Hilt’s law states the deeper the coal the higher will be the rank or the grade of coal formed. The law holds true if the thermal gradient is entirely vertical. But metamorphism can cause lateral changes in the grade of coal formed. At higher depths, the material is subjected to higher temperatures and pressure, and more buried plant matter is transformed into carbon. The Hilt rate is controlled by the chemical-physical factors and rates of sedimentation.
Hilt’s law was first observed by professor Carl Hilt in 1873.
