12 Transactions of the South African Philosophical Society. 



7,640 feet. The peak is a cone of black and red scoriae, with a 

 crater lake on the top ; the diameter of the crater is about a quarter 

 of a mile. From the coast other eminences of less height are 

 visible on the plateau that forms the centre of the island. These 

 hills are very probably also secondary cones of eruption ; several of 

 them, like the central peak, have crater lakes. 



The cliffs are formed of nearly horizontal beds of basalt, alternately 

 compact and scoriaceous, with intercalated layers of reddish volcanic 

 tuffs. The whole system of beds slopes slightly towards the shore, 

 as^can be seen to the east and west of the harbour. These beds are 

 traversed by dykes, generally vertical and of no great thickness. 

 Torrents and atmospheric erosion have worn gullies in these walls 

 of rock, and heaped together piles of debris, which have accumulated 

 to a height of 100 feet at the foot of the cliffs. This circle of 

 volcanic fragments is, in turn, edged by a belt of gravel of the same 

 nature, which is spread out on the narrow shore of the island. 



For nine months of the year terrible tempests run riot on the 

 island, and when the season of rains has ended, and the snow that 

 has accumulated on the top of the peak begins to melt, the water 

 rushes down in cascades carrying an immense quantity of debris. 

 These streams vigorously attack and demolish the less coherent and 

 homogeneous of the layers that form the horizontal strata ; they lay 

 bare the rocks of the dykes, and cut deep indentations in the ledge 

 of the terrace. The transverse dykes alone resist the erosion and 

 stand up like walls. 



The rocks were found by Mr. Buchanan to consist of basaltic 

 lavas containing porphyritic crystals of augite, plagioclase, mica, 

 titanic or magnetic iron, and in certain cases olivine. The ground- 

 mass was found to consist of microliths of the same species, especially 

 augite and felspar ; between these small crystals lay a vitreous base 

 which plays a wholly subordinate part. At certain points a yellowish 

 limonitic matter has been deposited as concretionary masses in the 

 pores. 



Above the basalt comes tuffs, the transition being effected through 

 rocks that are richer in glassyjxiaterials, but belong, nevertheless, to 

 the same lithological type. 



The tuffs covering the sheets are formed of fragments in which 

 the vitreous element predominates ; they appear, under the micro- 

 scope, to consist of a vesicular yellowish or brownish glass, passing 

 occasionally into the hydrated, reddish, resinoid product of decompo- 

 sition of certain basic volcanic glasses. The crystals that separate 

 out from these vitreous fragments belong chiefly to greenish pleochroic 

 augite, and are generally irregular in contour. The preparations 



