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TABLE 46.-Prospecting and Exploring Licenses granted in North-West Frontier Province during 1923.
TABLE 47.-Prospecting Licenses and Mining Leases granted in the Punjab during 1923.
THE SODA-BEARING ROCKS OF KISHENGARH, RAJPUTANA, BY A. M. HERON, D.SC., F.G.S., Officiating Superintendent, Geological Survey of India. (With Plates 2 to 12.).
HE interesting elæolite and sodalite bearing rocks of Kishengarh
were first known from specimens sent to the Geological Survey of India by Rao Bahadur Syam Sunder Lall, C.I.E., Diwan of Kishengarh State, and a short note on them was contributed by the late Mr. E. Vredenburg.1 Mr. Vredenburg draws attention to the colour change of the elæolite-sodalite-pegmatite as follows:
Moreover some of the sodalite exhibits an extraordinary phenomenon hitherto unrecorded in any mineral. While some of the specimens are of a bright blue colour similar to that of the mineral from many other localities, others appear under ordinary conditions transparent and colourless. But some of these colourless fragments when kept in the dark for a fortnight or three weeks assume a pink colour which disappears rapidly on exposure to bright daylight, and almost instantaneously in direct sunshine.
The phenomenon is particularly brilliant when the rock is first broken in the field, and the large blocks of elæolite (some of which are over a yard wide) appear, on fracture, as if suffused with blood. The colour seems to re-appear more completely in some specimens than in others, for while the disappearance of the colour is very rapid, its re-appearance, which constitutes the most remarkable feature of the change, is very slow. The precise nature and cause of this peculiar phenomenon are at present unknown. The following analysis gives the percentage composition of a specimen of the blue sodalite :
The other variety exhibiting the curious change of colour has been found so intimately intergrown with elæolite that it is not possible to obtain a complete analysis. Partial analysis indicates, however, almost exactly the same percentage as for the blue variety.
The elæolite-bearing rocks are surrounded by scapolite-gneisses and all the syenites contain scapolite associated with the elæolite and sodalite. The eleolitesodalite-pegmatite contains large crystals of ægirine, sphene and lime-iron-garnet."
In the annual report for 1903-4, Sir Thomas Holland draws attention to the same peculiarity:1
"When freshly broken, the patches of sodalite are carmine in colour and the fresh rock-face gives the appearance of being splashed with blood, but in daylight the colour rapidly, and in direct sunlight almost suddenly, disappears. Many of the specimens when kept in the dark for a few months recover their carmine colour, losing it again on exposure either to daylight or to electric light. The sodalite does not, so far as we can find, differ in chemical composition from the ordinary varieties of the mineral, and neither the loss nor the recovery of colour is affected by the humidity of the atmosphere."
The discovery of cancrinite, by Babu Baidyanath Saha, is also recorded from the north-east extension of the main syenite mass. During the field study and collection of specimens of the syenites I was accompanied by Dr. Murray Stuart, whose co-operation I gratefully acknowledge.
Within an area 10 miles long by 2 miles wide, lying north-east from Kishengarh City (26° 34': 74° 55'), are one large and nine small masses of syenite in the form of sill-like intrusions in the Aravalli schists, markedly elongated in the general direction of the strike of these rocks, the syenites themselves being strongly banded and foliated in the same direction.
The rocks of the Aravalli System here consist chiefly of micaceous thin-bedded quartzites, quartz-mica-schists and mica-sillimaniteschists, all highly metamorphosed and intruded by amphibolite,2 granite and pegmatite. A mile east of Kishengarh Railway Station the road crosses an anticline of impure vitreous quartzites, inter
1 Rec., Geol. Surv. Ind., XXXII, p. 158 (1905).
2" Epidiorite" according to the recommendations of the Joint Committee on Petrographie Nomenclature.
bedded with which is a bed of graphitic mica-schist, about 4 feet thick, from which a mineral paint has been made and used on railway rolling stock. A mile or two still further to the south-east vein granites and pegmatites are so abundant in the Aravallis that the result is a banded rock or injection gneiss ("migmatite" of Sederholm1) in which ramifying and interfoliating veins penetrate the rock to such a degree that over large areas there is not a square foot free from them.
Besides these, numerous lenticular bosses of intrusive granite occur, some of large size, but those in the immediate vicinity of the syenites are insignificant. These granites are often foliated and in general resemble in their minerals the vein granites. The final result, especially where pressure has acted upon and rolled out the vein granite, is a banded rock which may be termed a "gneiss," interspersed with, and often merging imperceptibly into, areas of "gneissic granite" or granitic gneiss". The complex is highly puzzling at first sight, but some of its obscurities are resolved by close study.
The evidence available goes to prove that the soda-syenites and their associated pegmatites were intruded previous to the deposition of the overlying Delhi System, the base of which lies a mile to the west of Kishengarh City. As the syenites are, so far as was seen, not penetrated by any of the very numerous granites, pegmatites and amphibolites (epidiorites) which are in such force in the surrounding rocks, it is at least possible that their absence may be due to the syenites being later in their period of intrusion than any of the others. Above, i.e., to the west of the Delhi-Aravalli unconformity, no exposures of syenite are found, and, as the accompanying geological map shows, the line of the unconformity appears to cut across the strike of the syenite sills and the Aravalli schists at a low angle.
The basal beds of the Delhi system, to a thickness of more than 2,000 feet, are exposed in the high ridge which overlooks the plain of Aravalli rocks, and consists of felspathic conglomerates dipping away from the Aravallis at angles of 80° and higher. From Kishengarh City the unconformity has been followed to the south-west as far as my survey has proceeded, for nearly 80 miles; 6 miles to the north-east the ridge is interrupted by alluvium but is met
1" On Regional Granitisation (or Anatexis)," Congrès Géologique International, 12th session (13).
with again along its strike-continuation, in the great mass of conglomeratic quartzites at Khakirdi, on the southern shore of the Sambhar Lake, and again to the north of the Lake in the conglomerates of Marot in Jodhpur. All along, wherever exposed, the unconformity is accompanied by arkose and conglomeratic beds, but their thickness is very variable from point to point, partly owing to original inequalities of accumulation, and partly due to repetition in some places and cutting-out in others, by multiple thrust-faults and slides along the unconformity.
In connection with the faulting may be noted the puzzling discovery of five or six large fragments of syenite along the base of a ridge parallel to, and at a distance of 4 miles to the west of, the range which marks the unconformity. Ruling out human transportation, which is quite unlikely, these fragments may have come either from a body of syenite close at hand but concealed below alluvium, or may have weathered, as boulders, from an irregular conglomerate which appears in the ridge and is derived from the disintegration of Aravalli rocks. In either case, taking into account the fact that the facies of the rocks of this ridge suggests that they are at or near the base of the Delhi System, it is possible that the lowest beds of the Delhis, with a section of the Aravallis, are here repeated by a strike-fault; unfortunately alluvium is so extensive and so deep that other field evidence cannot be obtained.
In the Kishengarh neighbourhood the Delhis above the basal beds are almost entirely concealed, but further to the south-west, towards Ajmer and Beawar, good sections are obtained, which show them to consist of thick quartzites, mica-schists and impure limestones folded synclinally into the Aravallis, with a great development of intrusive pegmatite, granite and amphibolite (epidiorite). The rock types are more definite in the Delhis than in the Aravallis, and in the former the various congeries of quartzites, mica-rocks and limestones, as the case may be, are thicker and more uniform within each set, whereas the Aravallis have a certain indefiniteness of character and such quartzites and calcareous bands as occur in them are thin and intercalated with the predominant rock-type-micaschist.
The scenery reflects the geological structure. In the Delhis we have high continuous ridges of quartzites and, to a less extent, of limestone, alternating with valleys eroded along the softer rocks, while the Aravallis form a monotonous "gneissic plain."