coasts are of granite of various kinds, with white, red and green felspar. In all, the felspar appears to be predominant, and mica deficient.” Dr. Ward says of the Malacca leterite--"In all its properties it agrees exactly with the rock common on the Malabar coast and described by Dr. Buchanan under the name of laterite.” I was now therefore, for the first time, in a position to bring my theory to the strongest test, for I had not seen any specimen of Indian laterite, and could only compare some of the apparently analogous Singapore rocks with it from descriptions. Captain Newbold, in one of the latest of his numerous papers on the Geology of Southern India, describes very minutely the often mentioned laterite of Beder and makes some remarks on the long debated question of origin. IIe combats the idea that it is a contemporaneous rock associating with trap, or a product like trap of igneous fusion. He also casts doubt on the theory, advocated by several Geologists, of the laterite being "nothing more than the result of the recent disintegra
tion of the granitic and trappean rocks in siti," and, without giving a decided opinion, says “the beds of lignite discovered by General Cullan and myself in the laterite of Malabar and Travancore and the deposits of petrified wood in the Red Hills of Pondichery, in a rock which, though differing in structure, I consider as identical in age with the laterite, and other facts too long for enumeration here, points rather to ils detrital origin like sanılstone."* (Journal of the Asiatic Society of Bengal, Vol. XIII. p. 995, 1814.) Mr. Darwin, I may mention in passing, seems to lean to a similar opinion with respect to analogous rocks noticed by him. “The origin of these superficial beds," he says, “though sufficiently obscure, seems to be due to alluvial action on detritus abounding with iron.” (Volcanic Islands, p. 143).
The first lateritic locality which I visited on my arrival here was the Island of Pálo Upæ, from which much laterite has been removed for building purposes, and where it continues to be cut. The first fragment which I knocked off the rock at once satisfied me that my theory was correct. It was a rock totally different in its original character from any which I have found at the southern extremity of the Peninsula, but which, by the same agency that altered the ordinary sedimentary rocks there, had been transformed from a common argillo-micaceous schist into a rock undistinguishable, save on minute inspection, and, where the alteration has been great, absolutely undistinguishable from some of the altered sedimentary shales and clays of Singapore. Upon careful examination I found, as I expected, in the sections afforded by the coast of this little islet, the original unaltered micaceous rock with great bands or dykes and overlying masses, exhibiting abundant varieties of transformation from a rock slightly discoloured by the ferruginous action through several lateritic types, to the calcined slaggy form in which the original composition and structure are wholly obliterated. I cannot enter into further particulars. My subsequent examination of about fifty miles of the coast from Pulo Arang Arang (P. Arram) southward, and of a portion of the interior of Malacca, has proved that the whole of this region has been originally composed in a great measure of the same argillo-micaceous schist. I shall hereafter give
* I have read all Captain Newbold's papers with the attention which they deserve, and I think every fact which he notices in his notes on laterite tracts is reconcileable with the theory which I maintain,
its numeralogical characters, for I have not time nor means at present to ascertain them carefully. It is soft and glistering like silk, and leaves a powder on the fingers which exactly resembles in appearance the fine glistering powdery down from a butterfly's wing. In some cases it is less dry and more argillaceous. With the exception of Cape Rachado* it has almost everywhere been more or less penetrated in bands (and broad spaces occasionally) by ferruginous gas which has transformed it into one or other of the forms before described or some intermediate forms. Dykes and veins of pure quartz and of quartz with numerous fissures filled with an iron crust are frequent in some localities, while in others they are wanting. Wherever these dykes and veins occur the foliation of the schist is much contorted. In some localities the surface is covered with black shining mamillated scoreous blocks passing down into a lateritic mass, in which the schist is often not greatly altered but is penetrated by ramifying dykes and veins of a ferruginous, quartzose, or quartzo-ferruginous character. Isolated pseudo-crystals and isolated plates of quartz occur in the schist in some places, and, on the other hand, patches of the schist are found in the hearts of large pieces of quartz. But it would require other 20 pages to give even an outline of the varied and irregular manner in which the rock has been altered. If we did not every where come upon portions of the original rock unaltered, or find traces of it in the altered tracts, it would be almost impossible to believe that all the varieties of the latter have had a common origin. I must briefly allude to Cape Rachado. This is a bolder and higher range than any found elsewhere along the coast and projects far into the Straits. It is the only locality which I have yet seen where the quartzose has predominated over the ferruginous action of the plutonic gases. The rock every where ex. hibits unequivocal evidence of its having been originally the same argillomicaceous schist which prevails over the rest of the region. In some places the cliffs are almost wholly quartzose, -in others the rock is a congeries of quartz veins and foliæ,-in others the seams between the quartz foliæ have a coating of the original mica,—in others the original mica predominates and the quartz is more sparingly scattered through it. Broad dykes of compact quartz, of quartz mixed with a ferruginous crust, of numerous parallel veins with quartz crystals springing from
* Where the plutonic action has been of a silicifying more than a ferruginating nature.