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IV.-—On the Strata of the Jumna Alluvium, as eremplijied in the Rocks and Shoals lately removed from the bed of the river; and of the sites of the Fossil Bones discovered therein. By Serjeant EDMUND DEAN.
[The Specimens alluded to are deposited in the Society's museum.]
It has always been a matter of speculation with me, since my first acquaintance with the Jumna, that presenting the obstacles to navigation, which it, undoubtedly, does at the present day, after seven years’ application of great talent, and a very considerable expenditure, what a gigantic work it must have appeared at its commencement. Experience, however, and a careful research have confirmed me in the opinion, that many of these impediments in one shape or another, were then, and are now, not only such as, their existence once known, could easily be removed, but there is every probability of some of the most dangerous of them being at this instant in a state of active formation and increase.
Taking a general view of the whole, as they occur between Agra. and Allahabad, I have found it convenient to class the obstacles most to be dreaded by navigators, as follows :
lst, Clay-banks or shoals; 2nd, Rocks ; 3rd, Kankar shoals, and 4th, Sunken trees. This classification is adopted with reference to the supposed degree of danger to the navigation that may be attached to each, a detailed description of which I have endeavoured to arrange in this order. ' e
The grand and perfect section of the Delta of the Jumna and Ganyes, (or I should rather say, from experience lately gained to the westward, of the immense general alluvium of Hindustan, opened by the channel of the former,) presents a regular alternating stratification of the different modifications of which the general Ddab alluvium is formed ; which consists (as far as the section has allowed me to examine), of five distinct strata, interspersed with irnbedded substances which from their irregular growth, positions, and occurrence, cannot be classed among the more regular strata. The regular strata occur as follows, namely, lst, Superior sandstone ; 2nd, Shale, and 3rd, 4th, and 5th, Alluvial, (fig. 1. Pl. XIII.)
Only two strata of the superior sandstone occur within the above bounds that I am aware of. The elevated positions of both decidedly have been produced by volcanic irruption, and will be described under the head of rocks.
The Shalewhich approaches nearest to d, var. of A . in the first division Of McCnLL0cn’s synopsis, described as passing into clay, appears very seldom. Note. The specimens marked “ y, l, 2, and 3," all stand the
test of adhering on being applied to the tongue or lips. (Specs. 3/. lst. Alluvial Clay, corresponding with a, var. of C., first division; is much intersected with seams of kankar >§ of an inch in average diameter, colour dull yellow, grey, and dirty white, and is interstratified with beds of nodule kankar varying between 20 yards, and half a mile in length (as exposed by the river), and from one foot to 15 in thickness. 2nd. Alluvial Compact Sand would form 21 var. e, of C., first division; does not agree with c, of the same division, as there is no portion of clay, and it is only partially consolidated by the pressure of superincumbent strata. It occurs both above and below the lst alluvial stratum from 3 to 18 inches thick, and of indefinite length and breadth; in some places a few yards, in others several miles.
3rd. Alluvial Clay, with a large proportion of sand b, var. of C., first division of Mr. McCu1.Loon’s synopsis. This stratum is frequently varied in colour, giving it an appearance of divisibility; but on examination, this difiierence will be found to extend to colour only, which varies in many places between dull yellow and grey.
lst. Of Clay Banks or Shoals.
These banks (fig. 2. Pl. XIII.) sojustly dreaded by navigators ofthe Jumnn, are quite as unwelcome to those engaged on the Jumna works, as their removal is both troublesome and expensive. They are formed of isolated and detached portions of the lst alluvial stratum, by an accumulation of sand forcing the stream into a new channel, formed by the whole of the 2nd and 3rd alluvials, and least tenacious parts of the lst alluvium, having been swept away at high levels, leaving such portions of the last as were sufliciently compact to withstand the force of the stream, which are generally those where the natural toughness of the clay is increased by the seam kankar before mentioned, (spec. a.',) which runs in every direction through it, literally lacing it together, and giving the clay a durability which the action of the strongest current has, perhaps, less effect upon, than it would have on a similar mass of stone of average texture.
The stream, which is generally confined in its course by these obstacles, rushes past them with violence, polishing (as much as clay is capable of such an operation) all those parts exposed to its action.
It was in the crevices formed by the washing away of the softer parts of a bank of this description, (figs. l & 2, Pl. XIV.) that the specimen of fossil bones, which were, I believe, presented by Capt. SMITH, and the tulwar, by Lieut. Bum, were found, whilst the clay bank was being removed, the whole upper surface of which was covered with from two to four feet of kaukar, of the conglomerate formation. I should wish this to be remembered, as I consider finding the latter iu such a.
situation as peculiarly corroborative of my remarks relating both to these banks and to the kankar formation. No instance, however, has ever been known of petrified or fossil animal, or vegetable remains, having been found fairly imbedded in or under this stratum.
Another formation of these banks is occasioned by the current sapping the high and abrupt banks of the river, by washing out the strata of compact sand, when such large masses of stiff clay are detached and thrown into the channel, as to defy the efforts of the stream to dislodge them, which if not speedily effected, a suflicient time has only to elapse to clear the outer parts of the earthy matter which may have fallen with them, which together with sand immediately deposits itself in rear, when every hour secures and strengthens them in their position against the stream, (fig. 3. Pl. XIV.) The interstices (should there be any) are soon filled up with any extraneous substances that may be lodged by the current. Those organic remains which may happen to be imbedded, or rather buried under this sudden deposit, if petrified in that situation, may be easily distinguished, as they invariably adopt in the process of petrif-action, the hue of the mass with which they are in contact, and which, when the process is complete, nothing will remove, and the porous parts of the bones either remain empty, or are filled with carbonate of lime, infiltrated, whilst in solution. The same remark applies to wood or any other substance. In every other situation the interstices of the fossil to which the water has unrestrained access, is filled with either silicious or argillaceous matter, and frequently with a composition formed of both. For the proper consolidation of either of which, however, the presence of the carbonate of lime is necessary.
Both these formations may be, and frequently are, instanced in one specimen, where from fracture or decomposition, sand or clay may be admitted to one part, when the composition is formed, whilst it is excluded from those more perfect, the pores of which will be either filled with crystallized carbonate, or remain empty as above stated.
By the continual cutting away, and falling in of the banks of the river, the accumulation of alluvial matter in some places is necessarily very extensive. The strength of the current preventing its deposit in the channel, it is carried down to the bend of the river, next below whence it has been dislodged, in the shape of thick sediment, and deposited there; the sand which accompanied its removal is from its greater specific gravity deposited in the bed of the channel. This alluvium forms in banks from 6 to 14 feet thick, and composes, on a rough calculation, not less than 80 or 100,000 acres of arable land, of the first quality, between Agra and Allahabad; producing by