I did uncover a deeply buried doc online about marbles being used in Buoys, wish I could remember or wade through my bookmarks and find it
On another site I run called Seaglasslovers,,,,we have one member who collects glass, well not marble, more like golf ball size glass from the railroad tracks,,,,
Other members researched it and found that they were used to transport cargo in stations,,,,laying the marbles out and putting planks on top to roll the heavy cargo......sounded credible to me.....
Yes,,,,I am amazed at the "myth" idea and it is funny how it has spread through the sea glass industry since we first posted it on our site 10 years ago....guess that is how all "myths" start. LOL