3Q2 DONALD C. BARTON 



is only 24 C. and at those depths must be even more seriously 

 reduced in amount. The diurnal temperature range, furthermore, 

 should be entirely absent at those depths, especially on slopes such 

 as those in which many of the graves on El Hesa are cut, where 

 direct insolation is received only during the summer and then at 

 a low angle. Granite itself has a low coefficient of conductivity 

 and that of dry granitic sand must be much lower; it would there- 

 fore not seem surprising that a blanket of several feet of disinte- 

 grated granite is found to be an effective insulating agent for the 

 fresh rock beneath. 



At Aswan and at the pyramids of Gizeh, the only factor by 

 which the conditions of exposure of the exfoliating rock differ is 

 in the humidity. At Aswan there is no rainfall, there is only a light 

 dewfall at night, and the relative humidity at 8:00-9:00 a.m. varies 

 from 28 to 58, average 39; while at the pyramids of Gizeh there 

 are several light showers # each year, there is a moderately heavy 

 dewfall at night, and the relative humidity at 8:00-9:00 a.m. runs 

 from 64 to 87, average 72. 



In the case of the exfoliating statues, their sheltered positions 

 in the temples and the connection between the exfoliation and the 

 lower portions of the statues would seem to indicate that the cause 

 of the exfoliation lay not so much in the temperature changes as 

 in some factor connected with the ground, as for instance, in the 

 ground-water or moisture, and it is to such a cause that the exfoli- 

 ation is ascribed by G. Daressy of the Department of Antiquities, 

 Egypt, who says: "Les granites exposes continuellment a, l'eau 

 ou au soleil se conservent bien, mais ou ils se degradent, c'est 

 lorsqu'ils ont ete enfouis dans un sol humide. La formation de 

 sels nitrate et autre fait alors decomposer le granite, surtout lorsque 

 le terrain est alternatement sec et humide." The expansion con- 

 sequent upon the kaolinization of the feldspar is emphasized by 

 Merrill as the cause of the disintegration of the granite near 

 Washington, D.C. Although kaolinization is megascopically very 

 noticeable in these cases, it would scarcely seem to be of sufficient 

 amount alone to account for the observed disintegration and 

 exfoliation. 



