The basis for such a classification is the physical properties of rocks, the manner and arrangement of different particles and mass forming a stone. They are classified as follows:
- Stratified rocks
- Unstratified rocks
- Foliated rocks
(i) Stratified Rocks
Stratified structure is formed due to the splitting of a series of parallel layers of sedimentary rocks. They exhibit distinct layers which can be separated. The plane of separation is called a cleavage plane. Limestone, slate and sandstone are examples of stratified rocks.
(ii) Unstratified Rocks
They are of granular or crystalline structure and become solid on cooling. They do not show any sign of strata, e.g., igneous rocks like granite, basalt, etc.
(iii) Foliated Rocks
These rocks show definite parallel arrangement of minerals showing a tendency to split in a specific direction, e.g., metamorphic rocks like gneiss, schist, etc.
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