Rocks are mixture- of minerals and therefore their physical and chemical composition vary with characteristics of minerals present in them. On the basis of genesis and structure, rocks are grouped into three classes.
- Igneous Rocks.
These plutonic rocks are formed by solidification of molten mass of earth (magma). If they were cooled slowly deep in earth called intrusive and if cooled rapidly near the surface are called extrusive or effusive. The common igneous rocks found in India are Granite, Basalt, Diorite and Gabbro.
- Sedimentary Rocks.
These rocks are derived from igneous rocks and arc formed by consolidation of fragmentary rock material, and the product of their decomposition, which is deposited by water. The common sedimentary rocks are conglomerates, sandstones, shales, Dolomite, Quartzites and limestone.
- Metamorphic Rocks.
These rocks are formed from igneous or sedimentary racks on exposure to intense heat or a very high pressure, or both, which have changed considerably in structure and mineral composition. The common metamorphic rocks are — Gneiss, Quartzite, Conglomerate, Talk, Serpentine, Soapstones, Slates, Marbles etc
Reference: Handbook of Agricultural sciences by Dr. A.K. Gupta