This is the curious Boston Stone on the Blackstone Block near Faneuil Hall. Most references to it have no citation at all. After doing some digging I learned that this was brought over from London in 1700 and used as a millstone for a paint mill in order to grind substances used for pigments. Sources differ on this but somehow that mill was destroyed and when it came time to rebuild on this site, somebody found the hollow stone and decided to include it on the foundation of the new building. It became a survey point.
Old Ironsides' masts against a dimming summer sky.