What is the X in X-mas?
1830 with reference the geological period, from a word formed in English in 1799 to mean "coal-bearing," from Latin carbo (genitive carbonis) "coal" (see carbon) + -ferous "producing, containing, bearing," from ferre "to bear" (see infer). The great coal beds of Europe were laid down during this period. As a stand-alone noun (short for Carboniferous Period) from 1940s.
The period of geologic time from about 360 to 286 million years ago. The term is used throughout the world, although this period of time has been separated into the Mississippian (lower Carboniferous) and Pennsylvanian (upper Carboniferous) in the United States. During this time, widespread swamps formed in which plant remains accumulated and later hardened into coal. See Chart at geologic time.