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The rock types show considerable variability of physical characters and mineralogical composition. Micaceous schists with well pronounced foliation and often studded with small garnet crystals, closely laminated, corrugated phyllitic formations, grey slaty rocks of varying hardness and finely laminated, soft siliceous shales, all forms have been noticed from place to place. of the country does not allow one to trace these variations systematically it is hard to delineate them and determine the progress of the metamorphic processes with any degree of accuracy.
As the nature
Slaty forms of the argillaceous metamorphics occur in close association with the phyllitic schists of the area. These are generally soft, and tlteir cleavage or lamination is usually too close to render them of any use as building stones. Instances have, however, been recorded where slaty tiles and slabs yielding good roofing and flooring material are available amongst the argillaceous metamorphics. A number of diggings have been recorded near the village Therka (23° 5' 74° 12′) south-east of Jhalod town (23° 7': 74° 9'). The cleavage is less pronounced and the slate in this locality has developed a slabby character suitable for quarrying in tiles and slabs of varying sizes. The rock is distinctly siliceous and often faintly micaceous at the partings.
Extensive quarries of slates have been developed about one and a half mile south of Jhalod town (23° 7′: 74° 9′). The rock is well traversed by master joints and yields fair-sized slabs and tiles in abundance. The colour varies between grey and brown. Incipient siliceous growths are often seen on the parting planes of the slaty formation.
The phyllites are generally dark grey in colour; the rock varies in hardness and fissility. In its typical form the phyllitic formation. is characterised by pronounced foliation and by its softness. Thin intercalatory quartzitic bands often stiffen the phyllites. Intrusive veins of white quartz also sometimes bring about the same effect. Generally, however, the phyllites crumble readily under the attack of weathering agencies and are very prone to lie buried under the debris derived from their own disintegration.
The formation is best exposed in the extensive rocky tracts of southern Rajputana and the adjoining Rewa Kantha States of the north. The argillaceous metamorphics along the Jhabua-Panch
Mahals frontier have also been mostly represented by the phyllitic schists.
The colour of the schists varies between shades of grey and green. Reddish brown staining of the schists, due to decomposition of the ferruginous contents, has also been noticed. Development of sericite between the laminae of the phyllitic formation has on rare occasions given the rock a silvery white tinge. Occurrences of incipient garnets and of small crystals of pyrite have occasionally been noted in the phyllites of the Southern Rajputana and Kadana States. In most cases the rock is very soft and devoid of tenacity or sheen and splits easily, yielding laminae of inconstant thickness and undulating, smooth surface.
The strike of the schists is generally N. N. W. - S. S. E. and the dip is either vertical or steeply inclined towards W. S. W. Local variation in dip is, however, frequently met with, and often evidences of local distortion to a remarkable degree.
Instances of fairly slabby forms of the phyllites have been noticed in several places. The occurrence at Tandladara (22° 53′: 74° 28') in Jhabua State is noteworthy. Slabs of quite convenient size, often measuring six feet by four feet with a thickness of four to six inches, have been extracted in considerable quantity by superficial digging. Quarrying, however, is difficult and risky owing to the steepness of the prevailing dip, which varies between 70° and 80° to the W. S. W.
Highly siliceous types of and Kadana States. These weather dark on exposure. characteristically uneven and hummocky topography.
The argillaceous metamorphics classed under the general name
of micaceous schists' show considerable variation of form in the area under review. They are, as a rule, highly foliated, the foliation planes being invariably marked by lamellar or flaky micaceous constituents set in a granular quartzose groundmass. The proportion of these siliceous and micaceous elements in the rock is remarkably variable. From finely laminated, readily cleavable typical mica-schist to fairly slabby siliceous types marked by parallel orientation of the disconnected flakes of mica in them, all forms have been noticed. The colour of these rocks varies with that of the predominating micaceous element in them. When highly sericitic the schists
phyllite are common in Lunavada are pale grey on fresh fracture and On weathering the rock forms a
appear dull white, but generally the colour varies within different shades of grey on account of the differing amounts of biotite usually present.
The schists are generally soft, easily denuded and lie mostly hidden under their own debris, except where exposed by stream sections or road cuttings. The outcrops are usually. of a crumbling and decaying nature, yielding sandy masses, spotted and freckled with black or bronzy mica and brown ferruginous stains.
In the Southern Rajputana States instances have been noticed where the phyllitic or slaty formations have gradually passed through a kind of knotted schist into biotite-garnet-quartz-schist.
Near Dohad (22° 50′: 74° 15') in the Panch Mahals district the schist is fairly siliceous. Microscopic examination shows (44/138; 22082) a schistose arrangement of granular quartz and patches of biotite. Rounded crystals of garnet, showing minute quartz inclusions, are distributed irregularly in the rock. The garnet is often enclosed in a rim of magnetite. Dark blotches of iron ores, mostly decomposition products of biotite, are also noticed.
Farther to the south, at the Panch Mahals-Ali-Rajpur frontier, the rock grades into a micaceous quartz-schist in which small crystals of brown garnet are occasionally discernible in the hand specimen.
The schists are much intercalated with quartzite bands in almost all the Rewa Kantha States. The schists as a rule occupy the strike-valleys sheltered by long chains of quartzite ridges and are mostly buried under a soil mantle.
Generally the schists are highly micaceous, soft and crumbling. A specimen (46/518; 23607) from the Kotharia (22° 42′ : 74° 1′)— Bar (22° 46′ 74° 1′) valley of the Bariya State shows under the microscope much bleached biotite passing into muscovite and sericite, quartz grains often rolled out along the foliation-strike of the micaceous constituents, and garnet in porphyroblastic crystals. The garnet crystals are much cracked. Concentration of brown, limonitic material is noticed around the garnet crystals, as well as filling the cracks in them. Microscopic rods of tourmaline have been observed in fair abundance. Magnetite is present in subordinate quantity.
The micaceous schists of the north-western Rewa Kantha States of Lunavada and Balasinor show well pronounced foliation and are often studded with incipient garnet crystals. The rock is usually
greyish in colour and shows a finely schistose disposition of the specks and spots of biotite, muscovite and sericite in a siliceous groundmass.
Besides the fissile and yielding schists, readily crumbling into glistening sandy dust, occurrences of fairly thick-bedded, slabby types have also been occasionally noticed. They often yield easily to quarrying along the bedding planes and are excavated for building purposes.
Typical instances of these slabby formations are seen in some force north of Chamaria (23° 4′: 73° 36') in Lunavada State. In view of the marked preponderance of the granular matrix and the parallel streaky orientation of the micaceous elements in it, these rocks may more properly be called gneisses than schists.
Similar gneissic slabby types have been met with in the central plains of Bariya State. A specimen from north of Pipodara (22° 43′ : 74° 3') shows under the microscope (46/485; 23609) a granular quartzose groundmass, containing flakes of biotite arranged with their longer axes parallel. The biotite is sometimes bleached. almost entirely into colourless mica. In close association with the biotite, and to all appearance owing its origin at least partly to the alteration of the biotite, is noticed some epidote and zoisite, along with calcite. A few small rods of tourmaline also occur. A laminated or thin-bedded but fairly compact type, intermediate in composition and physical characters between the soft, crumbling mica-schist and the hard, grey quartzite, is met with near Limkheda (22° 50′ : 74° 0′). Under the microscope the rock (46/489; 23612) shows patches of biotite and garnet crystals in a groundmass consisting of a mozaic of fine-grained quartz. The biotite is often partly bleached and shows pleochroic halos. Chlorite, due to the alteration of biotite, has been noticed in places. The garnet crystals are ragged in outline and contain innumerable quartz inclusions. Slender needles of tourmaline and muscovite, as well as occasional patches of magnetite, are seen in the groundmass.
Instances of compact, highly quartzose forms have occasionally been noticed in the midst of the soft micaceous schists. Exposures occurring about three miles east of Edalwara (22° 41′: 74° 4') furnish typical illustrations. The rock under the microscope (46/487, 23611) shows biotite, occasionally chloritised partly or entirely, in a granular groundmass of equidimensional quartz grains. Sericite
is seen in the cracks of the quartz grains. Black iron ore, magnetite, with well defined outlines, is present in some force. Small stumpy crystals of tourmaline and apatite are also present. The rock may be termed a biotite-sericite-quartz-schist.
Quartzite intercalations in the Aravalli schists.
Intercalated quartzite bands are very common in the Aravalli schists. On account of the superior resistance they offer to the denuding agencies, in contrast with the crumbling schistose formations which adjoin them, the outcrops of these quartzite intercalations often constitute parallel chains of steep strike ridges sheltering the low-lying valleys occupied by the schists.
The bands of the quartzitic formations vary widely in size and extent. From small intercalatory ribbons measuring not more than a few inches in thickness to massive belts measuring more than 2,000 feet across, from locally limited exposures too small to be recorded on the one inch to the mile scale field-map, to impressive chains more than twenty miles in length, all have been met with in the area.
When fresh the quartzite is subtranslucent with a bluish or pinkish grey tinge. The weathered rock is dull and opaque and of a grey or brown colour. Generally the quartzite is compact, thickbedded and coarsely re-crystallised. Somewhat irregular block jointing is common in the rock. The outcrops are usually much broken and dislocated, affording but few opportunities for measuring the dip, which, wherever discernible, has been found to be in conformity with the foliation-dip of the schists in which the quartzites
The joint-planes of the quartzites are often marked by veins. of white quartz, occasionally containing irregular sheaves and booklets of white mica. Several instances of much fractured grey quartzite, with the cracks filled with white quartz of secondary origin, have been noticed. Often the veinlets develop into aggregates of small pyramidal crystals lining the cracks and crevices of the rock. The best examples of these much fractured quartzites, irregularly veined with secondary white quartz, are to be seen south of Metral (23° 4′: 73° 49′) in the Panch Mahals district.