Hi, In oceanography and geology we use the term terrigenous to mean "continental land derived" as in terrigenous sediments, etc. Terra incognita is the "unknown land," not the unknown Earth! I was thinking pelagic is a good antonym for terrigenous. To my colleagues, and most others in both the vocational and applied aspects of oceanography, the open ocean and things living in it or derived from it are termed pelagic, e.g. pelagic sediments, pelagic birds or pelagic waters (as opposed to coastal or neritic waters). To Earth scientists terra means land or even continental (or felsic) land (not oceanic (volcanic) islands, no matter how big they are). Of course terra has other meanings as well (means planet Earth (a noun!) in lots of science fiction and it also is a term used for some highlands of the Moon. So, properly constrained, pelagic and terrigenous are antonyms when used as adjectives. I believe this is the best fit solution, in my book anyway. Perhaps a new word such as pelagenous or a word like interrigenous can be encouraged or used as antonyms for terrigenous.