442 Scientific Proceedings, Royal Dublin Society. 



The lower beds of the nunimulitic limestone seem to be largely 

 the home of the cherts, jaspers, and agates. (Russegger and 

 Newbold.) 



The igneous rock, to which I have referred above, is dioritic in 

 appearance, and is found, upon microscopic examination, to be a 

 Lamprophyre. It probably comes from an area of crystalline 

 rocks, similar to those which make their appearance, for instance, 

 near the First Cataract (Assouan), where " we pass from the 

 unaltered and nearly flat Nubian sandstones to rocks ■ highly 

 crystalline, greatly disturbed and penetrated with multitudes of 

 igneous veins." ' The latter are dykes of granite, felsite, diorite, 

 and basalt. Sir W. Dawson observed that one of the diorites 

 exhibited a porphyritic character, induced by crystalline patches 

 of white felspar. Now, the rock I have before me has a tendency 

 to develope the same structure — the porphyritic constituent being 

 a pinkish felspar. Still, the structure is generally evenly crystal- 

 line, with a grain which allows the constituent minerals to be 

 recognised with the aid of the lens. The colour of the rock is 

 very dark green. 



Under the microscope the felspar is seen to be mostly an un- 

 striated variety. Striated felspar is very rare, but the striae are 

 always very fine. The non-lamellated felspar does not always give 

 straight extinction. In section this constituent is lath- shaped, 

 being from 3 to 5 times as long as broad. The outline is gene- 

 rally well-defined. Still, at times it is irregular in appearance. 

 It is usually considerably altered ; but the decomposition is always 

 confined to the centre, and a thin external zone then remains quite 

 fresh and adular-like in appearance. This would lead us to expect 

 some difference in the chemical constitution of the two zones. The 

 alteration consists in the formation of cloudy granular products, 

 some of which belong to the mineral epidote, while others appear 

 to be micaceous in character. There is, unfortunately, not suffi- 

 cient material for a chemical analysis ; so no definite conclusions 

 can be drawn as to the nature of the unstriated felspar. Where 

 straight extinction occurs, it is, no doubt, an orthoclase ; but 



1 Dawson, I. c, p. 439 ; cf. also Lord Talbot de Malahide, Notes on the Geology of 

 Egypt, Journ. of the Geol. Soc. of Dublin, v. 1850-53, pp. 277-279 ; and C. B. Klun- 

 zinger, Upper Egypt, &e., London (Notes on Eocks of Desert Mts., pp. 229-235.) 



