Underlie

The old British convention for describing how far a vein or lode departs from vertical, expressed not as an angle in degrees but as a horizontal distance -- inches or feet -- per fathom of vertical depth.

Historical usage

The convention is recorded across 19th-century mine surveys nationwide, from Cornwall to the Yorkshire Dales: an 1856 survey recorded Dolcoath's main lode as running close to east-west and dipping 2 feet south in the fathom, and Condurow's as dipping 18 inches in the fathom; contemporary reports on the Grassington and Craven Moor lead mines in Yorkshire likewise describe veins whose 'underlie' changed unpredictably with depth.

How it worked

A vein's underlie was established by survey and reported alongside its strike (the compass bearing of the vein at surface), together telling a mine captain exactly how far a shaft or level would need to deviate from a straight vertical or horizontal line to stay within the ore as workings went deeper.

Why it was used

A vertical shaft sunk straight down, or a level driven straight in, would drift away from an inclined vein as depth increased; knowing the underlie let engineers plan a shaft's angle, or an 'underlie shaft' following the vein's own incline, so that workings continued to intersect the ore rather than losing it.

Risks & limitations

A vein's underlie could change abruptly and unpredictably with depth -- as recorded at several Yorkshire Dales mines, where an increased or reversed underlie could throw out workings planned on an earlier survey, sometimes requiring a new shaft to be sunk to recover the lost ground.

Regional variation

The same measurement is expressed today as dip, conventionally given in degrees from horizontal rather than as a distance per fathom -- the Strike and Dip geology record covers the modern convention and the underlying geometry it describes.

Mines associated with this term

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this might plausibly apply.

• Wheal Mary Ann — Cornwall (well documented example)
  The mine's own recorded workings give a precise underlie for the worked lode -- an average of about 15 inches east per fathom.

Sources

1. (primary) Memoirs 1965: Extracts from Old Editions of the Mining Journal — Northern Cavern & Mine Research Society
2. (secondary) A Cornish mining glossary — Cornish Mining World Heritage Site (UNESCO)

Record created: 2 September 2026 · Last researched: 2 September 2026