GEOLOGY OF JASINGA BY JONATHAN RIGG

Sketch

Of the

GEOLOGY OF JASINGA

The report, that Mr. Horner had, early in 1836, discovered Granite in the rivers Chimadur and Chiara, on the south coast of bantam, suggested the idea, that the rock might also exist among the defiles of the mountains, south of Jasinga, not withstanding that the Gunung Kendeng intervenes and rises to the height of at least 5000 feet above the sea.

A rather doubtful and partially decomposed speciment, picked up in Desember in the Chiberrum, at the western base of the gunung gede, helped to strengthen this opinion and a profusion of boulders, observed in the month of February following in the Chimanganteung, which flows along the southern side of the same mountain, seemed to confirm the fact.

It was however the 16th of July before the weather and other circumstances admitted of any regular search for the situs of the granite.

It had been previously observed, that the blocks, from which all former specimens had been struck, decreasein size in proportion to their distance from the gede. It was therefore determine to commence a search from the Kampong Chisusu, at the S.W. corner of that mountain, and to trace the river Chimangeunteung upward to the mouth of the Chiserua, the larges rill, that pours down from the precipitous sides of the gede.

The Chimangeunteung rises at the S.E. foot of the gede, where it is mountain locked on all sides, except the deep and narrow defile, through which it winds its way, between the southern base of the gede and Gunung Limbung, an inferior range due south.

Proceeding upward, the Chirempag is the first streamlet worth mentioning, that is met with, coming from the gede. From Chisusu to this stream the bed of the river is formed mostly of a black schistose rock, which in the first instance dips towards the gede, or appears sometimes to stand on end (as at the small cascade of masigit) and latterly dips from or rises towards it. This rock contain a small portion of lime, as it slightly  effervesces with nitric-acid. At the spot , whre this change of inclination takes place, a stratum of above 12 feet thick of a white, soft, earthly rock protrudes itself between the black schist and occupies both sides of the river; the two rocks are merely in simple contact, without any adhering or welding. On this part of the river no other rock appear in situ; but the bed is strewed with a profusion of blocks of trachytes, of sedementary rocks,of conglomerates, of immense masses of scoriae, of many ton weight; some of which stand 12 or 15 feet high and contain in their crevices aggregations of rock crystal and of plenty of granite. Of the last stone the larges block observed was an oblong slab of about 10 feet long, 5 feet broad and 2 feet thick. It was among this that a cube of 2 feet of granite was found, cut through in the middle by a seam of very compact trachyte, about 8 inches. Thick, so that the granite on it side was entirely separated; still the whole adhered most perfectly and form one solid block. This streaks cross both stones in the same continuous and uninterrupted line, as it were lines of fracture, that had gaped and closed to again; as the rock now shows no disposition to break in that direction.

From Chisusu to the mouth of the Chirempag is about a mile in a straigh line, and from this to the Chiserua a little les than ½ mile. On passing the Chirempag, all trace of the granite was immediately lost, and every attention failed to discover a single bit of it, which led to the supposition, taht its course must be down that stream; as it is  nowhere visible on the sides of the Limbung , along which a small road winds and which had previously been examined.

Continuing up the Chimangeunteung, the volcanic rocks are found in situ and present a variety of position in their dip and beds; the river has in some places worn its way into them to the depth of several feet; whilst at others, where the texture is more compact, it is still forced to cascade over their rugged surfaces. The black schist and white earthy rock, mentioned in the early part of this account, again suddenly appear, standing together nearly on their ends between the Trachyte beds, with which they almost form a right angle, as if they had stood in their present upright position before the volcanic matter had been poured out againts them ; they are respectively of the thickness of 10 and 6 feet.

The bed of the Chiserua is cut out of the former black schist; as also a more decided lime-stone, which is however quite black. This rill is a mere ravine, worn out of the slope of the gede and presents little interest and no variety ; the granite certainlydoes not exist here ; the water dissapears sooner than most mountain streams and about half way up the gede a wall of perpendicular and impassable rocks rises up to the heiht of 30 feet. Crossing the mountain ridge at this elevation and proceeding Westwards, the Chirempag is met with at about  800 feet above, where it falls into the Chimangeunteung and 1000 feet below the summit of the gede. This is very interesting little stream ; the rocks standing out boldly in all directions, of which the principal are sedementary. At the highest point reached these rocks seem to have undergone a general decay in situ and though they present to the eye every appearance of a solid mass, the hammer, instead of knocking off a fragrament, sinks into a stiff yellow clay. Lower down, where here and there the volcanic rock thrust them-selves out to day, all sympton of decomposition again desappears ; as if the contact had imparted a durability, which the others do not posses. In one place a vein of trachyte is observed inserted in a mass of sedementary rock, in the same manner as that of the Granite above mentioned; in this case however the igneous rock has imparted a darker color and greater hardness to its immediate neighboorhood, than what is presented by the same rock, at a little distance, in another spot, what appears to be a fine and close grained Granite, it seen ; but as the igneous rocks and particularly the syenitic are known to assume accasionally the appearance of one another, according to the condition, under which they are produced, the position seems rather to favor the idea of its being merely a variety of thachyte. The sedementary rocks are sometimes seem horizontal, then again perpendicular, or inclined at some angle to the horizon ; as if the volcanic matter from below had so disposed them, after breaking up their original continuous crust. The Chirempag next falls into into a wide chasm of the rocks and, after running about 400 feet under ground, again flows out further down. Below this, the ordinary boulders or syenitic Granite again made their appearance for the first time in this stream; they were blocks of about 4 feet square and lying more than 100 feet above the bed of the Chimangeunteung.

The original situs of the Granite has thus not been discovered ; wherever that spot may be, yet it cannot be far distant. The blocks of all the Granites met with do not bear evidence of having been much rolled, certainly not more than the other igneous rocks, with which they are associated. The fact of the boulders increasing, both in number and size, as we approach the Chirempag, as also their totalabsence above its mouth, seems to point out that ravine with its mountain torrent as the path, along which its fragments have been conveyed. The head of the Chibereum, in which the first speciment was discovered, is only separated by a narrow ridge from that of the Chirempag, and though the former river contains very few of these stones, yet the fact helps to confirm the where about of the Granite; as neither the Chikeam, which flows from the North of the Gede, nor the Chidurian, which sweeps round its Eastern base, countains a single vestige of this stone. The Granite inside may be higher up the Chirempag than what has been examine; it may be concealed by the earth and dense forests; of the fragments, which we now behold, may have been thrown down at the period of the paroxysmal explosion of the Gunung Gede, which has evidently been much bulkier at a former period and which now only the ruins remain.

During the period, that the Gunung Gede of Jasinga was an active volcano, it appears probable, that the sea washed at least its base, if it did not entirely cover it. The volcanic liquid seems mostly to have flowed towards the South and West, and land side, where the trachytes are still found pilled up; if any discharge took place towards the North, it is entirely covered over by later formations; the round and isolated hill Gunung Angsana, about five miles North from the Gede, being the only spot, where the volcanic rocks are found in situ. That the sand and other light tufaceous matter, ejected from the crater, have fallen into water, and by it is been distributed, is proved by the regular strata, in which they are still observeed, though no longer in a horizontal position, having been broken up and displaced by their subsequent elevation. Some of the lower beds of these volcanic ashes, resting upon limestone, contain a number ofsea shells. The limestone itself abounds with similar productions and again reposes upon more ancient and black, submarine and tufaceous formation; numerous sea shells, as oysters and cockles being plentiful throughout.

In these lower black strata, near Kampong Munchang, were discovered two tusks appearing to belong to some animal of the genus Sue, but too fine for any of the swine tribe, now existing in the forests. Unfortunately they were in a very brittle state and almost crumbled in the hand; the enamel was still perfect, showing no marks of abrasion from water; so that they cannot have travelled far; proving, that at the early period some habitable land existed in the neighboorhood, as is further confirmed by the numerous impressions of leaves and carbonized wood, which some strata afford.

The strata of the different tuffs and lime are not conformable; that is, the different beds are not exactly parallel to each other, from which we may infer, that their elevation was not simultaneous ; but that the risingsand deposits were alternate and successive ; long periods of tranquility sometimes intervening, during which the limestone was formed. The commencement of the latter is plainly seen, where the black strata have been partially denuded ; it is at first mixed with the subjacent black matter and by degrees becomes purer as it ascends, whereas its upper surface terminates abruptly and hears what rests upon it, without any intermixture.

Limestone is one of the most common minerals found in this part of the country ; it abounds in every part of the lower lands, as well as on the flanks of the mountains. The Gede it self must have originally burst its way, not only through masses of Granite and sedementary rocks, as the seems or veins, they contain of trachyte, prove; but also have borne up with it large beds of limestone, as in the case of the Chiserua, which is shut up in an elevated mountain defile, precluding the idea of its having been formed there since the existence of the Gede. The highes point, ot which shells have been observed in limestone, is in a black and very hard description, at the Western foot of the Gede, near the kampong Kembang Kuning, in which the forms of Pectens and other bivalve shells may be seen at an elevation of about 1,000 feet above the Sea.

The range of the Gunung Seribu runs out N.N.E. from the eastern base of the Gede and extend in that direction for 10 or 12 miles, thus forming a snug and quite bay of the parts now called Jasinga, which during the intermission of volcanic activity would be highly congenial to the habits of Testaceae, whose numerous remains still exist, and to those zoophytes, by which lime is supposed to be elaborated. The supposition, that Jasinga has formed a bay of the Sea at no very distant geological period is further borne out by the little elevation, which it yet possesses above the Sea, although at the foot of the mountains and removed above 30 miles from the North shore, towards which the slove is very gradual. It is similarly situated, with respect to the Western ocean, about Welcome and Papper bays, at which point the Sea is supposed to have formerly enterd and, running between the Southern hills and the mountains as islands accross the bay, to break the force of the N. W. Mousoon.

According to the Mr. Horner’s observations, the Demang’s house at Jasinga is only 352 feet above the Sea ; at Sajira, on the Chiberang, only 11 miles West, this height has decreased to 187 feet and at Lebak, 16 miles further in the same direction, the elevation of the residents-house is only 76 feet ; the waters from this place still find their way to the North shore. Westward still from Lebak, the whole district of Panimbang is reported to be so low, that in the wet mouseen the Chiliman readily overflows its banks and a great part of the country becomes little better than a swamp, the couse of so much fever and mortality to the native inhabitants. The Chiliman flows into the Western ocean.

Volcanos have a tendency to destroy themselves and, after remaining for centuries in a state of great activity, to lose of outward appearance of energy, their vents becoming closely shut up by the accumulation of their own debris. The volcanic forces however go on increasing with-in and, finding no longer their accostomed egress, open for themselves fresh vents at more distant points, or else swell and gain additional space by upheaving en masse the solid strata, by which they are shut in ; an operation of this kind may have coused the rise of the Westen end of Java, therby excluding the sea from its ancient do-minions. That the igneous agents are not yet extinct under Jasinga, may be inferred from the hot springs, that are occasionally met with.

On the Westen slope of the Gede two separate apertures discharge waters of the temperature of 97° fahrenheit, one of which has a very strong smell of Sulphur and about five miles West of this near the Kampong Sea in Bantam, is a well, whose waters indicate a temperature of 134°. The water of this latter well, when drawn out and left to cool, affords no other taste than that of ordinary water after boiling and only a very slight smell of sulphur is experienced. The waterbubbles up through a ledge of rocks, to which it imparts a white color, which however shows no disporition to effervesce with acids. When silence is observed and attention paid, a low, dull, rumbling sound is head at intervals and this is immediately followed by an emission of air bubbles and quite again successds. Though the taste of the waters is insipid to man, the Buffaloes and other animals are fond of sipping it, when it has cooled. All over the country there are a number of springs, known by the same of seuseupan, at which, in preference to the river, all animals drink; these springs are now cold and have no extraordinary smell or taste, yet must possess some peculiarqualities, as the buffaloes are known to thrive better from frequenting them. The water of all these springs may have one common origin ; but from the nature of the rocks and strata they pass through, some part with their beat more readily than otehrs; while again those, which make their way accross the more decided submarine formations, arrive at the surface at the ordinary temperature of the atmosphere.

Large quantities of silicified wood are every where met with, known by the name of Batu Sumpur, as the natives suppose, that this hard wood alone is fossilized ; but the variety of grain, which is still preserved entire, does not countenance the belief, that the Sumpurs (COLBERTIA and CAPELLIA) are the only trees, on which this process has acted.

There are positions; which seem more favorable for silicification than others. A good drainage of percolating waters appears necessary, so that when that medium has deposited the silex, which it holds in solution, it may readily run off and make way for a fresh supply. Consequently the greatest quantities are found lying above the more compact tuff and covered by enveloved in the loose, porons, superficial earth or volcanic sands, and many of these emit fire when struck by iron. On the contrary such wood, as has been entangled in the more compact and solid stuffs, still remains soft and may be readily cut away with a knife and, when fresh, presents every appearance of common charcoal saturated with water and easily crumbles in the hand ; these however, when preserved dry for a few months, attain considerable hardness and then offer effectual resistance to the knife, which extracted them from their original position ; cases have been observed of the two varieties being almost in contact, when the one imbedded in the close tuff was soft and friable, whereas its neighbour, that merely reposed on that formation, had become perfectly hard and solid.

All these woods may have been originally carbonized by volcanic action and thus preserved from putrefaction ; this supposition will account for the logs of stone of 10 to 12 feet in length being found cracked across the grain, in irregular lengths, yet still lying in the same continuous straight line. A mass of charcoal would easily give way under an increase of pressure ; but that the hard silex stone, often of 1½ or 2 feet diameter, should so break, can hardly be conceived. That pressure has been experinced, when in a soft state., is shown by the flattened appearance of some speciments. The black color has drained off with the water percolating through them : many lumps, which have become externally white or of a reddish hue, are still black at the centre.

Jasinga, 27th November  1837                                                              

JONATHAN RIGG




Source : Verhandelingen Van Het Bataviasche