2.3 Group III and IIIa

Keiller et al. describe IPG Group III as:

'Hornblende, feldspar and decomposed ilmenite. Green hornblende forms large plates of needles roughly parallel to each other. Ilmenite as scattered grains, decomposed to leucoxene. Few stout needles of apatite are also present.' (Keiller et al. 1941, 63).

Stone and Wallis (1951) introduced IPG Group IIIa as having amounts of brown mica and epidote in addition to the mineralogy found in IPG Group III.

Thin-sections from eleven of the twenty axes assigned Group III and two thin-sections assigned to Group IIIa were examined. As for Group I, petrographic differences were found in the thirteen thin-sections, allowing six sub-groups to be recognised. It is suspected from the work carried out by Keiller, Piggott and Wallis that Group III was founded on axe county reference Wiltshire 4 (Wi4/4), as this is the thin-section they compare to a thin-section from a quarry near Perranuthnoe (Keiller et al. 1941). This quarry is not precisely located but suspected to be a disused quarry approximately 1km south-east of Perranuthnoe Church. The resulting petrological match between the two thin-sections allowed the authors to deduce that this exposure was the probable material source for IPG Group III.

Group III Petrography


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