I have just come across this reference.
My GG grandfather was James Glanville who was I believe a son of William Glanville of Colan and Elizabeth, possibly Elford. In a box of family papers my cousin has come across a copy of a letter from a Thomas Glanville to his mother. The return address is Mr Stephen Maddox near St Louis, he appears to be a farmer. Thomas refers to his elder brother John and his american wife, no name, and children.
Is this likely to be of interest to you?
I have been unable to trace this John and Thomas so far though there is a will of a William Glanville of Colan in the Cornwall Records Office which I hope to look at in the next couple of months and may give some names.