circle vs mind fog

I swept my gaze around the store. Cold grey mist rolled forward, slow and steady, in every direction. "Looks like," I said. I nodded down a tall, narrow aisle containing auto parts. "Down there, quick."
"What's down here?" Murphy asked.
"Cover. I have to get us a defense against that mist." We reached the open space at the end of the aisle, and I nodded to Murphy. "Here, stop here and stand close to me."
She did it, but I could still see her shaking as she asked, "Why?"
I looked up. The mist had reached the far end of the aisle and was gliding slowly down it. "I'm going to put up a circle that should keep it off us. Don't step out of it or let any part of you cross outside."
Murphy's voice took on a higher, more tense pitch. "Harry, it's coming."
I twisted open both salt shakers and started pouring them out in a circle around us, maybe three feet across. As I finished the circle, I invested it with the slightest effort of will, of intent, and it closed with a sudden snap of silent, invisible energies. I stood up again, holding my breath, until the mist touched it a moment later.
It roiled up against the circle and stopped, as though a cylinder of Plexiglas stood between it and us. Murphy and I both let out our breath in slow exhalations. "Wow," she said quietly. "Is that like a force field or something?"
"Only against magical energies," I said, squinting around us. "If someone comes along with a gun, we're in trouble."


Summer Knight Chapter 19, Page 225-226