TER'TIARY, adjective Third; of the third formation. tertiary mountains are such as result from the ruins of other mountains promiscuously heaped together.

Tertiary formation, in geology, a series of horizontal strata, more recent than chalk beds, consisting chiefly of sand and clay, and frequently embracing vast quantities of organic remains of the larger animals. It comprehends the alluvial formation, which embraces those deposits only which have resulted from causes still in operation; and the diluvial formation, which is constituted of such deposits as are supposed to have been produced by the deluge.