This is a somewhat belated response to this line in inquiry.  If it
     duplicates what someone has already said, I apologize.

     The glass block floor that I know best is in the stacks at the Library
     of Congress.  In the good old days when researchers could get stack
     passes, I remember being VERY nervous about walking around on these
     odd, translucent floors.  The Library dates from some time in the
     1890s, I think.

     I don't know whether they are still there, since the Library has
     recently been "restored" at great cost.

     I have a feeling that I have seen these glass block floors in other
     library stacks as well.

     Marilyn