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a degree of hardness, and approaches a conglomerate ; the small rounded pieces being agglutinated by a clayey paste, resembling a puddingstone. This is particularly the case in those localities where it overlies the iron ores, so abundant on these hills. When the subjacent rock is the hematitic iron ore, the conglomerate resembles exactly the pisiform, or oolitic iron ore, and in some places it is hard enough to be used for architectural purposes. The conglomerate in this state of aggregation is similar to some varieties of laterite found in the plains of the Carnatic. But this pisiform iron ore is not to be confounded with another rock, which also resembles laterite, and is met on these bills in enormously thick beds, hereafter to be described.
Below the detritus, in almost all places on the hills, we find a thick stratum of an ochraceous red earth, which occasionally assumes both the appearance and the composition of lithomarge, and for this reason, I shall call it hereafter indiscriminately either lithomargic, or red earth. In some of the lower hills, this stratum is above 40 feet thick, as it is near the bund of the lake. It is this red earth, which, filling up the interstices among the original inequalities of the projecting rocks, has given the hills their rounded appearance, by smoothing all the asperities and irregularities of the original rock ; or, to speak more correctly, the projecting points themselves have been smoothed down by their own decomposition into lithomargic earth.
In general, this red earth is of a mottled colour, or streaked with ditferent hues of red, yellow, crimson, white, and grey or brown. It feels unctuous to the touch, and crumbles into dust when pressed between the fingers. It does not form a paste with water, but subsides to the bottom of the vessel. The different colours of this earth are separate and distinct, having a decided line of demarcation, so as to show that they are produced by the decomposition of I separate and distinct minerals. We occasionally find in it thick veins of pure white felspar decomposed into porcelain earth, traversing it in all directions; precisely as we observe the same veins of felspar, in an un-decomposed state, traversing the hard rock, which forms the hills.
This red lithomargic mould is evidently the result of the decomposition of two of the rocks, which almost exclusively form the Neilgherries; viz. the sienitic granite, and the hornblende rock, or primitive greenstone ; of both which we shall speak hereafter.
It seems that before the rock is transformed into red earth, it passes into a dry friable substance, which sometimes has consistence enough to be cut and used for architectural purposes; many of the stones used in the construction of the Kiiniir bridge, are of this nature. The second stage of the decomposition is that, in which it becomes of a. soft consistence and earthy texture : the minerals composing the rock still retaining their relative position as before. Thus we see in the lithomargic earth, what was hornblende, changed into a red ochrey substance; the felspar into a white clay ; the numerous garnets into a crimson-coloured clay ; the quartz alone remaining unaltered and undisintegrated, which, after all, occurs but in a very scanty proportion in the rock (No. 12).
It is curious to observe, that the substance of the crystalline rock is not protected from decomposition by the thick layers of its own decomposed substance ; and notwithstanding its being buried many feet beneath the surface of the soil, under athick stratum of vegetable earth detritus and lithomargic earth, the decomposition appears to be going on without the concurrence of the atmospheric air.
In many places the entire block has undergone the process of decomposition, and in the sections for the roads, we occasionally see many concentric layers of the decomposed rock, like the coats of an onion when cut transversely. It is not rare to observe, that these coats have, in many localities, a kind of crust (enduit) of a black substance, probably oxide of iron (No. 13). The decomposition of the rocks takes place from outside inwardly, and appears to proceed, or to have proceeded gradually. It seems that the felspar and the hornblende are the first to be decomposed, the one (losing the alkaline matter P Sir H. Davv) becomes opaque and whitish ; the other, by the hyperoxidation of its iron, is converted into an ochreous clayey substance : the garnets do not resist decomposition long; but the only change that the quartz seems to undergo is in its degree of compactness; becoming friable, and easily reduced into sand by the fingers.
If observations and facts were wanting to prove that this thick mass of lithomargic earth is owing to the decomposed granitic rock of these hills, the following is conclusive. The original undecomposed rock is, as Ihave said, traversed occasionally by thick veins of quartz. These veins resisting decomposition (which affects the remainder of the ingredients of the rock) are seen in a continuous course, penetrating from the hard crystalline undecomposed nucleus of the rock into the lith0margic earth, and into the concentric layers of the already decomposed rock. Therefore, it is impossible to avoid the conclusion, that the red earth and the rock were, at one time, one mass, traversed by the quartz vein, which is still seen continuous and entire, notwithstanding the transformation of one-half of the rock into red earth.
The appearance I have just described, is seen on the N. bank of the road, which descends from Ootacamund to Kaiti valley, after the steepest descent of the Kaiti pass is finished; and, I dare say,
may be found in many other places, which I have had no opportunity of visiting.
What I have said of. the quartz veins is also applicable to the more numerous felspathic veins, which traverse the rock ; with this difi'erence, that they are decomposed, and converted into porcelain earth, while those of quartz are entire and unchanged. But the continuity of the vein is evident, although one-half of it has changed nature.
An additional, although negative, proof regarding the transformation of the granitic rock into lithomargic earth, is, that on those hills where no rocks containing hornblende are found, this earth is wanting. This is the case on the summits of Dodabetta, Elk Hill, Kaiti pass, &c., in which places the protruding rock being either granite, or pegmatite, it exfoliates in laminae like granite, instead of decomposing into red lithomargic earth.
It would be worth ascertaining, whether the crimson-coloured dots and streaks in the lithomargic earth be owing to the decomposition of the numerous garnets contained in the original rock. I have had opportunities, more than once, to remark, that in those localities where the sienitic granite abounds with garnets, the lithomargic earth, resulting from its decomposition, has the crimson coloured dots similar to those in the undecomposed rock (No. 14). I have made the same observation in the decomposed gneiss in the Northern Circars, where it abounds with this mineral.
A question naturally presents itself after the above remarks, regarding the decomposition of the granite, and hornblende rock of the Neilgherries. The same identical rocks are found in many parts of the Peninsula, particularly along the chain of the eastern ghfits ; and yet their decomposition does not give rise to the same results. As I have visited but very few localities in India where these rocks prevail, Icannot positively say whether or not the result of their decomposition in both localities be the same*. But, this is certain, that the causes, which may have contributed to decomposition in one place, do not exist in the other: of that class are cold, damp, frost, eleva
_ vation, &c., which are not found in the low lands. Besides, is this
decomposition the effect of misting causes, or the consequence of time and revolutions gone by 3'
Here I must remark, that in some localities, such as near the bund of the lake, on the road below the church, above the bazar, &c. the red earth assumes the composition, texture, and appearance of real lithomarge.
* Doctor Harm: says, “ a red soil prevails where sienite forms the apparent ground rock.”—-Tracts Historical and Statistical on India, page 349.
As I have proposed to abstain from speculations, and from farfetched theories, I shall not enter into any hypothesis respecting the causes of this decomposition. It is enough to have noted a geological fact, which requires but simple inspection to be certain of its existence. I shall therefore proceed to describe some minerals, which are found imbedded in the red earth; some of which might prove very useful and advantageous in the arts. Such is the porcelain earth, found in enormous beds, and of the greatest purity, in this locality.
This mineral is evidently derived (as it is almost in -all places where it is found in Europe) from the decomposition of the pegmatite or graphic granite, which is chiefly met with in primitive districts. As this rock does not appear to be common on the Neilgherries, I found it ditficnlt, at first, to account for the origin of the numerous and thick beds of porcelain clay. It was after visiting and examining the summits of some of the highest hills, that I found a variety of pegmatite forming many of the most prominent rocks on them. Such are the summits of Dodabetta, Elk Hill, Kaiti pass, some of the peaks of the Kimdas, and probably many other places which I did not visit.
It is undoubtedly to some of the erratic blocks and rolled masses of this rock, or to the decomposition of those beds of pegmatite, ‘into which the true granite of the high hills seems to pass, that the porcelain earth is owing. Of these blocks, still in an undecomposed state, we see many in the valley of Kaiti derived, in all probability, from the summit of Dodabetta, or from that of the rock of Kaiti, where the pegmatite is seen in situ.
By comparing a piece of this porcelain earth, just taken out of the bed, with a piece of the hard pegmatite rock, one cannot but be convinced of their being the same rock; the one in a hard, the others, in a decomposed state. (No. 15.) The pieces of the crystalline smoky quartz (which is the only other mineral entering in the composition of the pegmatite, besides felspar) are still visible in the same situation, as when the rock had not undergone decomposition, having become more brittle, and easy of disintegration.
The porcelain earth is not to be confounded with that which results from the decomposition of the pure felspar veins, so frequently seen in the sienitic granite. By simply looking at both specimens, the difference is discovered (No. 16). The latter has no sandy particles in its composition, such as are found in the other, which by such addition is better adapted for the manufacture of pottery, in which silicious sand is a necessary ingredient.
I speak with some hesitation regarding a mineral I found only in one place on the Neilgherries, and I am doubtful whether it exists in
any quantity in those bills. It is a brown ferruginous clay, very closely resembling amber, particularly that kind which is exported from the Island of Cyprus (No. 17). I found it between two large blocks of decomposing sienitic granite, or rather hornblende rock, with garnets, close to the bund of the lake.
The next rocks to be described are two metallic ores, in all probability, originally imbedded, as veins, in the rock: which last being now decomposed, they are left imbedded in the lithomargic earth: indeed, one of these ores is still seen as a vein, in the undecomposed rock.
The first is the magnetic iron ore, so common in many parts of India, and which, besides the metal, contains variable proportions of quartz (No. 18). The places where I have met with this iron ore are marked in the map : in some of them the ore is imbedded in the lithomargic earth, while in others it is like a vein in the rock. I saw it in this last position in the road descending to Kaiti valley, where the metal is very little in quantity, compared with the granular quartz, which in some parts of the vein predominates to the almost entire exclusion of the metal (No. 19).
The two places on the Neilgherries, wherel have seen this ore very rich in metal, are, one near the village of Vartsigiri (Vrotagherry), and the other close to, and traversing, the Lake of Octacamund in two places. The specimen from Vartsigiri (No. 20) is very compact and rich in metal. I took it from a large block, probably the outgoings of a thick bed at the southern extremity of the valley, at the other end of which the village stands.
Generally speaking, the quartz is larnellar, very rarely granular, and it seems to alternate with the metal in parallel laminae. The appearance, composition, and proportion of the ingredients of this magnetic iron ore are very different in different places ; nay, in the same vein. For instance, the vein seen just below the building called Gradation Hall, between the road, and the margin of the lake, in its N. E. extremity, has a compact, metallic structure, highly magnetic, with hardly any quartz (No. 21) : a few yards to the southwest, the vein contains a good deal of quartz; the metal is more oxidated, although maintaining still its magnetic powers (No. 22). Following the vein in the same direction, we see it appear in the opposite side of the lake, in the banks of the road, which goes round and close to the lake. There the ore has lost a good deal of its quartz; the iron is more oxidated, and the rock assumes a kind of columnar structure (No. 23). This is the appearance of the vein in the section for the road. But the out-croppings of the vein at the