Andesite at Slag Hill consists of 70% dark brown volcanic glass with varied degrees of trachytic texture in the plagioclastic matrix and less than 5% of the andesite comprises vesicular textures. Plagioclase, hornblende and augite are mostly in form as phenocrysts and comprise 1% to 10% of the andesite. Orthoclase crystals are found occasionally and they likely represent xenocrysts.