It's true that some rock types like granite have low conductivity, and safety earth connections in granite country need alternative methods (eg relying more on the Neutral wire). But most rock types give reasonable protection.That's beside the point. Whenever an electric circuit is grounded or a conductor is grounded it can't build up a charge. Try. It won't work. The earth is a charge sink. I used to be an electronics technician. That was when I was first acquainted with the subject. Every time I worked on an electronic device I always grounded the chassis. Not once in those years did I ever get zapped. You won't get sapped from a conductor which is conducted to ground with a conductor. - Just as the text said.