Avon

At least 79 small diatremes and dykes occur in the counties of St Genevieve and St. Francois. The majority are of peridotite with pyroxene and phlogopite but one pipe 1.6 km north of Avon consists of alnoite. The pipe is about 60 m in diameter and cuts Bonneterre dolomite, which is metamorphosed for 6-7 m from the contact. The rock of the pipe consists of up to 50% olivine phenocrysts, largely serpentinized, phlogopite, abundant melilite, and accessory apatite, magnetite, chromite and perovskite with secondary carbonate.