Killas

Killas is a Cornish mining term for metamorphic rock strata of sedimentary origin which was altered regionally by the Variscan orogeny and then locally by heat from the intruded granites in the English counties of Devon and Cornwall.

The sediments are not evenly spread over the county, with the Carboniferous beds only found in the north of Cornwall.

The depositional environments of the killas were very varied, as is revealed by the fossil content and the sedimentary sequences.

The deformed brachiopod fossil Cyrtospirifer verneuili,[2] known to quarrymen as the Delabole Butterfly, was found in the upper Devonian beds of North Cornwall.

Killas underlies two thirds of Cornwall and on and around Dartmoor in Devon, and, as a result of hydrothermal mineralization from the granites, contain the majority of the mineral lodes or veins which at one time provided up to half of the world's tin and copper, and economic prosperity to Cornwall (and to a lesser extent Devon).