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ASIATIC SOCIET Y.
No. 44.-August, 1835.
1.-Geological Sketch of the Neilgherries, (Nil-giri.) By Dr. P. M. Benza, Surgeon to the Honorable the Governor of Madras.
[Read at the Meeting of the 5th instant.] The group
of hills, called the Neilgherries, may be considered as the southern termination of the Western Gháts, which at this place end in abrupt, lofty, and almost vertical precipices; the extensive valley of Coimbatúr, dividing them from the Palghát chain, which, in the same direction as the Gháts, extends down to Cape Comorin.
The Neilgherries form an elevated plateau, projecting in an easterly direction, from the line of the gháts, in the form of a triangle, the base of which is the continuation of the gháts themselves,
They rise abruptly from the table-land of Mysore, in stupendous cliffs, with an elevation of many thousand feet. Two rivers encircle them, as it were, running round their base. The Bhowání river, rising in the western side of the Kúndas, and among all the hills of that group, runs in an easterly direction along the foot of the side of the Neilgherries, and, just below the apex of the triangle, is joined by the Moyar, which together with the Paykar, having their origin in the Noddimatty range precisely opposite the sources of the Bhowání, and making a sharp curve after leaving the hills, runs an easterly course, joining the Bhowání at Dánikncottah, and under that name, after running about 30 miles, they discharge their water into the Cavery.
The Neilgherries*, being the highest hills in the whole of the peninsula, south of the Himalaya, possess a greater degree of geologi. cal interest than any other group in this extensive region.
" The Neilgherry Hills are situated between the parallels of 11° 10' and 11° 32' N. latitude, and 76° 59' and 77° 31' E. longitude from Greenwich ; their greatest extent in an oblique direction, from S. W. to N. S. is from 38 to
Their being almost in the middle of a district, in which one of the most interesting rocks in the Indian formations (the laterite) is found developed in all its characteristic features, adds not a little to their importance in a geological point of view. On account of their superior elevation, they ought to be carefully examined by the geologist, before he extends his researches to the other parts of the chain, of which they forın the most elevated point.
It was, undoubtedly, after consideration of this kind, that the late Doctor TURNBULL CHRISTIE, of the Madras Medical Establishment, had begun his geological survey of the peninsula from the Neilgher. ries, as from a point where the rocks, found at a lower level, are seen in their original state, unmodified, and unaltered by formations and deposits, which events and revolutions, subsequent to the elevation of the whole chain of the western ghats, must have produced ; and had his life been spared, he would, undoubtedly, have given to the public the most accurate and comprehensive account of the geological formations of this interesting part of India, and would have settled many doubtful points in Indian geology, which now keep many of the ablest geologists in a state of uncertainty and suspense.
The few memoirs he published regarding the geology, not only of India, but of those places through which he journeyed, particularly of Sicily, show what was to be expected from a man, who evinced so much information and accuracy of observation on those subjects. Unfortunately for Indian geology, he was cut off at the very beginning of his labours on these very hills, which had in preference attracted his attention and researches.
We are told that the experienced eye of the geologist can easily guess the nature of the rock composing a bill or a system of hills, by the simple inspection of its outlines : thus, spiry peaks show the formation to be primitive ; rounded smooth outlines are indicative of calcareous mountains ; while the castellated ruin-like appearance of a mountain, is proper to the sandstone formation.
This criterion, however, would lead into error regarding the nature of the rocks forming the Neilgherries. Although their contour is even, smooth, rounded, and, as it were, undulating, the fundamental rocks of which they are composed belong to the primitive class.
Their outline resembles those hills and eminences we meet in districts, resulting from tertiary or alluvial deposits. What the rock
40 miles, and their extreme breadth 15 ; taking in account the great undulations of the surface, and the breadth above stated being pretty constant throughout, their superficial extent may be fairly estimated at from 6 to 700 square geographical miles.-Baikie's Observations on the Neilgherries.
is, which gives those hills the rounded form they exhibit, will be shown hereafter.
With the exception of some vertical cliffs and mural precipices, seen in the boundaries of this elevated plateau, and a few projecting masses of the fundamental rocks on the summits and declivities of these hills, the whole group is uniformly covered by a thick stratum of vegetable earth (No. 1*), which overlying a thicker stratum of red earth, (to be described in the sequel,) supports numerous plants, chiefly grasses, which, growing most luxuriantly in thick contiguous tufts, give the surface a smooth carpet-like appearance. This vegetable earth in general is clayey, and of a grey colour, and very friable. On this soil we occasionally see small rounded pieces of the decomposed subjacent rock, bestrewed particularly on those spots where blocks of the decomposing rock are seen jutting through the soil.
This vegetable soil is replaced in the low valleys and flats at the foot of the hills, by a black soil, such as we frequently see forming the peat. bog in swampy grounds, in which a large quantity of vegetable matter is decomposing (No. 2).
This soil is of a black, or deep brown, colour ; of tenacious consiste ence, when moist; crumbling into powder, and often splitting into prismatic masses, when dry. At first sight, it resembles the black soil of the plains of India. From this last, however, it seems to differ greatly, in containing a large quantity of carbonaceous matter, and much oxide of iron.
To deprive this black soil of the greater portion of its humidity, I exposed it to a heat, sufficient to melt lead, and after having weighed a certain quantity of it, subjected it to an intense heat for an hour ; after this, it had lost more than 25 per cent. of the original weight, and had changed into an ochrey red powder (No. 3), without undergoing any vitrification, as is the case with the black soil of the Deccan, (Voysey.) It would therefore appear, that the loss is owing to the oxidification and consequent volatilization of the carbonaceous matter.
This soil, although more frequently found in low situations, is often seen in a thick stratum on the declivities of the hills, such as on the slope of one of the Dodabetta group, facing the cantonment; on that of the Elk Hills, (S.) above South Downs; near the Kaití Pass, and in many other localities, where I have remarked about it, a most luxuriant vegetation of innumerable ferns, of which the roots are decaying into a black powder.
In many places below this black soil, and sometimes under the * The figures refer to specimens deposited in the As. Soc. museum ; the letters to the accompanying Map.-Ed. .
vegetable earth, we see thick beds of a yellow ochraceous earth abounding with silica (No. 4). Indeed, in some places, as at Kotagherry, it resembles very much the yellow Venetian Tripoli, previous to undergoing preparation for the arts (No. 5). The geological position, however, of the two, differs very much-the Venetian Tripoli, which is brought there from Corfu, and from the neighbouring coasts of Epirus, is found (as I have had opportunities of ascertaining) in the sandstone formation, which alternates with the magnesian limestone*. The kind of Tripoli I met with on the Neilgherries, seems to be the result of the disintegration of a species of iron flint found in primitive formations; some of the specimens I collected, have a great resem. blance to the Eisnkeissel of WERNER (No. 51). Some varieties of the finest white Tripoli arise from the decomposition of silicious rocks, such as calcedony, in Corfu and in upper Italy; but in general, the Neilgherry specimen is not so silicious, and seems to contain a good deal of alumina and iron. It is in this yellow clay that we occasionally see some tubular bodies, formed by concentric layers of the same clay, round the numerous roots of plants that grow on the soil above (No. 6). But what attracted my attention most was, to see (at Kotagherry) those tubular bodies traversing the thick stratum of black earth, which overlies the yellow clay, without having a particle of it in their composition. As if the roots, by a kind of capillary attraction, sucked up through the black soil, without mixing with it, the particles of the yellow clay which, undisturbed by the vicinity of the black soil, arranged themselves concentrically to the root; and the latter decaying has left the cavity of the tube emptyt.
* It seems to be an argillaceous iron ore, similar probably to the one at Ash. burnham, used for the manufacture of Tripoli, and belonging to the Hastings sands.-See FITTON's Geological Sketch of the vicinity of Hastings, page 50.
p" BROGNIART alludes to sometbing similar to these tubular bodies, enclosing the roots of plants in sandy places, where the iron appears to aggregate the sand round the roots ; and he concludes the paragraph by confessing his inability to assign the cause producing it. et dans ces-ci la cause qui a accumulé l'oxide de fer à l'entour de la racine .... est encore difficile à assigner.--Tabl. des Terr. qui composent la Surface du Globe, page 56.'
My friend Mr. Malcolmson, Secretary of the Madras Medical Board, writes to me as follows : On the banks of many of the streams in the Deccan, the black soil is seen penetrated by tubular incrustations, resembling kankar; they are evi. dently formed round the roots of plants, the decay of which leaves a cavity which may sometimes be seen to divide and ramify. Some of those in the banks of the Kanar river, Kamptee, near Nagpore, are more than an inch in diameter.-B.
Sergeant Dean's Jumna collection exhibits many incrustations of calcareous and ochreous matter of a similar nature.-ED.
Immediately below the vegetable soil, in almost all places, we find a stratum of detritus (in general not above a few inches thick), which is different in different localities, according to the nature of the rock on which it rests. Thus, it is ferruginous on those places where iron ores are found : quartzy and silicious above the thick veins of quartz, which intersect these rocks. But in general it is composed of small fragments, sometimes rounded, and sometimes angular, of the decomposed rock (of which we shall speak hereafter), being identical with that we see on the surface of the soil (No. 7).
The simple inspection of this detritus, overlying, and correspond. ing in position and nature to the subjacent rock, forces upon us the conclusion, that it does not belong to the alluvium (terrains de transport), but that it has its origin in the disintegration of the rock in situ, without any, or any material displacement from the rock which has given rise to it.
Another fact that proves this detritus to arise from the decomposition of the underlying rock, previous to its becoming lithomargic earth, and while in the dry friable state which seems to have preceded it, is, that the porcelain earth, wherever this last earth is found in large beds below the vegetable earth, is never overlaid with detritus ; because the rock is all at once converted into porcelain earth, without the intermediate passage into the dry friable rock, from which the detritus arises,
This detritus is seen almost in all localities on these hills; the numerous sections that have been made in their declivities, for the new roads, show it clearly every where. On looking at the banks on the sides of those sections, we observe the detritus adapting itself to all the irregularities and zig-zags of the subjacent rock, or stratum. Fig. 2 of Pl. XXXI. shows this conformity better than any description. It is taken from the bank of the road round the lake near the bund.
That this detritus has not been transported from any distance is further proved, by observing it on the surface of the soil in those places where the protruding rocks are either decomposed or decomposing. We often see the still undecomposed nucleus of the rock protruding through the soil, surrounded and enveloped by the numerous concentric layers of the decomposed rock, the bassets of which we see level with the soil, the upper portion of them having been disintegrated into a detritus, which is scattered on the soil in the vicinity of the blocks. As far as I know, no organic remains have hitherto been found either in this detritus, or in the black soil.
In some places the detritus, for causes difficult to guess, assumes