Basalt scoria from Sannome-gatau sually shows a
porphyritic appearance due to the presence of abundant
quartz and feldspar xenocrysts derived from
granitic basement rocks. Under the microscope,the
commonest phenocrysts are olivines,which occupy
less than 5% by volume, and range up to 0.8 mm in length but are mostly less than 0.4 mm. They are euhedral tabular or elongated parallel to the c
crystallographic axis