In general, the most desirable coals are bituminous and anthracite because they have the highest carbon concentration, and pressure at which it forms-beyond anthracite, the coal will metamorphose into the mineral graphite. Figure 13.7 shows the distribution of different coal deposits in the United States. Notice how coals in the West, and that anthracite is restricted to a narrow belt within the Appalachian Mountains. The differences in rank are due to differences in geologic conditions (temperature and pressure) under which the coals formed