repair/attach Marble (http://www.lightemprador.com) slab

The Marble (http://www.lightemprador.com) slab under an inner door has cracked and (http://maplered.com) separated from its mount.  The slab rests on a 5 1/4" wide white cement base with 1/2" Stone (http://g603.net) floor Tiles (http://www.g682.net) on each (long) side. The Marble (http://www.lightemprador.com) is about 6" wide, 42" long, 1" thick, and (http://maplered.com) is now in two pieces (each 6" by 20+") The door separates the outer vestibule from the inner vestibule in a three-family house, and (http://maplered.com) the doorway sees MUCH use.  This house was built in 1910.  The slab has been cracked for years (at least 30).  It detached only recently, and (http://maplered.com) I don't know if it was ever reset before that.  How can I rejoin the pieces, and (http://maplered.com) how can I reattach the slab?