I have done a little more research about seaglass marbles. So, to be complete, I should mention two other theories about why we find marbles on the beach. One is that ships used to use marbles as ballast, and then when there were shipwrecks, the marbles fell into the sea. The other theory is that children used to play with marbles down by the seashore, and throw them into the sea. Believe them if you wish. I do think the seaglass marble bottles are the most probable explanation -- but who knows. Obviously, someone has lost his (or her) marbles!