Mylonite
Fine-grained foliated rock produced by intense deformation, predominantly ductile crystal-plastic flow and recrystallisation.
Language and route: English learned geological term from Greek μύλων (mýlōn), mill or mill-house, via Lapworth’s Scottish tectonic terminology. English learned geological term from Greek μύλων (mýlōn), mill or mill-house, via Lapworth’s Scottish tectonic terminology
Literal gloss: milled rock
The name and its history
Charles Lapworth · 1885 in north-west Scotland
- What does the word literally mean?
- Immediate linguistic pathway: English learned geological term from Greek μύλων (mýlōn), mill or mill-house, via Lapworth’s Scottish tectonic terminology. Relevant original form: μύλων (mýlōn); conventional gloss ‘milled rock’. The linguistic gloss should not be mistaken for the original scientist’s stated motive unless directly demonstrated.
- What did the original author mean?
- Lapworth described very fine, layered rocks at the Moine Thrust as if intensely milled, challenging their prior interpretation as original sedimentary layers.
- Why was the name needed?
- The apparent inversion of geological strata forced him to explain mechanically how old rocks had moved over younger rocks.
- What evidence supported the original concept?
- Grain-size reduction, foliation, tectonic contacts, repeated strata and field relations; the mill analogy was morphological and mechanical.
- How has geology changed the concept?
- Modern mylonite generally records crystal-plastic deformation, not simple brittle grinding; grain-size reduction need not mean literal milling into powder.
Further historical context
Lapworth described very fine, layered rocks at the Moine Thrust as if intensely milled, challenging their prior interpretation as original sedimentary layers.
The apparent inversion of geological strata forced him to explain mechanically how old rocks had moved over younger rocks.
Grain-size reduction, foliation, tectonic contacts, repeated strata and field relations; the mill analogy was morphological and mechanical.
Modern mylonite generally records crystal-plastic deformation, not simple brittle grinding; grain-size reduction need not mean literal milling into powder.
Editorial qualification
Lapworth 1885 is supported by BGS retrospective; exact original quotation not reproduced.
Related names
Foliation · Gneiss · Phyllonite · Cataclasite
Sources
- BGS geological conservation review, Lapworth and mylonite — Historical account identifying Lapworth 1885 and the Scottish thrust controversy
- BGS Loch Eriboll field guide — origin of thrust and mylonite — Historic sequence; Geikie thrust 1884, Lapworth mylonite 1885
- BGS Northern Highlands: mylonite definition and field evidence — Distinguishes original mill analogy from predominantly ductile recrystallisation