270 F. W. Rudler — Rocks from Arabia Petrcea. 



dominate in some of the sections, and the pinacoids in others. The 

 characteristic cleavage parallel to {110} is well displayed. Some 

 sections of rhombic shape, cut approximately in the plane of sym- 

 metry (010), give extinction angles of nearly 40°. 



Two sets of felspars may be noted. Those of the first consolida- 

 tion occur in groups of large crystals, some of which give sections 

 reaching a size of 2 mm. x 0*6 mm., three or four such crystals 

 forming a cluster. All the felspar is profoundly altered. By reflected 

 light it presents a dead white colour, but by transmitted light a red- 

 dish tint. Notwithstanding its advanced state of decomposition, the 

 repeated twinning of a plagioclase may be detected, and the wide 

 extinction angles of the broad lamella? suggest a felspar of high 

 basicity. The small felspars of later consolidation occur in crowds 

 of slender interlacing crystals, with irregularly-terminated lath- 

 shaped sections, measuring on an average 0-16 X 0-04 mm. The 

 meshes of this reticulating mass of microlites are occupied by a 

 green decomposition product, of fibrous and tufted structure, with 

 strong depolarizing action between crossed Nicols. There are also 

 large irregular patches of a green mineral, chloritic or serpentinous, 

 following irregular cracks in the rock, and associated with calciteand 

 epidote. The calcite also forms large crystalline masses and ill- 

 defined patches, with aggregate polarization. Olivine appears to be 

 represented by crystals which have suffered much alteration, and are 

 mostly converted into green pseudomorphs with a brown margin. 

 Magnetite is disseminated through the rock, and red and brown 

 oxides of iron occur in patches. 



It does not appear easy to bestow upon this rock an appropriate 

 name. It is very similar to the rock No. 13, from Jebel esh Shomreh, 

 described in the Appendix to the Memoir as a hornblende-augite 

 andesite, but I am anxious to withdraw that name, as the rock is 

 decidedly basic. Notwithstanding the large proportion of horn- 

 blende, the affinities of the rock appear to lie rather with the 

 dolerites and diabases. Its structure is in no way related to that of 

 the diorites. With a little more olivine and a little less felspar it 

 might perhaps claim a place among the hornblende-picrites, which 

 have been so well described by Professor Bonney, D.Sc, F.B.S. 



VI. Dolerite from Gehat es Shomrah. 



A dark-brown, nearly black, rather fine-grained rock, with large 

 porphyritic crystals of greyish felspar with greasy lustre. The 

 ground-mass, viewed in the microscope, consists mainly of a confused 

 network of felspars, yielding lath-shaped sections, mostly binary 

 twins, with an average size of 0-03 mm. x 0*007 mm. The augite 

 occurs in crystalline grains and small crystalline masses, of brown 

 colour, occupying the spaces between the felspars, and evidently of 

 later consolidation. Indeed, the penetration of the augite by the 

 felspar gives rise to an indistinct ophitic structure. Olivine is 

 present, partly serpentinized, in the form of amber-coloured grains, 

 with here and there a small crystal. One well-marked crystal, of 

 irregular six-sided shape, measured 0-8x0-3 mm.; like the other 



