Killas
A historic mining term for slaty, metamorphosed sedimentary country rocks around Cornish granite; meaning varied by place and period.
Language and route: Regional mining usage in Cornwall. Killas is attested in Cornish geological and mining English. Its ultimate linguistic derivation has not been established; usage in Cornwall does not, by itself, prove a particular Cornish-language root.
Literal gloss: unresolved
The name and its history
Cornish miners (vernacular term; individual origin unknown) · In published geological debate by the early nineteenth century
- What does the word literally mean?
- Recorded as killas (also kyllas) in Cornish mining and geological usage. A secure derivation into an earlier lexical root has not been demonstrated; it would be misleading to manufacture a literal translation.
- What did the original author mean?
- For Cornish miners it denoted practical kinds of fissile country rock, not an established mineral species.
- Why was the name needed?
- Different workers were trying to match local field terms with broader British and European rock classes.
- What evidence supported the original concept?
- MacCulloch explicitly contrasted broad Cornish usage with proposed restriction to greywacke.
- How has geology changed the concept?
- Present-day definitions commonly focus on metamorphosed country rock around granite, while no secure literal derivation is known.
Further historical context
An early geological discussion by John MacCulloch warned that killas should not simply replace the German-derived name greywacke. He observed that Cornish miners used it for a broad range of fissile country rocks, including clay-slate and greywacke-like material: local field usage was not coextensive with the imported rock classification.
Nineteenth-century attempts to align miners’ practical rock distinctions with systematic petrography altered its interpretation. Modern Cornish descriptions usually use killas for variably metamorphosed sedimentary country rocks around granite intrusions, but it does not determine a single protolith or grade.
Editorial qualification
Historical coinage and context have not yet been fully researched.
Related names
Elvan · Granite · Sedimentary
Sources
- Royal Geological Society-era primary source (MacCulloch) — Historical usage
- Poldark Mine geology glossary — Local usage and etymological uncertainty
- Historical English Dictionary: killas — Early attestations