THE DISINTEGRATION OF GRANITE IN EGYPT 383 



coarse. Syene red granite with the base of the overlying Nubian 

 sandstone (Cretaceous) (Fig. 1). They form a zone of what Ball 

 designates as "broken-down granite, a kaolinic mass with quartz 

 grains." The zone is 1 to 1^ m. in thickness, has a relatively 

 sharp even contact with the overlying sandstone and conglomerate, 

 but below grades through less and less disintegrated granite into 

 comparatively unaltered rock. The upper part of the zone is 

 composed of material that has suffered slight rearrangement, but 

 the middle and lower portions consist of the untransported debris 

 of disintegration. The feldspar of the upper portion is almost 

 completely kaolinized. In the middle portion, the kaolinization 



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Fig. i. — Diagrammatic section across the region of the Aswan Cataract. 1, Dis- 

 integrated and decomposed zone at the base of the Nubian Sandstone. 2, The massive 

 granular disintegration. 3, Disintegration of the present. 



is much less and in the lower portion it is not megascopically 

 noticeable. Although the disintegration was seemingly not of the 

 exfoliation type, 1 it nevertheless took place roughly parallel to the 

 very level upper surface of the granite. The surface is so level as to 

 suggest a peneplain surface. The disintegration would seem to have 

 taken place contemporaneously with or immediately preceding the 

 deposition of the Nubian sandstone, and the kaolinization may 



1 According to the present use of the term, it is possible to distinguish several 

 types of disintegration. The term is applied in some cases to the breaking up of a 

 rock-mass into blocks and in other cases to the breaking up of a rock-mass through 

 the loss of cohesion between the constituent grains. The former process may be 

 termed "block-disintegration" and the latter, "granular-disintegration." Allied 

 to the block disintegration is what may be termed the "exfoliation" type of dis- 

 integration in which thin plates of rock rift off parallel to the surface of a ledge or 

 block. The process seems to involve considerable loss of cohesion between the grains 

 and readily goes over into granular disintegration. The term disintegration is also 

 used in a loose sense to denote the chemical breaking up of the rock-mass. But as 

 Merrill advises, it would seem better exclusively to use for that process the term 

 "decomposition" and to reserve the term "disintegration" for the process of mechan- 

 ical breaking up. 



