70 



DR. J. BALL ON THE 



[Feb. 1903, 



action of the stream in flood. Frequently a number of potholes 

 cut one into the other, and masses of rock are thus detached which 

 fall into the cavities arid continue the grinding. The rocky bar is 

 of course quite bare of debris, except for the boulders lodged in the 

 numerous potholes and hollows ; and even where the rock is 

 untouched by the annual flood, it is of marked freshness, though 



Fig. 2. — Sketch showing poiholincj of the gneiss-barrier in the Nil<\ 

 Semna Cataract. 



covered with separated blocks and flakes. Here, as in all arid 

 regions, the process of weathering is mainly one of disintegration 

 without decomposition, the great diurnal variation of temperature 

 being the principal agent in the process. A small amount of 

 chemical decomposition certainly goes on, but its products are 

 removed by water or the sand-blast with great rapidity. The specific 

 gravity of the gneiss is 2*61. 



A microscopic examination of the rock shows it to be a highly- 

 crushed biotite-granite, consisting almost entirely of quartz, ortho- 

 clase, and biotite, with very minute amounts of apatite, sphene, 

 and iron-oxides. The usual evidences of crushing, in the shape of 

 parallel disposition of the constituents, cracked crystals, cataclastic 

 structure, and undulose extinction, are strongly marked. The 

 variations of colour in the mass are due to variations in the relative 

 abundance of pink orthoclase and dark biotite. The gneiss extends 

 for some distance on either side of the barrier, but here appears to 

 have been more highly crushed, and thus more easily eroded, than 

 the band which forms the barrier itself. 



