After a bit of research, I think I found the same “Angel with Millstone” that Cecil mentions in his answer to the question, “Why do Saints have halos?” (13-April-1984).
http://www.straightdope.com/classics/a1_040.html
It is from the Bomberg Apocalypse:
http://www.apocalyptic-theories.com/gallery/millstone/bamberg.html
It illustrates the scriptural passage:
“And a mighty angel took up a stone like a great millstone, and cast it into the sea, saying, 'Thus with violence shall that great city Babylon be thrown down, and shall be found no more at all” (Revelation 18:21).
The millstone, by the way, looks a heck of a lot more like a real millstone (or a Frisbie) than the other medieval illustration of the “Angel casting a millstone into the sea” I found. The other millstone looks more like a round loaf of bread or possibly a cushion.
The monk who drew that one had probably never seen a millstone, since he didn’t have any problem drawing a circular halo. Then again, he lavished such care on drawing his angel and Saint John (the man taking notes across the sea), that maybe he simply didn’t have time to do a proper millstone.
http://www.getty.edu/art/collections/objects/o3396.html
I have seen a couple of millstones in my time. In fact I worked in a (restored) grist-mill one summer (1979). Actually, Charlie ran the grist-mill. He was a farm boy with the mechanical skills the rest of us lacked. Miraculously, he kept his $100 (Canadian) car running almost all of the time.
We had a buckwheat pancake picnic for our families with the grain we (Charlie) ground.
It was one of those Government-funded “make-work-for-students” dealies.