444 



Scientific Proceedings, Royal Dublin Society. 



and on the other with diorites. The syenitic Lamprophyres he 

 separates into two groups, the Minettes and the Yogesites, whilst 

 the dioritic are divided into Kersantites and Camptonites : — 



Lamprophyre 1 



Syenitic 



Dioritic 



orthoclase + biotite : = Minctte 



hornblende 



orthoclase + or = Vogesite 



augite 



plagioclase + biotite = Kersantitc 



hornblende 

 plagioclase+ or = Caniptonite. 



augite 



The dominant felspar in our rock appears to be an orthoclase. 

 Hence, as hornblende is the dominant ferro-magnesian constituent, 

 the rock is a Yogesite ; on the other hand, a dark mica (biotite) is 

 also present, but in minor quantity. The rock must, therefore, be 

 allotted an intermediate place between the two types, and receive 

 the name " mica-bearing Yogesite." 



It is interesting to note that Professor Bonney 2 who micro- 

 scopically examined the series of rocks brought from Upper Egj^pt 

 by Sir William Dawson, found amongst them a holo-crystalline 

 trap, which he has described as a quartziferons Kersantite. The 

 constituents are given as biotite, hornblende, felspar (commonly 

 plagioclase), quartz, apatite, iron ores, and sphene; the locality 

 being Biggeh (Assouan). 



The distance between the cataract at Wady-Halfa (the 

 second) and that at Assouan (the first) is about 214 miles. 

 As the average fall of the Nile between these two points is, 

 according to L. Horner, 3 not 9 inches in a mile, it is not to be 

 expected that much coarse gravel can be carried forward. The 

 greater portion of the heavier detritus thus falls down in the 

 higher parts of Upper Egypt. It is a well-known fact that the 



1 Biotite is used as synonymous with dark mica. 



2 Notes on the Microscopic Structure of some Rocks from the Neighbourhood of 

 Assouan, &c, Geol. Mag., 1886, vol. iii., p. 106. 



3 An Account of some Recent Researches near Cairo, undertaken with a view of 

 throwing light upon the Geol. History of the Alluvial Land of Egypt. (Philosophical 

 Transactions of the Roy. Soc. Lon., 1855, vol. cxlv., p. 117.) 



