Extract

This paper re-interprets certain exposures previously mapped as Lower Old Red Sandstone, which, together with new exposures, constitute an extension of the local Silurian succession; and briefly describes, in the light of more recent faunal knowledge, that part of the Silurian succession already known to be present.

The Craighead Inlier is a lenticular mass of Lower Palaeozoic rocks in the form of an anticline pitching to the north-east. It is bounded on the south-east by the Kerse Loch Fault and on the north-west by an unconformable contact with the Lower Old Red Sandstone. The Ordovician rocks are therefore confined to the core of the anticline in the south-west end of the inlier. The uppermost beds of the Ordovician, the Drummuck Group, have already been described by A. Lamont (1935).

The Silurian rests with unconformity on the Drummuck Group, the basal bed being a purple conglomerate containing pebbles of jasper, black chert, and spilite. This conglomerate, together with the green and buff sandstones above, constitutes the lower part of the Mulloch Hill Group. The green and buff sandstones are calcareous, and contain an abundant fauna, chiefly of brachiopods but with some trilobites, especially in the upper buff sandstones. The lower green sandstones are characterised by the brachiopods Dalmanella biconvexa Williams, Cryptothyrella angustifrons (M’Coy), Stropheodonta mullochensis Reed and others all typical of A1, A2, A3, and possibly A4 of the Llandoverian at Llandovery (Jones, 1925, 1949; Williams, 1951) (See table below).

The trilobites Calymene blumenbachi, and Phacops sp. are found in the

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