Lignitic dial was discovered on or near the coast of Coos Bay as early as 1855, and this is still the only productive coalfield within the state, although there are outcroppings of the mineral all along the Coast Range N.
The true bituminous coal produced, which in 1897 was only equal to that of the lignitic and semi-bituminous varieties (1.75 million tons), had come by 1902 to constitute three-fourths (5.46 million tons) of the entire coal output.
Coal and lignitic coal are the principal economic minerals met with in this central plain, though natural gas occurs and is put to use near Medicine Hat, and " tar sands " along the northeastern edge of the Cretaceous indicate the presence of petroleum.
The coal underlying the east half of the state, the " Great Plains," is lignitic and of inferior quality, but that in the mountain districts is bituminous and generally suitable for coking.
The oldest, bordering the Lower Carboniferous, is the Tuscaloosa formation of clays and sands arranged as follows: dark clays, thin lignite seams, lignitic clays, sands and chert, and light clays; this formation is 5-15 m.