BANDED ANDESITE.
Gebel Sufra.
VEINED KERSANTITE.
Gebel Fereyid.
NATURAL SIZE.
In places, the rock of Gebel Zergat Naam takes on a finer grain and a greyer aspect in the mass. A slide cut from this variety [11,512] shows essentially the same composition, but the felspars are here porphyritic in a crystalline ground mass of felspar and hornblende. The rock thus passes into syenite-porphyry. The porphyritic felspar crystals show the same lamellar structure as those in the more coarsely crystalline rock.
Trachyte.
Fig. 17.—Trachyte, from a dyke at Gebel Kahfa [11,537 A] × 10. A clear crystal of orthoclase felspar is seen in the centre of the field, surrounded by a finely crystalline ground mass of hornblende and felspar.
Trachyte, the volcanic representative of syenite, occurs in dykes [11,537 A] seaming the granite of Gebel Kahfa. It is a light grey rock, of very fine grain, breaking with a rough surface, containing pores and small stumpy white pearly-looking porphyritic crystals of orthoclase (see [Plate XXIII]). The sp. gr. is 2·56. The microscope shows the ground mass surrounding the porphyritic idiomorphic orthoclase crystals to be a holocrystalline mixture of hornblende and felspar. The felspars of the ground mass are usually in more elongated forms than the porphyritic crystals, and appear to be partly plagioclase. The hornblende, in small and very irregular crystals of dark olive-green colour, often clouded and dirty looking, is scattered plentifully among the feslpars of the ground mass. Accessory primary minerals appear to be entirely absent.
A rock which occurs in a hill at the head of Wadi Amba-ut [10,375] appears to be essentially similar to the above, but the porphyritic orthoclases are more numerous and the ground mass contains small quantities of augite and magnetite. This rock is in a highly altered condition, the felspars being full of kaolin and epidote, while the hornblende, which is here of a paler colour than in the trachyte of Gebel Kahfa, is highly chloritised.