Doc wrote:

What would be the right stuff to set a marble ledge onto the 2 x 4 at
the bottom of a window frame? The old stuff was something white that
looked like mastic. Should that do the job or is there something
better?

I've used silicone caulk as have most of the installers I've seen, but you could
also use construction adhesive as long as you don't over do it. Keep in mind you
do want to be able to remove the ledge under some circumstances without damaging
the frame or the ledge.