A geological term that states the deeper the coal, the deeper its rank (grade). The law holds true if the thermal gradient is entirely vertical, but metamorphism may cause lateral changes of rank, irrespective of depth. Increasing depth of burial results in a decrease in the oxygen content of the coals. The phenomenon was observed by professor Carl Hilt (1873).
Coal Geology by Larry Thomas, [1], # ISBN 0471485314, # ISBN 978-0471485315