THE DISENTEGRATION OF GRANITE IN EGYPT 391 



The maximum rate, shown by the Granite Sanctuary, would seem 

 to be about 1 cm. in two thousand years, and the minimum rate 

 would seem to be so low that the effects are not apparent in three 

 thousand years. In addition to this variation in the- rate of dis- 

 integration apparently corresponding to a variation in the conditions 

 to which the granite is exposed, there is apparently also a variation 

 depending upon the orientation of the disintegrating surface in 

 reference to certain directions within the rock, possibly the rift and 

 the grain, or possibly a faint schistosity which is almost universally 

 present in the Syene granite. 



THE CAUSES OF THE DISINTEGRATION 



The conventional explanation of disintegration in a region of 

 desert climate like that of Egypt is that the disintegration results 

 from the racking to pieces of the rock through the contraction and 

 expansion consequent in the high-temperature ranges. In the 

 case of Egypt, there would, however, seem to be serious objections 

 to this explanation, although some disintegration undoubtedly 

 does take place in that manner. The first objection is that, 

 although the temperature range is of the same magnitude at both 

 Aswan and at the pyramids of Gizeh, the rate of exfoliation is very 

 much less at the former place than at the latter, and furthermore, 

 that, although the statues in the temples are exposed in many cases 

 only to very low temperature ranges, the rate of exfoliation in 

 many of these cases is of the same magnitude as that at the pyramids 

 of Gizeh. The second objection is that the massive granular dis- 

 integration of the Aswan region penetrates to a greater depth than 

 appreciable temperature changes can be expected to extend. The 

 depth of the zone of warming at midday in desert regions is given 

 by Walther as the result of many observations as only about 19 cm. 

 The annual temperature variation is said by Sir William Thompson 

 to be reduced at a depth of 8 m. (25 ft.) to one-twentieth of its 

 superficial amount. The mean annual temperature range in 

 Egypt is less than 20 C and, at the depths to which disintegration 

 can be seen to have penetrated at Aswan, 3 to 4 meters, must be 

 reduced to amounts which are essentially negligible, especially 

 since the period of the range is so long. The mean monthly range 



