How is coal formed?

How is coal formed? Describe the qualities of four different types of coal found in India.

Coal is formed due to the compression of plant and animal material over millions of years.
The four qualities of coal found in India are :
(i) Peat has high moisture content and low heating capacity.
(ii) Lignite is a low grade brown coal, soft and with high moisture content.
(iii) Bituminous coal is buried deep and subject to high temperature. It has good heating power and Is usually black in colour.
(iv) Anthracite coal is the hardest variety and has the highest heating power.