Hilt's law describes how 
Hilt's law describes how depth affects the quality of this substance, and iron sulfide catalysts were originally used in the Bergius process of hydrogenating this substance. Another form of this substance used in Wootten fireboxes is produced under metamorphic conditions, and a large quantity of substance is found in the Powder River Basin. (*) Sub-bituminous and anthracite are varieties of this substance, much of which originated from the Carboniferous period. It can be mined from namesake seams and beds. For ten points, identify this group of black, carbon-based sedimentary rocks commonly burned to produce energy.
Hilt's law describes how depth affects the quality of this substance, and iron sulfide catalysts were originally used in the Bergius process of hydrogenating this substance.
coal [reverse prompt on specifics such as "peat" or "lignite" or "bituminous" or "anthracite" before mention]