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Mixed gneisses. The 'composite gneisses ', a much metamorphosed and highly injected facies of the Aravalli schists recorded in the southern area of Central Mewar,' gradually pass into more or less unmixed slaty, phyllitic or micaceous schists in the Southern Rajputana States of Dungarpur and Banswara. The granite and pegmatite intrusives, interfoliar injection of which in the schists is responsible for the formation of these mixed gneissic rocks, have been less and less permeative in the south. For some distance the intrusives appear as small dykes and veins till they altogether cease to be the constant associates of the argillaceous schists.
In the south, the Aravalli schists mapped near the south-eastern tracts of the area under review are bounded by an extensive exposure of a gneissic formation. Its junction with the schists is very much blended. No gritty nor conglomeratic formation has been anywhere at the junction, which has been closely followed for a length of about fifty miles. On the other hand, rich development of garnets has been frequently noticed in the phyllitic and micaceous Aravalli schists along their contract with the gneiss. The junction, in all probability, is one of eruptive unconformity.
The gneiss near the junction is generally characterised by very close and pronounced foliation. The rock is soft and crumbling. Outcrops are often covered with alluvial soil of varying thickness. Highly banded biotitic occurrences, suggesting interfoliar injection of granitic matter in schistose formations, have sometimes been met with in the midst of the gneisses. When broken along the foliation these have all the appearance of a schist, but across the foliation their gneissic character is evident. The pink felspathic constituents of these sometimes occur as 'augen’ wrapped in foliaceous biotitic material. The occurrences about four miles north-west of Bhabhra (22° 32' : 74° 20') furnish noteworthy instances of these highly foliated biotitic gneisses.
The gneissic exposures generally strike north-west and southeast, with foliation dips either vertical or highly inclined towards the north-east.
Farther south in the southern Rewa Kantha States the gneiss has been traced into homogeneous granite, greyish pink in colour and medium-grained in texture. It is composed of pink or grey felspars, greyish, granular, subtranslucent quartz and small specks of mica, both dark and white.
1 Mem. Geol. Surv. Ind., LXV, Pt. 2, pp. 152-153, (1934).
INTRUSIVES IN THE ARAVALLIS.
Discontinuous lenticles of dark amphibolite are often found in the gneissic formation near its junction with the Aravalli schists.
These have well defined junctions with the Amphibolite.
highly foliated gneiss, and, occurring as dark massive or schistose bands in the foliated, crumbling biotite-gneiss, they have all the appearance of metamorphosed intrusive dykes.
Intrusive granite in the midst of the Aravalli schists and limestone has been recorded between Visalpur (22° 55' : 74° 26') and Ram
bhapur (22° 55' : 74° 30') on the northern bank Granite.
of the Anas. The outcrops do not rise much above the general ground level, and weather into grey knolls and flat domes. The rock is hard and breaks unevenly under the hammer. In the hand specimen it shows greyish quartz and pink felspar. The ferro-magnesian mineral, biotite, is characteristically scanty. Under the microscope (22075) the quartz is seen in allotriomorphic granular texture, the plagioclase felspars, belonging to the soda end of the series, mostly oligoclase, have a tendency to form phenocrysts, and are more or less clouded. Calcite and zoisite, derived from the decomposition of the plagioclase felspars, are present in considerable proportion. Biotite, present in flakes and small patches, is partly chloritised.
The granite is obviously intrusive in the Aravallis. Tongues and apophyses have been traced into the adjoining phyllites. In the main outcrop the granite is characteristically variable in texture, Both coarse, and fine-grained forms are seen. The rock is often distinctly foliated. The foliation-strike is invariably parallel to that of the country rock,-generally north and south.
An extensive occurrence of intrusive granite has been mapped around Godhra (22° 47' : 73° 37'), the headquarters of the Panch Mahals district. To the north the granite extends across the southern boundary of the Lunavada State well into its interior. In the east it covers a large area in the south-western tracts of the Bariya State. In the west the granite crosses the Mahi into the Balasinor State and is mostly represented by scattered groups of outcrops in the midst of the Recent alluvial deposits. Southwards the outcrops of granite extend across the Kalol taluka of the Panch Mahals into the Bariya State.
The granite is clearly intrusive in the Aravalli schists. Although generally the junction is soil-covered, instances have been noticed in Lunavada and Bariya States where tongues of the granite are traceable into the adjoining schists, and aplite and pegmatite dykes, evidently representing the residual products of differentiation of the magma, have been seen to traverse the surrounding countryrock as well as the granite masses. Xenolithic occurrences of the schists and gneisses have also been noticed in the granite.
The rock is remarkably free from dynamic effects. Its mode of occurrence and association with the Aravallis are clearly indicative of the fact that it has at no stage been involved in the tectonic movements responsible for the folding of the ancient sedimentaries. It is therefore quite probable that the granite is post-Delhi in age, although there has been no contact with that series in the area under review. In fact, it appears to be the Erinpura granitel. In this connection it may be noted that the thick muscovite and tourmalinebearing pegmatite dykes of this granite are remarkably similar to the post-Delhi pegmatites of north-eastern Rajputana.2
The granite is generally unfoliated, although occasional instances of incipient primary banding are not altogether absent. In texture the granite varies between medium- and coarse-grained types, with a marked tendency towards porphyritic forms. The colour varies between pink and grey, depending on the colour of the dominant felspathic constituent.
Pronounced block and mural jointing are usual in the outcrops. The progress of differential weathering along the joint-planes has often resulted in fantastic 'galleries' and ' arches' besides the usual smooth rounded blocks heaped in characteristic mounds and “tors ’.
In view of the close similarity of mineral composition and physical characters, as well as mode of occurrence and association, of this granite to the Erinpura granite, scattered exposures of which are met with not many miles away from the extreme north-western outcrops of the granite under description, the latter may safely be correlated with the Erinpura granite.
Aplite and pegmatite dykes and veins are in frequent association with the granite. The north-western margin of the granite is particularly marked by extensive dykes of coarse-grained pegmatite
1 Trans. Nat. Inst. Sci. Ind., Vol. I, No. 2, p. 32, (1935). 2 Mem. Geol. Surv. Ind., XLV, Pt. 1, p. 98, (1917).
often showing tabular blocks of white or creamAplite and pegmatite.
coloured felspars, white
felspars, white translucent quartz and books of stainless muscovite. Well developed crystals of black tourmaline have often been noticed in the pegmatite.
DELHI SYSTEM. Highly re-crystallised, massive quartzites representing the Alwar basal series of the Delhi system appear in the north-western corner of the area. These are in strike continuity with the Alwar quartzites of Mewar and north-eastern Rajputana. The detailed description and lithology of these rocks have been given by Mr. C. S. Middlemiss in “The Geology of Idar State”.1
INFRATRAPPEANS. The Palæozoic era, following the deposition and development of the Archaeans and the Puranas, is entirely unrepresented in the area under description. Patchy formations, belonging to the upper systems of the Mesozoic group, overlie the ancient metamorphics with marked unconformity. The eroded surfaces of these, again, have been overrun by the lava flows of the Deccan trap.
The infratrappean formations are generally thin and lenticular. They form irregularly eroded horizontal shelves capping the ancient metamorphics and often crop out from below the Decean trap. The infratrappeans are characterised by considerable heterogeneity of composition and variability of respective thickness of the component beds from place to place. Instances of considerable lateral variation in the lithological characters of the beds are also common, and it is indeed difficult to draw up an accurate succession of the series, which may reasonably claim general application in the extensive area under description.
In view of fundamental differences in origin and distinctive lithological characters, the infratrappeans of the area have been classified under (1) the Ahmednagar sandstone, (2) the Lameta beds, (3) the Bagh beds, and (4) the Ninar sandstone.
These names were originally introduced into Indian geology by different workers at different times in the course of their work in widely separated parts of the country. Although the last three probably represent more or less contemporary series of sediments, these local names have still their use in descriptive geology.
1 Mem. Geol. Surv. Ind., XLIV, Pt. 1, p. 74, (1921).
The Ahmednagar sandstone may be correlated as Lower Cretaceous, the Bagh beds as Upper Cretaceous ; the Lameta beds and the Nimar sandstone are here unfossiliferous, but may be regarded as equivalent to the Bagh beds.
The Ahmednagar sandstone. The Ahmednagar (Himatnagar) sandstones were first described by Mr. C. S. Middlemiss in Idar State. These are generally composed of thick beds of undisturbed, horizontal freestone intercalated with shales and conglomerates. Several outcrops have been mapped in the western tracts of the area. These vary in colour between white and different shades of pink, red and brown.
Fossil plants, of Lower Cretaceous age, have recently been discovered in the Ahmednagar sandstones by Dr. A. M. Heron and the second author, the latter being responsible for the palæontological portion of this paper : Himmatnagar (Ahmednagar) (23° 36' : 73° 2') is just beyond the western limit of the area under description.
Mr. Middlemiss first drew attention to this locality, but he was able to obtain only unrecognisable plant remains. He correlated the Ahmednagar sandstone with the Umia stage of the Upper Gondwanas, formerly thought to be Upper Jurassic, but now regarded as Lower Cretaceous, as the Umia plant-bearing beds are interstratified with marine deposits homotaxial with the Wealden of Europe.3
Professor B. Sahni4 has described the plant remains from Himatnagar (Ahmednagar) as Matonidium indicum, sp. nov. and Weichselia reticulata, two Wealden genera. There is therefore little doubt that the Ahmednagar sandstone is Lower Cretaceous.
The Lameta beds. A conglomeratic formation with a siliceous or calcareous matrix forms the bottom band of the series. This grades into progressively
gritty, siliceous or calcareous types, with the Lithology.
result that the bulk of the upper formation 1 Mem. Geol. Surv. Ind., XLIV, Pt. 1, p. 137, (1921). 2 Rec. Geol. Surv. Ind., 71, Pt. 1, p. 28, (1936).
3 Op. cit., XLVIII, p. 32, (1917), Proc. Asiat. Soc. Beng. (New Series), XVII (iv), pp. clxvi, (1922).]
Rec. Geol. Surv. Ind., 71, Pt. 2, pp. 152-165, (1936).