Furnace causes this?

P8080009, originally uploaded by Lipwak.

Yesterday I filled the birdbath with clean water. Last night it rained. Today, as often happens, the birdbath is full of this dirt.

I used to think it was from whatever animals were using it (raccoons, woodchucks, who knows what). Now I think it is soot from my furnace. The furnace would have run during the rain, causing the soot to not get far. (The furnace really smells when it runs. It ought to be replaced.)