<P> A considerable area of peninsular India, the Indian Shield, consists of Archean gneisses and schists which are the oldest rocks found in India . The Precambrian rocks of India have been classified into two systems, namely the Dharwar system and the Archaean system . </P> <P> The rocks of the Dharwar system are mainly sedimentary in origin, and occur in narrow elongated synclines resting on the gneisses found in Bellary district, Mysore and the Aravalis of Rajputana . These rocks are enriched in manganese and iron ore which represents a significant resource of these metals . They are also extensively mineralised with gold most notably the Kolar gold mines located in Kolar . In the north and west of India, the Vaikrita system, which occurs in Hundes, Kumaon and Spiti areas, the Dailing series in Sikkim and the Shillong series in Assam are believed to be of the same age as the Dharwar system . </P> <P> The metamorphic basement consists of gneisses which are further classified into the Bengal gneiss, the Bundelkhand gneiss and the Nilgiri gneiss . The Nilgiri system comprises charnockites ranging from granites to gabbros . </P> <P> Rocks of the earliest part of the Cambrian Period are found in the Salt range in Punjab and the Spiti are in central Himalayas and consist of a thick sequence of fossiliferous sediments . In the Salt range, the stratigraphy starts with the Salt Pseudomorph zone, which has a thickness of 450 feet (137 m) and consists of dolomites and sandstones . It is overlain by magnesian sandstones with a thickness of 250 feet (76 m), similar to the underlying dolomites . These sandstones have very few fossils . Overlying the sandstones is the Neobolus Shale, which is composed of dark shales with a thickness of 100 feet (30 m). Finally there is a zone consisting of red or purple sandstones having a thickness of 250 feet (76 m) to 400 feet (122 m) called the Purple Sandstone . These are unfossiliferous and show sun - cracks and worm burrows which are typical of subaerial weathering . The deposits in Spiti are known as the Haimanta system and they consist of slates, micaceous quartzite and dolomitic limestones . The Ordovician rocks comprise flaggy shales, limestones, red quartzites, quartzites, sandstones and conglomerates . Siliceous limestones belonging to the Silurian overlie the Ordovician rocks . These limestones are in turn overlain by white quartzite and this is known as Muth quartzite . Silurian rocks which contain typical Silurian fauna are also found in the Vihi district of Kashmir . </P>

Geological system in india in terms of age