"Hammarberg, Eric" wrote:

> True cavity wall construction is generally a post WWII technology. If the
> building is 100-years old there may be a few voids but the collar joint
> should mostly be filled with mortar.

Again- I'm thinking it is entirely possible that the collar joint has so
deteriorated that the masonry has turned to dust, and like sands in the
hourglass dropped into the basement or out the weep holes onto the street.