Reading about them at the moment, the Cornish solution basins nearly always occur in the hardest granite rock and over the centuries water deepens the first hollow, dissolving the quartz out of the matrix of the granite leaving a shiny quartz layer at the bottom of the basin.
This can be seen as a 'libation' basin, Borlase (when he was not thinking druids) thought that this pure water, formed from rain, snow or dew would have been used by the elders in their ceremonies, me I like to think that Daniel Gumb's family used Stowe Pound's 15 solution basins. Always on the flat rocks on top of tors, there is even a possibility that Trevethy Quoit's holed stone could be a solution basin worn through, and that it may have been brought down from a tor.......