Web of Science is a standard source for all scientists.
Despite it's name, it can be used effectively in the social sciences, humanities, and fine arts.
It's strength is that you can find out how many times an article has been cited AND easily access all the articles that cite an article you are working with.
It is not geology specific and should never be used as a substitute for GeoRef.
JStor is a journal archive. It contains issues of journals all the way back to the the first day they published. For any journals this means the 1800's or even earlier. If you are interested in some of the seminal works of historical figures in geology, you will be able to find a lot in JStor. It's also good for first person accounts of historical events - think the 1906 earthquake in San Francisco.....
GeoRef is THE database for geologists.
It selectively indexes and provides access to the geology literature of North America from 1785 to the present. It covers the literature of the world from 1933 to the present.
Journal articles, books, maps, conference papers, reports, and theses are included. Where present, author-supplied abstracts are included.