In the wikipedia article about vitrified forts they say the heat actually weakens the structure.

Yeah - I didn't mean to suggest it was a good step - they just seem stupendously mysterious sites (I grew up fairly near one and was always fascinated by it).

There is a simple explanation for finding vitrified rocks in a fort. Throwing lots of burning oil from the top over a rock of the correct type. They probably found some practical uses for nordic petroleum