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.
The Wilcox formation (called Lignitic by Hilgard, and named by Safford the Lagrange group) lies to the west of the last, and its western limit is from about 32° 12' on the Alabama boundary about due north-west; in its north-westernmost part it is on the western edge of the Tertiary in this state.
It is marked by grey clays and sands, lignitic fossiliferous clays, beds of lignite or brown coal, sometimes 8 ft.
The Jackson formation south-west of the Lisbon beds, is made up chiefly of grey calcareous clay marls, bluish lignitic clays, green-sand and grey siliceous sands.
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.
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 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.
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.