before vowels hydr-, word-forming element in compounds of Greek origin, meaning "water," from Greek hydro-, combining form of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet"). Also sometimes a combining form of hydrogen.
1795 as "science of the past and present condition of the Earth's crust," from Modern Latin geologia "the study of the earth," from geo- "earth" + logia (see -logy). German Geologie is attested by 1785. In Medieval Latin, geologia (14c.) meant "study of earthly things," i.e. law, as distinguished from arts and sciences, which concern the works of God. Darwin used geologize as a verb.
There rolls the deep where grew the tree.
O earth, what changes hast thou seen!
There where the long street roars, hath been
The stillness of the central sea.
The hills are shadows, and they flow
From form to form, and nothing stands;
They melt like mist, the solid lands,
Like clouds they shape themselves and go.
[from "In Memoriam," 1850]
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Harper, D. (n.d.). Etymology of hydrogeology. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/hydrogeology