Which is NOT a stage in the formation of coal?

The formation of coal involves several distinct stages, each characterized by different types of coal that result from the increasing heat and pressure applied to organic material over time. These stages include peat, lignite, bituminous coal, and finally anthracite, which represents the highest grade of coal.

Lignite is the earliest stage of coal formation and is composed of somewhat decomposed plant material. Bituminous coal follows lignite and is formed under greater heat and pressure, resulting in a higher carbon content and energy density. Anthracite is the final stage, characterized by its hard texture and very high carbon content due to the prolonged exposure to heat and pressure.

Siltstone, however, is a type of sedimentary rock made up of fine-grained particles and is unrelated to the coal formation process. It does not represent a stage in the transition from organic material to coal but rather indicates a different geological process involving sediments. Therefore, identifying siltstone as not being a stage in the formation of coal is appropriate because it does not participate in the coalification sequence that begins with plant debris and progresses through various forms of coal.