386 DONALD C. BARTON 



to the outcrop and surrounding detrital slopes of similar types of 

 rocks in New England. 



The fine-grained granite of the region, somewhat similar in 

 composition to the coarse red granite, although with a lower 

 content of colored silicates, is not so severely affected by the dis- 

 integration. Exfoliation takes place very slowly, and although 

 the edges and corners of exposed blocks have in most cases been 

 rounded, the general form of the blocks is angular. The outcrops 

 in general aspect are not unlike these of the more jointed phases 

 of the coarse red granite. The fine-grained granite is, however, 

 itself much jointed. It was not seen massively disintegrated and 

 small dikes cutting the massively disintegrated coarse granite 

 showed merely slight exfoliation of the edges and corners of the 

 joint blocks into which the dike is broken. Flaking and the loosen- 

 ing of single grains on exposed surfaces do not seem severely to 

 affect the fine-grained granite. 



THE RATE OF THE DISINTEGRATION OF THE GRANITE 



The rate of disintegration of exposed surfaces of granite at 

 Aswan is not as rapid as at first might seem. Many of the numer- 

 ous hieroglyphic inscriptions of this region show noticeable disin- 

 tegration and on this account relatively rapid rates of disintegration 

 have been postulated. These inscriptions almost without excep- 

 tion are carved on bowlders of exfoliation, and in but few cases 

 was there seemingly much effort on the part of the ancient Egyp- 

 tians to remove more than the most readily detachable plates of 

 exfoliation. The greater number of the inscriptions therefore 

 were carved on surfaces that were already partially disintegrated. 

 In the few cases in which the writer was able to satisfy himself that 

 the inscriptions had been cut in surfaces dressed back into fresh 

 rock, there was no disintegration noticeable and the inscriptions 

 were entirely fresh and sharp. Such inscriptions can be seen on one 

 of the two natural obelisks on the island of El Hesa. The inscrip- 

 tions date from the reigns of Mentuhotep I, about 2100 B.C.; 

 Thutmoses IV, 1420-1411 B.C.; Amenhotep III, 1411-1395 B.C.; 

 and Psammeticus II, 588-583 B.C. The inscriptions show no 

 noticeable disintegration, and tapping with the finger or hammer 



