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be critical in reducing volatilization and moisture and, consequently, increasing the rank of coal. ~~ong these are (1) depth of burial with associated pressure and heat, (2) time, (3) structural defomation (usually through metamorphism), (4) possible heat of nearby intrusive igneous rocks, and (5) type of plants decomposing into the coal. N """ ~ W In the coal picture for the entire United States, western coals are far younger in depositional age thun are the eastern coals, and they tend to be covered by less overburden. Thus, most western coal is lower in rank--subbituminous up through the lower levels of the bituminous ranks. There is one known important anthracite deposit in the Upper Colorado River Region in Gunnison County, Colorado. Much western coal, particularly in Wyoming, is neither deeply buried by overburden, nor has it been subjected to complex folding, faulting, heat, and pressure of metamorphism, which would have lowered the moisture content and increesed the rank. . Grade of coal is a quality deternination based on content of ash, sulfur, and other constituent materials and is a basis for evaluating the various coals. Most coals currently mined and included in total resource estimates contain less than 33 percent ash, by weight. The average ash content from a study of several ~ine samples from the encire United States was 8.9 percent, which is higher than the average ash content of most western coals (Averitt, 1975). In the Averitt study, sulfur content averaged 1.9 percent, much higher than the sulfur in the majority of western coals, which makes the Upper Colorado River Region coals more desirable from the standpoint of air pollution control and reduction. The economic recoverability of coal, and estimates of the total available resource, depends on the thic~ess of coal beds. For higher rank, higher value eoals, beds as thin as 14 inches may be considered economically feasible for mining. For lower ranks of coal, such as subbituminous and lignite, beds are considered minable if they are fro~ 2~ to 10 feet thick. (Averitt, 1975). Thickness of overburden is also a factor in the recoverability of coal and in determining the method of mining used (e.g., whether or not strip mining is feasible). Coal resource data are usually categorized by the thickness of overburden. Deposits to an overburden depth of 0 to 3,000 feet are classified as a surface deposit, and in the upper layers to about 1,500 feet are eonsidered minable by surface methods, while overburden thickness between 3,000 and 6,000 requires underground mining. ESTIMATING THE RESOURCE The coal resource, which represents the total quantity of coal in the ground including identified and hypothetical resources, is often difficult to identify. In estimating the coal resource, the terms "measured, II uindicated, II dnd "inferred" are used. The Cerm "reserve" . B-3