Imagens das páginas
PDF
ePub

accuracy of most of Mr. Wynne's work in the Salt Range and elsewhere, his failure to see more than a general resemblance between the two is difficult to understand.

Vertical Section No. 3, Plate 1, is of the cliffs east-north-east from Bhal, east pole, Nurpur. In this the Boulder-bed is observed resting directly on the Obolus beds, that is to say, the Magnesian Sandstone is absent. The Olive series is very thin, and the Speckled Sandstone fully developed. I have indicated, by different methods of shading, the remarkable colour banding of this section,-a character pervading all the sections in the Salt Range, and giving to them such a realistic appearance from the geologists' point of view.

The Obolus Shales. One of the principal objects of our presence in the Salt Range was to verify the find of trilobites made by Dr. Warth. The account of this find is given by Dr. King in the Records (Vol. XXII, Part. 3). The very small fragments, three in number, were identified by Dr. Waagen as belonging to the genera Conocephaliles and Olenus. I never saw the specimens myself, and as Dr. Warth was a little uncertain as to the exact locality from which they had come, we had to make a new search in the Obolus beds of Khusak Fort for fresh proof.

After some fruitless efforts we were at length successful, Mr. Datta turning out the first fragment on the ciiffs north of the Khusak fort hill from a dark purple, calcareous, shaly band (zone B), very firm within, and ringing under the hammer, Numerous specimens of Hyolithes wynnei, and of small brachiopods, were also obtained. After one or two more fragments had been found, we transferred our attention to the Fort hill itself. At a point on the east side, where a natural gallery runs round the base of the Magnesian Sandstone cliff, I found a finely laminated, hard shale of dark colour, an inspiration to search which resulted in a large collection of some better preserved fragments of trilobites, and numerous small brachiopods. Many of the latter were beautifully preserved in the hard, thin-bedded rock. We then proceeded to work systematically the whole of the section through the Obolus beds, from the top of the Purple Sandstone up to the Magnesian Sandstone.

Many pleasant memories are connected with the days we spent on the slopes round Khusak fort. For the first time in the annals of the Geological Survey two of its members were consciously engaged on Cambrian rocks, unearthing some of the oldest known forms of trilobites. But the chief marvel to me during the many hours a day we spent cramped or blistering on the steep slopes, was that these very ancient fossiliferous rocks should have escaped all the disturbance attending upheaval of the earth's crust during the geological ages, epochs, and eras that have passed over them.

In Plate V I have outlined a view of the hill on which Khusak fort stands. It will serve to give the reader an idea of the style of rock-sculpture, bare of vegetation, so universal in the Salt Range. Its geological structure, stratigraphy, and fossil zones will be understood from the profile view of the same in Plate II, and from the Vertical Sec!ions Nos. 4 and 5 in Plate I.

Infra- and Supra-Nummulitic Rocks. In one respect this season's work was signally barren in any valuable results. I refer to the examination of the beds immediately underlying the Nummulitic Limestone. Their position is unfortunate, on account of the tendency of the Nummulitic limestone to spread for a considerable distance in the form of a talus down the slopes below the Nummulitic scarp. This is the one blot in the otherwise perfecily exposed sections of the Salt Range. What the Boulder Clay in many parts of England is, in obscuring the true relation of the rocks below it, so is the Nummulitic limestone of the Salt Range. Its nodular character, which causes it to weather into egg-shaped lumps, is doubtless a great assistance in spreading it far beyond its in situ limits into incongruous positions above other formations. Only in a few isolated places has a landslip or watercourse cut away the talus, leaving the rocks visible beneath. Even then, owing to the changeable nature of the deposits, which are shales, marls, and sandstones, lateral continuity is never found, and the correlation of one distant exposure with another is therefore well nigh impossible. I do not think, therefore, that the unorganized mass of local observations that we made in this connection would be worth recording as it is.

Dr. Waagen has given a classified arrangement of these beds in his last memoir (Pal. Ind., Geol. Results, pp. 51, 52); but he truly says that the subject will require much additional study before all the points are cleared up. My own impression is that such obscured sections could only be properly unravelled in the course of many years by the united efforts of a geological field-club.

Turning to the supra-Nummulitic rocks, the junction between them and the Nummulitic Limestone is one presenting some difficulties, and it gave rise to a little friction of opinion between Mr. Medlicott and Mr. Wynne. The chief questions were as to whether the junction between the two was conformable or not, and as to whether the apparent conglomerate at the base of the former was a real or a pseudo-conglomerate. Mr. Wynne, in his memoir on the Salt Range, declared that the two series were conformable, and the conglomerate not a real basal conglomerate, but of the nature of many of those semi-concretionary indurated clay-conglomerates of the Nahan Sandstone, which sometimes contain bone fragments. Mr. Medlicott had previously expressed an opposite opinion (Mem., Geol. Surv. of Ind., Vol. III, p. 91) that the sandstone lay on a denuded surface of nummulitic limestone. But in his Sketch of the Geology of the Panjab, written in 1883-84, though only published in 1888, he seems to have given up the former position, at least as regards the trans-Indus continuation of the Salt Range. He says (pp. 21 & 22): “ In the westerly portions of some of these ellipsoids the eocene limestones are closely associated with greenish clays like those in the eocene strata flanking the outer Himalayan hills, and these clay rocks sometimes contain sandy layers rendered conglomeratic by the presence of nummulitic limestone pebbles enclosing nummulites, so that the occurrence of an apparently derived detrital rock of this kind cannot always be taken as evidence of later general age than the rock of which it is formed. Bearing this in mind, it seems not unlikely that the conglomeratic or pebbly layers found at the base of the tertiary beds in the neighbourhood of the Sheik Budin gund represent some stage of the eocene period, a surmise that might even be extended to the more heterogeneous detrital accumulations of pretertiary rock, fragments of which form the floor of the tertiary series in the northern part of the Kiri Khasor ridge."

To me it seems that the sudden change from a great thickness of nummulitebearing limestone to a still greater thickness of sandstones, identical in con position

with the Nahan Sandstone, is in itself as telling a proof of discontinuity as could be desired in this part of the earth, where marked unconformabilities are absent owing to the fact that the upheaving and crumpling forces have not acted at all until modern geological times. The conditions that brought about the deposition of the limestone, and the entirely different ones that resulted in the Nahans, could only pass into each other thus suddenly by a revolution on the earth's surface, such as the cataclysmists were wont to picture to themselves. I cannot doubt that a break in time must be indicated here, sufficient to allow the one set of conditions to grow out of the other set.

The intervening conglomerate was observed by us in several places. In the neighbourhood of Choya-Ganj-Ali-Shah the conglomerate had nothing whatever “pseudo" about it. We found it to consist of a matrix of slightly hardened sandrock, containing well-rounded pebbles of Nummulitic Limestone containing nummu. lites.

Associated with these were a very large number of pebbles of a limestone very similar to the Nummulitic Limestone, but of an ochre tint outside and destitute of fossils. There were also found a number of pebbles of crystalline rocks, some of them not unlike the red granite of the Boulder-bed. The presence of the crystalline pebbles is itself a point of very suggestive meaning, for these same pebbles in the Boulder-bed, as we now know, form the base of a distinctly unconformable series. Another feature observed by us was, that, while in every case the conglomerate passed upwards into the normal Nahan Sandstone, it did not always lie directly on the Nummulitic Limestone. Near Choya. Ganj-Ali-Shah there is a small intervening thickness of finely laminated purplish and olive-grey shales that reminded me at once of the Sabathu nummulitic shales of Garhwal. Their appearance was quite distinct from the reddish purple shales inter-bedded with the Nahan Sand. stone.

Half a mile south of Kallar-Kahar on the Sardi road, a good junction section is exposed. On the top of the Nummulitic Limestone there is first a thin layer of purple shale, then a conglomerate containing numerous rolled pebbles of Nummulitic Limestone in a coarse sandy matrix. Above this are 1 or 2 feet of calcareous grit, and then normal Nahan Sandstone, though rather soft.

Dr. Waagen, in his Geological Results, equally comes to a similar conclusion, by indirect reasoning from the Indus river section. He writes (p. 24): “The chief results which we can deduce from this description of the section consist in the fact that on the whole the succession of the beds is not dissimilar to that established in the Simla region; and that on the Salt Range side of the Potwar plateau there exists a great gap in the tertiary strata, only the lowest (Nummulitic) and the highest (Siwalik) beds being there represented."

les whichole the s'that on

ORIGIN AND AGE OF THE Salt Marl. I now come to the difficult part of this paper, namely, the setting forth of sundry

fest reasons for the doubts I have felt with regard to the received treatment of the sub. explanation of the origin and age of the Salt Marl, or Red ject.

Marl, and its contained minerals. It is always a thankless task to indulge in destructive criticism, more especially in a case such as the

Peculiarit

present, where I cannot carry my demonstration beyond the range of probability into the sphere of certainty.

Owing to the restrictions now imposed on the Geological Survey, in consequence of a greater demand for positive knowledge regarding the economical resources of India, it has become impossible to give continued attention to any one subject in any one area of the empire. Change is the order of the day, and our visits to a place must be brief, and come to an end generally when the particular question, whether relating to coal, oil, or what not has been disposed of. The system of local head-quarters away from library, museum, and laboratory during the hot and rainy months of the year is also a serious handicap to complete scientific research. I must therefore content myself with a summary discussion of my field notes, merely pointing out in what direction I think conjecture lies, touching the deeper problems involved. On reading Mr. Wynne's memoir, and before arriving at the Salt Range, I was

es in the much struck by certain peculiarities which the Salt Marl structure and lie of the presented, as described by the author. At that time I drew Salt Marl.

up the following list of these peculiarities, and (as it seemed to me) inconsistencies :(1) Its soft and homogeneous nature, for the basal rock of the range of Lower

Cambrian age at least. (2) The absence of definite stratification in it. (3) The abnormal mode of its occurrence mixed up with different formations,

which Mr. Wynne accounted for by very extraordinary faults. (4) The presence, on the other side of the Indus, of important salt-bearing

beds separated from the Salt Range by a short distance only and yet

supposed to be of very much younger age, namely, doubtfully eocene. The suspicion taking shape in my mind at that time was that the Salt Marl, instead of constituting the foundation on which rest all the other formations of the range, might be of much more recent age, and formed otherwise than by evaporation and desiccation of salt lakes, such as that of Utah, or of inland seas like the Dead Sea and the Caspian.

In putting forward the present theory as to the origin and age of the Red Marl, Division of the subject.

hint I shall deal with the subject under the following three

" heads : (1) Structure and composition of the Red Marl; (2) its distribution and connection with other formations; (3) concluding hypothesis.

[ocr errors]
[ocr errors]

(1) Structure and Composition of the Red Marl. The Red Marl, with its immense masses of rock salt and gypsum, has been very

thoroughly described by Mr. Wynne in his memoir, pages M

70-84. I shall cull a few of his paragraphs as being of tion.

special interest, or as bearing on the present theory. Of the Red Marl itself, he gives Dr. Fleming's remarks on its chemical composition, which run as follows :

“It does not disintegrate when treated with hydrochloric acid, but in powder effervesces strongly, the greater part remaining undissolved as a red mud, composed of clay and sulphate of lime ; the portion soluble in acid consists of carbonate of lime and carbonate of magnesia in about equal proportions with a little alumina and peroxide of iron, to which it owes its colour."

Mr. Wynne concludes from the above that the term “gypseous marl” is not inapplicable. As to its petrological nature and aspect he writes

“The marl forms the most noticeable portion of the saline group, but in close' association with it are thick beds of gypsum and thicker ones of rock salt. It is tough rather than hard, but when very dry, possesses much the consistence of sun-dried brick."

He further writes

"Beyond the gypseous, saline, and dolomitic layers the red marl bears few original traces of stratification, or inter-stratiñcation, generally none at all; hence it is difficult to form any correct idea of its thickness. • * * * It may be doubted whether another example could be found of such à homogeneous, argillaceous and aqueous deposit of the same depth in which signs of stratification are equally absent. in strong contrast to this is the perfect lamination frequently seen in the enclosed salt, and in the p'aty dolomitic layers. From the contorted state of the latter and the curvature of the beds of salt in some of the mines, it may be presumed that, whether stratified or not, the salt marl is likewise disturbed.” It is necessary to emphasize some of the points quoted above. So far as I have

seen, I should say that the Red Marl, except when it encloses No divisional planos.

masses of the dolomitic rock, or when it is in combination with beds of gypsum and rock salt, never shows any stratification whatever; nor is there the slightest trace of any divisional planes in it. On taking a lump in the hand, the broken edges are precisely such as one would get in any structureless, amorphous mass. The likeness to a sun-dried brick mentioned by Mr. Wynne applies as much in the matter of homogeneity as in hardness. Seen in large masses on the hillsides, it equally displays a lack of structural planes. In this respect it differs from limestone and other similar, chemically formed rocks; for a compact amorphous limestone, though evincing but little trace of stratification in the hand specimen, very often gives to the surface of the earth either a scarp, cliff, or moulded slope, by which its obscure planes of original deposition can be discerned. But there is no such hint thrown out by the Red Marl. Denudation acts on it as it would on a heap of sand, making streamlets here, and rough hillocks there, the former winding in a capricious manner, and the latter strewing the slopes at random. With the absence of divisional planes there is also an absence of colour banding,

The Red Marl, though varying somewhat in the brilliancy • No colour banding

ing and depth of its colour, is of one general hue throughouil

of There is no parallel system of tints, such as the colour banding of slates (which reveals the original stratification of the slates, although the power to split along it has been taken away). Again, the Red Marl in itself, that is to say without reference to the dolomitic

ary layers of which I shall have to speak anon, is not a clastic, rock, but a chemical pro. nor even a sedimentary rock, but a chemical production. duction.

There are no grains of sand in it, no pebbles of foreign rocks-nothing to indicate that it ever grew layer by layer and stratum by stratum like a sub-aërial aqueous deposit.

The absence of all organic remains in it also points in the same direction.

Even the lenticular patches and layers, and the massive amorphous beds of gypsum, as well as those of the rock salt which are said to be bedded, stand on a

s, it equally d other similar, chetne trace of sır:

No

* With the exception of certain pale greenish-grey blotches, which will be referred to presently.

« AnteriorContinuar »