Professor Bonney has described, in the British Association Report for 1885, the rock specimens brought from Kilima-njaro by Mr. H. Johnston, and has called attention to the peculiar form of the felspar crystals which are porphyritieally developed in a rock referred by him to augite-andesite. It is at his suggestion that a more detailed account of these crystals is here attempted.

They are of a brown to yellowish-brown colour, full of cavities, and considerably weathered, and are so much rounded that it is impossible to determine their nature with certainty by means of the hand goniometer. They sometimes attain a length of 85 mm.