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Altered basic members of the intrusive complex, characterised by the presence of hornblende, sometimes replaced by chlorite, as the chief constituent, have been grouped together under the general term amphibolite. The range of variation in texture and composition of these basic rocks is, however, wide. Compact, massive dykes running parallel to the general foliation-strike of the gneisses are found in equal abundance with the foliated or highly schistose forms of the basic metamorphics occurring in close and often confusing intermixture with the gneiss. The foliated types weather comparatively easily and generally do not form any marked relief in topography. Their exposures are mostly to be found in
stream-beds and river-sections of the arca.
Metamorphosed basic dykes are frequent in the gneissic complex of the Partabgarh-Banswara frontier area. These almost invariably run parallel to the foliation-strike of the gneiss and are discontinuous. The longest ones could only be traced to a distance of about two miles. The dykes vary considerably in width, but nowhere exceed a hundred feet. The dyke rock is hard and compact, and is traversed by pronounced joints.
The dykes diminish in thickness and number when followed southward, and are often replaced by hornblende-schists. These are either coarsely crystalline or finely granular.
North-east of Pipalkunt (23° 48′: 74° 34′) the dyke rock has been found to have intruded into the gneiss; while three miles east of Kotwal (43° 59′: 74° 32') it is intrusive in the granite of the
Under the microscope the dyke rock shows mainly brown, pleochroic hornblende with subordinate amounts of granular quartz and plagioclase felspar. Epidote is occasionally developed along cracks and joint-planes.
Relation of the Gneissic Complex to the Aravallis.
The junction between the gneissic complex and the Aravallis in Mewar has been found to be marked by frequent bands of conglomeratic or gritty quartzitic formations, the latter often showing evidence of false-bedding'. These led to the conclusion that the junction between these two formations is one of erosion unconformity, the gneissic formation representing the basement rock,
upon the weathered and eroded surface of which the basal members of the Aravalli system were deposited1.
Further occurrences of conglomeratic formations, often containing pebbles apparently derived from the members of the gneissic complex, have been recorded in several places along the junction of the two formations in the area under review. These confirm the conclusion regarding the correlation already arrived at while working on the northern extension of these formations.
The Aravallis form by far the most wide-spread geological unit in the southern Rajputana and the northern Rewa Kantha States.
The system is represented by (1) a basal quartzitic formation, often conglomeratic, (2) an impure calcareous facies, generally dolomitic in composition and (3) an argillaceous series consisting of slaty, phyllitic and micaceous types along with arenaceous intercalatory bands.
It has not been possible to map the various types of the argillaceous metamorphics separately. The intermittently soil-covered condition of the country and the infinite gradation obtaining between the different types have rendered any attempt at separate mapping of these on the one inch to the mile scale maps impracticable.
On the south the Aravalli schists are bounded by an extensive exposure of a streaky and highly foliated biotitic gneiss. The junction is much blended and the formation has all the appearance of a composite or mixed gneiss.
South of the gneissic belt the Aravallis are again seen extending over the southern tracts of the Panch Mahals district and the neighbouring States of the Rewa Kantha Agency.
A portion of these southern exposures of the ancient sedimentary metamorphics has been described by W. T. Blanford under the local name of the Champaner series in his memoir "On the geology of the Taptee and Lower Narbadda valleys and some adjoining districts". He classed the Champaner beds with the Azoic group' of the area leaving the question of their geological horizon quite open 2.
Relation to the Champaners.
1 Mem. Geol. Surv. Ind., LXV, Pt. 2, p. 128, (1934). 2 Op. cit., VI, Pt. 3, p. 189, (1869).
L. L. Fermor examined the Champaners near Shivrajpur (22° 25': 73° 37') and drew attention to the similarity between the Champaners and the Dharwars1.
The northern continuation of Blanford's Champaner beds has been studied by B. Rama Rao in the Bariya State. He tentatively correlated these partly with the Raialo series of Rajputana and partly with the Upper Dharwars of Mysore, and placed them in the Purana group2.
In view of the continuous extension of the Aravallis southward into the Panch Mahals district and adjoining States and the close similarity in lithology and association of the Champaners with these, it has been definitely concluded that the Champaners are the southern extension of the Aravalli system3.
Both in the northern and the southern exposures of the Aravallis the argillaceous variants of the system form its prevailing representatives; outcrops of the basal quartzites and the limestone are very few in number and limited in extent.
General description and lithology. Basal conglomerate and quartzite.
Instances of conglomeratic quartzites have occasionally been seen in the midst of the Aravalli schists in Banswara and Jambu
ghoda States. Generally the occurrences are small, narrow and lenticular in shape. The highly disturbed and altered condition of these patchy exposures renders their stratigraphical significance obscure. They may either be truncated anticlinal crests of the Aravallis or may simply represent local shallow-water deposition of the Aravalli sediments.
Several discontinuous outcrops of conglomeratic quartzites flanking the pre-Aravalli gneissic complex and dipping below the Aravalli phyllites and schists have been traced in the Southern Rajputana States.
The exposure stretching between Padaria (23° 22′: 74° 18') and Potla (23° 11': 74° 26') in Kushalgarh, furnishes the most noteworthy instance. The band is exposed at intervals along the course of the Haran river. Its dip varies from 40° to 70° towards
1 Mem. Geol. Surv. Ind., XXXVII, pt. 2, pp. 281-282, (1909).
2 Geology of Baria State, p. 6, (1931).
3 Rec. Geol. Surv. Ind., LXVIII, pp. 25-26, 72, (1934).
the south-west. In its northern portion the conglomerate consists of pebbles of black mica-schists and quartzite imbedded in a dark matrix, which has itself become scnistose. The rock, occasionally, has a gneissoid appearance due to the quartz pebbles having been compressed into augen'. The matrix is often quartzitic and the rock may be devoid of pebbles for considerable distances, then appearing as a banded or massive quartzite, dark grey or black in colour. Following it S. S. E. along the strike, pebbles of gneiss appear, often as large as a man's hand, the black schistose fragments disappearing until, when the trap overlies it, the formation appears to grade into a banded gneiss. The foliation-strike of the gneiss is the same as that of the conglomerate and the transition seems gradual. This rock may either be metamorphosed arkose or may represent the original rock from which the conglomerate was partly derived. From field evidences, it appears the truth lies halfway between these two probabilities. The metamorphic action which occurred subsequent to the deposition of the conglomerate would be sufficient to effect coalescence between the two rocks in contact, as they would be similar in mineral composition, and partial remelting and intermixture would inevitably take place.
A series of outliers of the basal quartzites have been recorded near the boundary between Mewar and Partabgarh States in the midst of the banded gneissic complex. The rock consists of pebbles of grey, pink or white quartz held in a chloritic or sericitic matrix. A proportion of the pebbles, for small distances, is fairly uniform in size, though over long distances the range of variation in size is remarkably wide. The larger pebbles are flattened and lenticular in shape, occasionally simulating an augen' structure weathered faces of the outcrops.
No extensive, continuous outcrop of the Aravalli limestone has been mapped in the area. Generally the exposures are small and discontinuous. The outcrops are invariably rugged, much broken up and consist entirely of dislocated craggy blocks and rough boulders, affording absolutely no clue to the stratigraphical position of the formation in relation to the schists amidst which it usually occurs. Instances, however, have been recorded where the limestone occurs in juxtaposition with the basal conglomerate and underlies the argillaceous schists.
The limestone is generally of indifferent purity, containing argillaceous or siliceous impurities in varying proportion. A general characteristic of the impure Aravalli limestone is the fact that it is impregnated with reticulated siliceous veins, stringers or ribs, which on the weathering of the rock stand out as prominent interlacing ridges. When fairly pure the rock is either pink or greyish white in colour and more or less dolomitic in composition.
The formation varies widely in texture from place to place. In southern Rajputana and adjoining tracts the limestone is generally thickly bedded and coarsely crystalline. In the south, at the northern frontier of the Jambughoda State, it is mostly massive, compact or finely crystalline. The formation is invariably traversed by well developed joints and the outcrops weather dark grey and form characteristically uneven, dissected, rocky plateaus.
An interesting instance of the Aravalli limestone, interspersed by intrusive granite, has been noticed in Jhabua State, about five miles north of Anas Station (B., B. and C. I. Ry.) close to the village Rambhapur (22° 55′ : 74° 29') at the confluence of the Anas and the Pat rivers. The outcrops are much broken and generally covered with rough blocks and boulders. The rock is much intruded by granite, veins and stringers of which are seen in considerable abundance in the rock. The limestone is fine-grained, compact, highly siliceous and often varyingly ferruginous, the ferruginous mineral being limonite. On fresh fracture the rock shows a buff or grey interior. The weathered outcrops are dark brown. The rock is irregularly fractured and cracked. Traces of veinlets of secondary siliceous materials are seen in irregular ramifications standing out on weathered surfaces of the much fractured limestone. Often these preponderate over the calcareous contents, small pieces of which are then found held in the siliceous network. Sometimes the veinlets of the secondary silicates also show evidence of intense brecciation, and blocks, recemented with ferruginous cherty material, are seen in great force in the midst of these outcrops of the impure limestone.
The argillaceous facies of the Aravallis is represented by slaty, phyllitic and micaceous schistose formations.
Being usually soft and closely cleaved, these easily crumble on weathering and often lie buried under soil mantles of varying thick