[thejeff]

Stone/iron skin boost natural protection.
The Earth Bless boosts armor protection. It does nothing if you're not wearing armor.
Armor and natural protection combine in some complex way to get actual protection, so both the bless and the spells will add to your actual protection, but not linearly. So you are losing efficiency, but it's hard to get high protection without losing efficiency somewhere.
For thugs and mages, the reinvig may be more important than the protection anyway. And you should also Summon Earthpower. There is never too much reinvig.