Perlitic rock covers several square kilometres on the upper slopes of Terrace Mountain. It is grey on fresh surfaces, alters to light brown and is characterized by fractured, unaltered plagioclase phenocrysts (2 to 7 mm in diameter), comprising about 15 per cent of the rock. Accessory biotite, usually less than 3 millimetres in size is also set in a greenish grey glassy matrix. Anhedral olivine crystals less than 0.5 millimetres across, iron oxide (0.5 mm) and pyrite (0.5 mm) are trace components disseminated through the glass matrix. Perlitic onion skin and arcuate fractures are visible throughout the glass, which is, in places, partially converted to palagonite. The degree of palagonitization varies through the deposit, but is generally minor.