Hi, Bret,
I can only guess without seeing a picture. There are a couple of possibilities. If you are sure that the host rock was limestone, then the black smoky layer could be chert, especially if the layers of black are parallel to the limestone beds. Chert is a micro-crystalline silica that was a silica ooze layer on the bottom of a sea.
On the other hand, if the whole deposit is bedded, and you mention blocks laying around, then it could be a welded volcanic ash flow or lava flow, and the black layer could be welded volcanic glass.
Or, it could be something else!

From RockDr - October 22, 2009 at 20:55:30

Message: 68457