Also note that thre are also some "hidden" terrain types that have an effect on the province.
The random map generator use them : when you see on a province something unusual, like a volcano for exemple, it means that there's one of thos terrain types.
The volcano for exemple is for "firesite", wich I suppose must increase the chance of fire site (and probably the global chance of a site)
The small temple must be the type "priestsite"
etc ...
There are one such type for each magic path.
If you want to know what special terrain type are on the provinces in a random map, you can use my online map validator at
http://dominions.realites.org/
Just uncheck the "randomize" and "greater manysites" options (as you don't want to change the map, only to analyse it), and in the map resutls you will have added to eachprovince in comments the list of all its terrain types, including the hidden ones.