Minor intrusions in the Pentland Hills

Abstract

The field relations are described of a deuterically altered suite of Lower Old Red Sandstone sills and dykes intruded into the Silurian and Lower O.R.S. sediments of the Pentland Hills. The least altered of these intrusions are sills exposed at Fairliehope Farm and in the River North Esk. They are sparingly porphyritic rocks carrying completely chloritised olivine and less abundant chloritised phenocrysts of other mafic minerals. In the groundmass partly albitised andesine crystals give the rock a criss-cross texture; some unaltered augite remains, locally with sub-ophitic relationships. Interstitial chlorite is common, accompanied by a little quartz and albite-oligoclase. The effects of chloritisation, albitisation and carbonation are traced through the more altered sills and dykes. Comparison is made with related intrusions in Ayrshire. Two chemical analyses are recorded. In the light of the evidence now available it is concluded that the rocks described are likely to have been olivine-bearing andesine-dolerites originally, the dykes having been somewhat poorer in olivine than the sills.