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On Soundings recently taken off Barren Island and Narcondam, by COMMANDER A. CARPENTER, R.N., H. M. I. M. S. 'Investigator,' the Officer in charge of the Marine Survey of India.
In February 1884, the volcanoes of Barren Island and Narcondam, in the Bay of Bengal, were surveyed topographically and geologically by Captain J. R. Hobday and the undersigned, the results of our work being published in the Memoirs of the Geological Survey of India (Vol. XXI). In the report alluded to I pointed out1 that scarcely any reliable soundings had been taken off the shores, and that, although there was reason to believe that the volcanoes rose from very deep water, almost nothing was known on this point with certainty. The projected scientific cruise of H. M. I. M. S. Investigator seeming to afford a possible opportunity for examining the ground, I drew up a short note respecting the desideratum in question, which was forwarded by the Director of the Geological Survey to Captain A. Carpenter, the Officer in charge of the Marine Survey of India, in command of the Investigator. Our acknowledgments are due to him for the readiness with which he fell in with the suggestion offered, and for the charts, of which reduced copies are appended herewith, giving the results of soundings taken by him last May.
The following sections have been plotted from the above soundings, combined with the sub-aerial contours as given by Captain Hobday.
In respect to Barren Island, especially, they show very clearly how insignificant the visible portion of the volcano is, in comparison with the huge mass of ejecta which has been piled up beneath the waves. The volcano certainly rises from a depth of not less than some 800 fathoms, giving a total height from the sea-bottom of not less than 6,000 feet, as the volcano stands at present, or some 8,000 before the upper part of the outer cone was blown away.2 But the outward slope is still continued at the deepest soundings taken, indicating that the base of the sub-marine mountain (or the sea-floor) must be sought still further out, and that the entire altitude is still greater than that given.3
Most sections of the inner cone present an angle of 32°, while the average slope of the outer cone above water has been estimated at about 25°. Of the sub-marine slopes plotted B has, near the shore (i.e. inside the first sounding), the remarkably steep inclination of 320, the sub-aerial slope above being only 21°. At C the corresponding inclinations are nearly equal, or 28° and 30°. A shelves more gradually, or at 19°, which may perhaps be attributed to the latest outpour of lava through the breach into the sea.5 D is not steeper near the shore than considerably further out.
Taking the entire lines of sounding into consideration the mean inclinations for equal distances are fairly uniform with one exception. Thus the inclinations of A,
'Memoirs, Geol. Surv. of India, Vol. XXI, pp. 258, 281. 2 Ibid, p. 257.
Since the above was written, later soundings, given in the letter published below, have been received from Captain Carpenter, which indicate a probable total height of some 8,000 ft., at present, and 10,000 formerly.
• Op. cit. 257.
"Vide Map, Op. cit.
B, and C for the first 2 miles from shore (the total length of the shortest line B) are respectively 1610, 1810 and 2010.
The reason why the water is comparatively shallow to S.S.W. of the volcano (along the line D) is not very apparent. Any effect caused by the action of the wind, during the eruption of fragmentary ejecta, would be sought for on the opposite side of the island, i.e. to leeward of the south-west monsoon. According to the charts of the Bay of Bengal, just issued by the Meteorological Department, the currents are very irregular in that part of Bay, so that the effect in question can scarcely be attributed to a persistent set of detrital material in one direction. The shallow water is in the continuation of the line joining Narcondam and Barren Island, a fact which lends some little support to the idea that one or more sub-marine eruptions have occurred there; but in this case one would expect a more irregular sea-bottom than the soundings indicate. It may be, indeed, that eruptions of ash, &c., have occurred as suggested, and that the material has been distributed over the surface by the action of currents.
Both sections of Barren Island show along the sub-marine slopes that curvature of outline, due to gradually decreasing inclination, which is so common in sub-aerial volcanoes, and of which an admirable example is to be found in the great Japanese volcano of Fusiyama.
Samples of the material brought up by the lead from nine of the soundings 2 (including the deepest and furthest out) were sent to the Museum by Captain Carpenter. The stony matter consisted in every case of grains and small pellets of black volcanic ash. With reference to organisms Captain Carpenter writes—"I consider that there was a remarkable paucity of marine foraminifera deposit over the volcanic or pluvial detritus in the depths round Narcondam Island, and a complete absence round Barren Island for many miles, which appears to show the slowness with which marine deposits are made in that portion of the ocean. You will notice on the tracings of our soundings that I have noted the occurrence of globigerina and pteropod shells round Narcondam, but only in three cases was the major part of the formation composed of marine ooze." That there should be more organic deposit on the sea-bottom off the shores of an extinct volcano, like
The surface temperatures at two other soundings were 800 and 86° 5. The temperatures given above are corrected for pressure.
Narcondam, than round an active one like Barren Island, is what might perhaps be expected. Such deposits in the latter case are always liable to be buried beneath showers of ash.1
It will be noticed that the soundings off Narcondam are on the whole shallower than those round Barren Island. To a certain, but probably to only a small, extent this may be due to the raising of the sea-bottom through the wash of detritus off the flanks of the extinct volcano.
It would seem, however, that the minimum depth assignable to the sea-floor cannot be less than some 600 fathoms, which would give a total height to the volcano of about 6,000 feet. F. R. MALLET.
Since the above was written, the subjoined interesting letter, from Captain Carpenter, dated 22nd November, has been received.
"In continuation of my letter No. 903 of 1st November 1886, I have to acquaint you that on passage to Burma this month I took deeper soundings between Barren Island and Narcondam Island, obtaining these casts:
N. of Barren Island
On the tracing sent you last July there is a cast of 411 fathoms at 4 miles N.N.E. of Narcondam; and on the published charts there is about 50 fathoms at 70 miles N.N.E. of it, from which the depths shoal gradually to the delta of the Irrawaddy. It therefore appears that the deltaic shelf extends right out to Narcondam.
A thermometer sent to the bottom at the 1,010 fathoms cast shewed 41°2 corrected temperature Fahrenheit. The usual temperature of the Bay of Bengal at 1,010 fathoms is 37°6, and the temperature 41°2 is that suitable to a depth of 740 fathoms. If we look at a chart of the east side of the Bay of Bengal we see that there are three inlets into this partly enclosed sea. One is only 150 fathoms deep, viz., Preparis Channel; one is not marked with any depth, viz., the Ten degrees channel; and one has 760 fathoms marked nearly on the ridge between Acheen Head and Great Nicobar.
As you are doubtless aware, the cold of the great ocean depths is due to the gradual flow of Arctic and Antarctic waters towards the equator, and it has been shown by the Challenger expedition that where the cold flow rises to pass over a ridge it becomes warmed, and does not again lose its heat on descent into a deeper bed. This temperature then at 1,010 fathoms, equal to the normal temperature at 740 fathoms, seems to prove, as far as one observation can be a proof, that no greater depth than 740 fathoms exists on the ridges between Acheen Head and Great Nicobar, nor in the Ten degrees channel."
1 While on the subject of Barren Island, I should mention that Mr. Daley, Apothecary of the Investigator, landed on the 28th April, and noted the temperature of the hot spring as 110° F. This is interesting as showing that the decrease of temperature which has been going on during the last quarter of a century (Mem. Geol. Surv. of India, XXI, 275) still continues. Captain Carpenter remarked that "from the ship the thin column of steam (from the central cone) could be barely seen at 3 miles distance."
Note on a character of the Talchir boulder beds, by W. T. BLANFORD, F.R.S.
There is a character of the Talchir boulder bed to which I do not think attention has been called, but which appears to me to throw some light on the origin of the deposit. This is the combination of large size with thorough rounding of the boulders.
In many places boulders exceeding a foot in diameter are as common as smaller fragments, sometimes, if my memory is correct, more abundant, whilst thoroughly 'rounded masses of 2 and 3 feet in diameter are of frequent occurrence, and even larger rounded boulders may be found. I have measured more than one 6 feet in extreme length. The boulders are often almost, or quite, spheroidal in shape, evidently from the effect of being rolled,
Now rolled boulders of this size, exceeding a foot in diameter, are, I believe, of extremely rare occurrence on sea coasts. I have not for many years had an opportunity of visiting a stormy coast composed of hard rocks, but so far as I can learn from enquiry large rounded boulders are not often met with in any quantity. In large rivers such rounded masses are also infrequent, and are only found in very rapid streams, such as the Nerbudda. When they do occur, they are few in number, compared with the smaller pebbles. Large rounded blocks with but few associated. smaller fragments are, so far as my experience serves, characteristic of hill or mountain torrents.
I may be mistaken on these points, but I think the question is worth raising. If I am correct it seems probable that the Talchir boulders are derived from rapid streams. The beds themselves however in their fine texture and general absence of false bedding indicate deposition from still or slowly moving water. My impression is that they were formed in large marshes or lake-like expansions of great river valleys. The utter unconformity between the Talchirs and all underlying formations indicates a great change in the condition of the country at the commencement of the Gondwána period, a change by which an area of subaerial denudation was converted into one of partial deposition. If the change, as is probable, was one diminishing the fall in the river valleys, a natural result would be that in portions of those valleys there would still be a sensible though diminished fall, whilst other parts would be converted into large marshes or shallow lakes. In the latter the Talchirs may have been deposited, the boulders being floated in from stream beds in the surrrounding hills. The only known agent in floating such boulders is winter ice, especially when broken up by floods in spring; water weeds and roots of trees could not account for the number and frequent occurrence of the boulders in so many different places. The circumstance that the boulders are of irregular occurrence, abundant in places, in others absent, is quite in accordance with the theory put forward.
It should be understood that the Talchir beds to which I refer in this note are those of Bengal and the Central Provinces generally. Mr. R. D. Oldham has recently shown that the boulder beds noticed by myself some years ago in Western Rajputana are also probably Talchirs, but I quite agree with him that these differ from the typical deposits in several respects.
Analysis of Phosphatic Nodules from the Salt-range, Punjab, by H. WARTH, PH.D.
The proportion of phosphorus-pentoxide is equivalent to 66 per cent. of ordinary pure calcium phosphate.
These nodules occur in the shales above the coal in the eocene strata of the Eastern Salt Range in the Punjab.
They are covered all over with peculiar circular pores. Very often shells are also found metamorphosed into the same phosphatic mineral.
The nodules are very numerous about Dandot Colliery and the neighbourhood. The quantity of material (as far as it was followed up) was not found sufficient for practical utilisation, but the occurrence is so far of interest as it affords another proof of the presence of phosphatic mineral in the sedimentary strata of India.