Petrographical Notes on the Geological Collection from the south Central Sahara made by Mr. Francis R. Rodd

Abstract

There are four volcanic rocks in the collection, two basalts and two trachytes. They all come from the volcanic region north and north-east of Auderas (Central Air), 1 which, from Mr. Rodd9s mention of ‘isolated volcanic domes’ and ‘conical peaks’, appears to be of Recent, or, at the earliest, of Kainozoic age. The two basalts are labelled ‘Tighummar’, a locality about 14 miles north of Auderas on Rodd9s map. One specimen is of the porous character and pale-grey tint so frequent in Kainozoic and Recent basalts, and shows small phenocrysts of olivine and augite, the former weathering into yellow limonitic spots. The other rock is darker and more slag9gy in appearance, and is devoid of determinable phenocrysts. In thin section the pale-grey rock proves to be an olivinebasalt, with sparse and small phenocrysts of almost colourless augite, sometimes showing a faint purplish tinge, and olivines with thick investments of red iron-ore. The olivine forms some phenocrysts as big as the pyroxenes, but all transitions in size can be traced down to numerous, small, hæmatized crystals which form part of the ground-mass. The ground-mass consists of plagioclaselaths and microlites (labradorite, Ab₂An₃), granular augite, and magnetite-cubes, together with the above-mentioned olivines. There is a small amount of turbid isotropic base. The felspars show well-marked flow orientation, and range in size from microlites to microphenocrysts. The second rock approaches the ultrabasic basalts in composition. It shows numerous small phenocrysts of fresh olivine and augite. The latter is dictinctly publish, but many sections have