268 F. W. Mueller — Rocks from Arabia Petrcea. 



II. Bed Granitoid Bock, with Micropegmatitic Structure, from Jebel 



Watiyeh. 



This rock is an intimate aggregate of flesh-red felspar and quartz, 

 with grains of vitreous quartz and crystals of felspar scattered 

 through the ground-mass. It may be regarded as a fine-grained 

 binary granite, almost a microgranite, with porphyritic quartz and 

 felspar ; thus suggesting a transition to a coarse kind of quartz-felsite 

 with a ground-mass of phanero-crystalline texture. The felspar is 

 highly charged with reddish-brown granules, perhaps kaolin with 

 limonite, which by their comparative opacity render the section very 

 nebulous. The felspar is presumably a red orthoclase, and is asso- 

 ciated with large crystalline grains of clear quartz with numerous 

 pores. In places the felspar and quartz of the ground-mass are so 

 intimately united as to form a beautiful micropegmatite or grano- 

 phyre. The association naturally suggests contemporaneous, and 

 probably rapid, solidification of the two minerals. In some cases 

 the formation of the micropegmatite has started from a pre-existing 

 crystal of felspar, whence the interblended felspar and quartz diverge 

 in plumose forms. This is the rock referred to by Prof. A. Geikie 

 in his "Text-book of Geology," second edition, 1885, p. 635, foot- 

 note 3. 



The rock from Jebel Musa, No. 1, in the Appendix to the Memoir, 

 described as a gneiss, seems rather to be a fine-grained granitoid rock. 

 The faint tendency to foliation, suggested by a small specimen, is 

 illusory. The rock, though diverging greatly from the normal 

 granitic type, and presenting rather a granulitic texture, may pro- 

 bably be best described as a hornblende-granite. 



III. Bed Porpky rite from Jebel Mnsa. 



This rock presents a compact base of deep tile-red colour, with 

 subconchoidal fracture, in which are disseminated porphyritic crystals 

 of red felspar and irregular dark green patches of an altered mineral, 

 apparently a mica. Some of the large felspars reach a size of 4 mm. 

 X 1*5 mm. Even in thin section they present a strong reddish 

 colour, which is deepest around the inner border of each crystal, where 

 the granules of ferric oxide are accumulated. Some of the crystals 

 seem to be orthoclase, but others show indistinct polysynthetic twin- 

 ning, with broad lamellation. Zonal structure is presented by some 

 of the crystals. The other porphyritic constituents are green crystals, 

 measuring about 1-25 mm. x 1 mm. These seem to be pseudomorphs 

 after biotite ; at least such an origin is suggested by their strongly- 

 marked strias, which seem to represent the basal cleavage of a mica, 

 by the occasional curvature of the laminae, and by the hexagonal 

 form of some of the sections. This green epigenetic mineral is 

 strongly dichroic, and is flecked with an opaque oxide of iron, pro- 

 bably magnetite. The slide also carries some irregular patches of 

 chlorite. Apatite is present in rather stout six-sided prisms, both in 

 the felspars and in the altered mica. Epidote occurs in aggregates 

 of greenish-yellow grains, inclosed in both the porphyritic minerals. 

 Quartz may be detected in interstitial spaces, but is apparently of 



