calcareous (adj.)

also calcarious, "of the nature of lime, containing lime, chalky," 1670s, from Latin calcarius "of lime, pertaining to lime," from calx (genitive calcis) "lime, limestone" (see chalk (n.)).

The spelling in -eous, which appeared about 1790, is erroneous, influenced by words in -eous from Latin -eus. The etymological sense of calcar-eous would be 'of the nature of a spur.' [OED]