Reference:

Wood GH, Kehn TM, Devereux Carter, M and Culbertson WC (1983) Coal resource classification system of the U.S. Geological Survey. Geological Survey Circular 891, United States Geological Survey, Denver. Viewed 4 August 2014, http://www.pubs.usgs.gov/circ/c891/.

Last updated:
3 August 2016