coal measures

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun plural A stratigraphic unit equivalent to the Pennsylvanian or Upper Carboniferous Periods.
• noun plural Strata of the Carboniferous Period usually containing coal deposits.

from The Century Dictionary.

• In geology, that portion of the Carboniferous series in which beds of coal are found.
• In America and Great Britain a special significance attaches to this term in geological classification. In Pennsylvania the coal-measures are those strata which carry the productive coal-beds of Carboniferous age and all deposits intercalated between them, and they are divided into lower productive, lower barren, and upper productive measures. The coal-measures, with the millstone-grit at their base, constitute the upper division of the Carboniferous system. In Great Britain and on the Continent their relations are similar.