among miners, denotes a train of metal-line stones, serving to direct them in the discovery of mines.
SHOAD-Stones, a term used by the miners of Cornwall and other parts of this kingdom, to express such loose masses of stone as are usually found about the entrances into mines, sometimes running in a straight course from the load or vein of ore to the surface of the earth.
These are stones of the common kinds, appearing to have been pieces broken from the strata or larger masses; but they usually contain mastic, or marcasitic matter, and more or less of the ore to be found in the mine. They appear to have been at some time rolled about in water, their corners being broken off, and their surface smoothed and rounded.
The antimony mines in Cornwall are always easily discovered by the shoad-stones, these usually lying up to the surface, or very nearly so; and the matter of the stone being a white spar, or defaced crystal, in which the native colour of the ore, which is a shining bluish black, easily discovers itself in streaks and threads.
Shoad-stones are of so many kinds, and of such various appearances, that it is not easy to describe or know them; but the miners, to whom they are of the greatest use in the tracing or searching after new mines, distinguish them from other stones by their weight; for if very ponderous, though they look ever so much like common stones, there is great reason to suspect that they contain some metal. Another mark of them is their being spongy and porous; this is a sign of especial use in the tin countries; for the tin shoad-stones are often so porous and spongy, that they resemble large bodies thoroughly calcined. There are many other appearances of tin shoads, the very hardest and firmest stones often containing this metal.
When the miners, in tracing a shoad up hill, meet with such odd stones and earths that they know not well what to make of them, they have recourse to vanning, that is, they calcine and powder the stone, clay, or whatever else is supposed to contain the metal; and then then washing it in an instrument, prepared for that purpose, and called a vanning shovel, they find the earthy matter washed away, and of the remainder, the fliny or gravelly matter lies behind, and the metallic matter at the point of the shovel. If the person who performs this operation has any judgment, he easily discovers not only what the metal is that is contained in the flood, but also will make a very probable guess at what quantity the mine is likely to yield of it in proportion to the ore.