F. W. Riidler — Rocks from Arabia Petrma. 269 



secondary origin. Probably the history of the rock has been some- 

 what of the following character : — The apatite, the biotite, the large 

 felspars and the ground-mass successively solidified in the order here 

 enumerated ; the mica was afterwards transformed into a chloritic 

 mineral, with separation of magnetite, while further alteration of the 

 chlorite resulted in the formation of epidote, which may also have 

 partly resulted from the decomposition of the felspar, inasmuch as it 

 occurs in the heart of some of the larger felspar crystals. 



IV. Felsitic Tuff from Es Sliomrah. 



This rock appears macroscopically to be a porphyritic felsite, with 

 compact chocolate-coloured base and disseminated crystals of white 

 felspar. Under the microscope, however, it is seen to contain 

 angular fragments of various rocks, with broken crystals of plagio- 

 clase, forming a bi'ecciated mass. Mr. F. Eutley, F.G.S., to whom 



1 am indebted for having examined some of my sections, points out 

 the general similarity of the ground-mass to that of the rhyolitic 

 rock from Pont-y- Groin lech, which he described some years ago ; 

 but at the same time inclines to the view that in the present case 

 the ground-mass consists of fine dust, in great part felspathic, 

 perhaps associated with shreds of devitrified lava. Among the im- 

 bedded fragments, which vary in diameter from about 0*03 mm. to 



2 mm., are rhyolites, or devitrified rocks showing banded and 

 fluxion structure. Some of the fragments and imperfect crystals of 

 felspar reach a size of 2-25 x 1*3 mm., and though mealy in texture, 

 show traces of repeated twinning. 



V. Hornblende-augite BocTcfrom Es Safeh. 



A coarsely crystalline, almost black rock, weathering with a rusty 

 surface. Thin sections show large porphyritic crystals of brownish- 

 green hornblende, colourless augite, and decomposed felspar, im- 

 bedded in a pale brownish ground-mass of felspathic microlites, 

 with green products of decomposition. Some of the crystals of horn- 

 blende measure, in section, as much as 3 mm. x 1*2 mm. Most of 

 them are much fissured and carious, inclosing green alteration- 

 products, and having rather indistinct outlines, bordered by dark 

 greenish granules. The best-defined sections are those cut approxi- 

 mately normal to the perpendicular axis. These show the common 

 six-sided forms, bounded by the edges of {110} {010}, with inter- 

 secting lines of prismatic cleavage breaking up the section into 

 characteristic rhombs. Elongated sections parallel to the vertical 

 axis give longitudinal cleavage lines, and the extinction-angles in 

 relation to these lines vary from 0° in some of the sections which 

 are cut parallel to {100}, to a maximum of about 15°. The pleo- 

 chroism is not very marked : a = pale yellowish brown, ft = pale 

 bottle green, c = olive green. Much of the hornblende is twinned. 



The augite appears in very sharply-defined crystals, which in thin 

 sections are quite clear and colourless. Most of the sections are 

 eight-sided, being limited by edges of {110} {100} {010}. Some 

 of these measure nearly 1 mm. in the edge. The prismatic planes 



