A database with a wide subject coverage will give you a broad perspective on your topic.
The question of "what's the single best database for Subject X" can be a highly contentious question (these are Meg's picks -- she's a science & engineering librarian, just FYI). The databases below are a great place to start your research in a particular subject area, although searching one database alone won't give you the full picture if you're trying to create a comprehensive literature review.
Don't find anything in one database? Try different search terms, or try another of our 900+ databases.
Geology is an incredibly multidisciplinary topic -- the multidisciplinary databases will serve you well, as well as subject-specific databases that relate to your chosen topic. For geologists, you have a database idol, and its name is GeoRef.