THE DISINTEGRATION OF GRANITE IN EGYPT 385 



to Aswan. The disintegration in each of the cases is approximately 

 at the elevation of that on El Hesa. It is a distinctly noticeable 

 fact that in many of these cases, as for instance in the case of the 

 grave on El Hesa cut 4 m. back into a northerly facing 40 slope 

 lying at the foot of a cliff about 25 m. high, the disintegration has 

 penetrated to a depth of at least 3 to 4 m. in spite of the fact that 

 direct isolation is received only during the summer and then only 

 at a low angle. 



Disintegration taking place under the present conditions is 

 abundantly shown by most of the exposed ledges and loose blocks 

 of the region and is manifested in three ways: (1) Surfaces which 

 have been exposed for a relatively short while show a slight rough- 

 ening. Individual grains and fragments of feldspar and of quartz 

 become loosened and are removed. (2) Surfaces which have been 

 exposed for a longer time show in addition exfoliation of thin 

 superficial layers commonly of about two-thirds of a centimeter in 

 thickness. Cross-sections afforded by broken blocks show that 

 megascopically noticeable incipient exfoliation has penetrated to 

 a depth of 10 to 15 cm. from the surface. (3) Disintegration takes 

 place also by the spalling and splitting of large blocks and frag- 

 ments, but the amount of disintegration taking place in this 

 manner in the Aswan region is not very great. Of these three 

 methods of disintegration that by exfoliation is by far the more 

 important. In the excavation for the dam and navigation canal, 

 concentric disintegration and decomposition were found to have 

 penetrated to a depth of several meters below the high Nile 

 level and are probably to be considered as going on at the 

 present. 



The chief granite of the Aswan region is the famous Syene red 

 granite, a coarse red porphyritic granite composed chiefly of large 

 phenocrysts of orthoclase. Where the joints are comparatively 

 far apart, its outcrops under the effect of the concentric exfoliation 

 of the joint blocks resemble huge piles of bowlders. Where the 

 jointing is more pronounced and the joint blocks of much smaller 

 size, the concentric exfoliation is much less in evidence and the 

 outcrops are composed of, and surrounded by, detriial masses of 

 angular and subangular blocks and in appearance are very similar 



