A maiden "washing marble-white stone" in "Johnny Collins" seems nonsensical, but the mermaid is "washing silk upon a stane" in "Clerk Colvill" which makes all the sense, and the close similarity of these details is offered as compelling evidence for equating the two ballads.
In "Johnny Collins," a young woman is shown washing marble-white stone, but this seems absurd. In contrast, a mermaid is depicted as washing silk on a rock in "Clerk Colvill," which makes perfect sense and highlights the striking similarity between the two accounts.