To answer my own question, there is a specific FAQ at the website that calmon posted:
http://www.freewebs.com/dominions2/faq.html
There's lots of other cool stuff there as well, (thanks calmon), but that's specifically the FAQ.
It's for Dominions2 though, I don't know if the information is out of date.
Incidentally having reviewed the section on the chances of getting magic sites, I think I was extremely unlucky to have cast Haruspex on about ten provinces (including 6 mountains and a forest) in a middle era game, and got two bleedin' Farms of Plenty (i.e. no gems). Although I guess a lot of the Nature sites must be in a forest. How does that work? If the random roll gives a Nature site, and then another roll determines a site which should be in a forest, do you get no Nature site? Or does it only pick among the ones which could appear in that terrain? Or is there some other complexity I'm missing (has it changed from Dominions2)?