GENERAL

GEOLOGY

[This section on Geology has been contributed by Shri. V. R. Venkoba Rao of the Geological Survey of India, Poona.].

The geology of the entire district consists of dark-coloured volcanic lava flows and laterites. The lava flows were poured out of the long and narrow fissures in the earth's crust, at the close of the Mesozoic Era, approximately, 80 to 100 million years ago. These are spread out in the form of horizontal sheets or beds and constitute the innumerable spurs, hills and hill ranges; bold, flat topped ridges; lofty peaks and plateaus with impressive cliffs. These hill ranges and plateaus form a part of the famous Western Ghats. In the plains and valleys the lava flows occur below a thin blanket of soil of variable thickness. A characteristic feature of these flows is their horizontal disposition and considerable lateral extent with almost incredible uniformity in their composition and appearance.

Because of their dominantly basaltic composition and the tendency to form flat-topped plateau, the lavas are termed plateau basalts. Since these basaltic lava flows cover an extensive region in the Deccan and frequently present step-like appearance to the hills and ridges they are commonly termed as "Deccan traps." the word trap meaning 'step like'.

The traps attain a thickness of nearly 2.500 to 2,800 feet around Matheran and Raygad plateaus, respectively. The individual flows vary greatly in thickness from a few feet to as much as 75 feet or even more although the average thickness is about 40 feet. In a single hill a number of flows, sometimes as many as 10 to 20, could be seen resting horizontally one above the other. Vertical, inclined, prismatic and columnar jointings are commonly found in the hard and compact basalts. The rocks wither by exfoliation into massive, spheroidal boulders which are usually seen on hill slopes and foot hills.

Petrologically, the lava flows of the district are extraordinarily uniform in their composition and texture, corresponding to a dolerite or a basalt, with an average specific gravity of 2.9. In composition the basalts are composed of abundant labradorite feldspar, enstatite, augite and interstitial glass. Magnetite is the most common accessory mineral though, at times, a fair amount of olivine is also present.

The basalts are usually dark grey to grey and bluish grey in colour and are hard, compact and tough and fine to medium grained in texture. At places, these exhibit a porphyritic texture also. They generally form the hill tops, plateau and cliffs and show well-developed characteristic columnar and prismatic jointing. Next to this common variety of trap is found the comparatively softer, amygdular and scoriaceous traps, purple to greenish in colour, usually showing rounded and elongated or tubular cavities and geodes with infillings of secondary minerals like calcite, zeolites and a variety of secondary quartz like agate, jasper, chalcedony, etc. These generally occupy the lower portions of the ridges and their slopes and usually the valleys and plains. Associated with these common basic lava flows are also found, at times, acid lava flows represented by light coloured, trachyte, rhyolite, etc. Tuffaceous beds, volcanic ash and breccia beds are also noticed at places in the district. A red clayey bed, often termed as "red bole", representing an altered ferruginous flow, is occasionally present interbedded with the trap flows.

Sandwiched between the two trap flows, thin beds of grey to dark grey and dirty-green argillaceous and calcareous shales or clays and friable sandstones are sometimes known to be present in the trappean areas, although no such beds have been reported so far from the district. These are known as inter-trappean beds representing the sediments deposited in shallow lakes during the quescent periods of volcanic eruptions.

Beds of laterite, usually formed by the mechanical and chemical disintegration brought about by the atmospheric agencies on the underlying trap, cap the several peaks and lofty ridges in the district. They are also found at places in the lower regions. The beds vary in thickness from five to 50 feet or more. The rocks are usually mottled, reddish to reddish or yellowish brown in colour and show vermicular and tubular cavities often stained with dark brown ferruginous solution. The rocks are soft and show bright colours when freshly cut but become very hard and dull on exposure to atmosphere. The outer surface of the beds present a dark to dirty brown colour and a very rugged and pitted appearance.

The traps on weathering give rise to a greyish to dirty green, friable murum which on decomposition and decay yield a rich and fertile reddish-brown to coffee brown and black soil. The laterite on disintegration gives rise to a dusty, reddish to reddish-brown soil.

Economic Geology.

No notable economic mineral that could be exploited profitably has been discovered in the district, so far. However, the traps are considered as one of the best materials for use as building stones, road metal and ballast and there is no dearth of the same in the district.

Bauxite.

Isolated lofty hills, capped by laterite, are at times known to contain bauxite in some quantities. Though no significant deposits of bauxite are reported from the district so far, yet a very small quantity of aluminous laterite is known to occur on the hill tops around Matheran. They are also reported from Shekhadi, Wakalghar, Kandivli in Shrivardhan mahal and Ambegaon and Mandla in the Murud mahal. All these localities are about 15 miles from Janjira. These occurrences have not been examined in great detail with regard to their quality, quantity and workability.

Building Materials.

The numerous lava flows, particularly, the hard, compact, tough, fine-grained basalts constituting the several ridges and hills afford almost an inexhaustible source of good quality building stones, road metal and railway ballast. Being dense, hard and durable the traps are known to answer well to the tests for porosity, crushing strength, attrition, etc., and as such are among the best materials for building construction. The trap rocks are also well suited for use as road metal and railway ballast. When suitably selected, the fine grained basalts form an excellent aggregate for concrete.

Nodular Kankar, a concretionary lime carbonate, is usually found sporadically on soils and alluvium covering the Deccan trap in the district. The Kankar, on burning, yields good lime almost comparable in quality with the hydraulic lime. At present it is used for burning lime for local use in making plaster and white-washing. In addition to Kankar, the sea shells found along the coast at places, are used for the manufacture of lime.

Iron-Ore.

Sporadic occurrences of iron-ore, caused by the enrichment or concentration of ferruginous matter from the laterite, are known to exist in the laterite beds in different parts of the district. However the grade of ore is very poor.

Mineral Springs.

Along the north-south coastal tract between the sea and the Western Ghats there are three well-known groups of springs in the district, namely, the Unhere, Sav and Vadavli spings. The Unhere springs in Roha taluka arc 70 and 80 miles from Poona and Bombay, respectively. The Sav springs are situated on the southern bank of the Savitri creek to the north of the village Sav. They are about 120 miles from Bombay and 70 miles from Poona. The Vadavli springs are situated on the southern bank of the Savitri river at about two miles east of Mahad town. They are about 124 miles from Bombay.

The Unhere springs containing chloride and saline water discharges at the rate of 410 gallons per hour giving out plentiful bubbles of CO₂. The Sav springs containing lesser proportion of chloride and plenty of C0₂ and traces of H₂0 springs out at the rate of 600 gallons per hour. Both the groups of springs record a temperature of 41.5°C and are devoid of radon content. Another spring near Unhere is found to discharge at the rate of 1,500 gallons per hour giving out plenty of C0₂ and traces of H₂S. This has a radon content of 0.806 and records a temperature of 60°C. They are known to possess medicinal value in curing skin diseases and rheumatic complaints. They are also said to induce appetite. It is considered possible to develop some of the reputed mineral springs into spas and health resorts according to the Taleyarkhan Committee's Report.

Salt.

Salt is being produced by the direct solar evaporation of sea  water near Uran, Shewa, Karanja, Pen, and Panvel in Kolaba district. A statement showing the average annual production of salt at the above centres for five years is given below:-

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