This relic is a rude stone, forming a kind of chair, lying in a field adjoining the churchyard, and about thirty or forty yards from it. When it was removed to its present position is unknown. There was also a well below the church called Ffynnon Canna; and there is still a small brook available, if required, for following the rules prescribed to those who wish to avail themselves of the curative powers of the saint's chair. It appears that the principal maladies which are thus supposed to be cured are ague and intestinal complaints. The prescribed practice was as follows.
The patient first threw some pins into the well, a common practice in many other parts of Wales, where wells are still thought to be invested with certain powers. Then he drank a fixed quantity of the water, and sometimes bathed in the well, for the bath was not always resorted to. The third step was to sit down in the chair for a certain length of time; and if the patient could manage to sleep under these circumstances, the curative effects of the operation were considerably increased. This process was continued for some days, even for a fortnight or longer. A man aged seventy-eight, still living near the spot, remembers the well and hundreds of pins in it, as well as patients undergoing the treatment; but, about thirty or thirty-five years ago, the tenant carried off the soil between the well and the watercourse, so as to make the spring level with the well, which soon after partly disappeared, and from that time the medical reputation of the saint and her chair has gradually faded away, and will, in the course of a generation or two, be altogether forgotten.
There can be little doubt that the present church occupies the site of the old and original building of Canna, although there is, in the middle of the parish, a field called Parc y Fonwent, or the churchyard field, where, according to local tradition, the church was to have been originally built, but the stones brought to the spot during the day, were removed by invisible hands to the spot where the present church now stands, accompanied by a voice clearly pronouncing this sentence: "Llangan, dyma'r fan," or, "Llangan, here is the spot." Such miraculous removals of stones are reported and believed in many other parts of Wales; and in the present instance the story seems to have arisen from the circumstance of the field in question having been formerly church property.
More (on the inscription) here in Archaeologia Cambrensis (1875).
Coflein puts the stone at SN17701874 and says before 1925 it used to be here SN17751875. But how big is it? You'd think it was too big to move. And (my ultimate excuse for including this stone) surely it was around here near the spring and the special insisted-upon spot before the church turned up. (Perhaps it's smaller than I hope, as the RCAHMW puts it at 28 by 26 inches).
I can't find a photo (and I think Ocifant's tried to find the place in person without luck?) but the drawing in 'Lives of the British Saints' shows the slightly ambiguous lettering and the hollow "produced by the multitude and frequency of the devotees".