[Illustration: Fig. 22.—Mica diorite, from a dyke at Gebel Abu Hegilig [10,391], × 17. _f_, felspar (mainly plagioclase); _b_, biotite altering with formation of limonite; _h_, hornblende; _ap_, apatite; _m_, magnetite.] Most of the diorites of South-Eastern Egypt contain little or no biotite as an accessory constituent. An exception occurs, however, in a great dyke [10,391] of very fine grained diorite of sp. gr. 2·87, which traverses the granite of Gebel Abu Hegilig, and which contains more biotite than hornblende. The dyke is so decomposed that it is difficult to get a coherent hand specimen; but a slide cut from one of the less altered portions shows the rock to be a very fine grained holocrystalline one, made up of plagioclase, biotite, hornblende, apatite and magnetite, with abundant alteration products such as epidote, kaolin, and chlorite. All the minerals, except the apatite and some of the iron oxides, are allotriomorphic. The felspars are very much altered, but appear to be mainly plagioclase. The biotite is in little brown ragged-looking plates, strongly pleochroic, frequently altered with separation of flakes of limonite. The hornblende is green, in small and very irregular-shaped crystals, which show very little trace of cleavage and are frequently chloritised. The apatite is in long hexagonal clear prisms. Iron oxides, sometimes showing square or hexagonal outlines, and epidote in granules, are liberally scattered through the rock. From the abundance of biotite and the fine grain and manner of occurrence of this rock it was taken in the field for a decomposed lamprophyre; but the entire absence of idiomorphism in the ferro-magnesian minerals show that it should rather be placed with the diorites. =Diorite-porphyrite.= Rocks which may be somewhat doubtfully classed as altered diorite-porphyrites occur at Gebel Abu Hodeid as well as near the ruins of Um Eleiga and at Gebel Um Heshenib. [Illustration: Fig. 23.—Diorite-porphyrite, Gebel Abu Hodeid [12,143], × 40. _f_, porphyritic felspar (plagioclase); _h_, hornblende; _b_, biotite; _g_, ground mass, consisting chiefly of plagioclase and hornblende.]