108 Study of the New York Obelisk as a Decayed Boulder. 



which, though in a sheltered angle of the rocks and less exposed 

 than Ousertasen's, was much further gone. It may have been of 

 the Pyramid period" (IVth dynasty, 3124-2840 B. C, Lepsius). 

 This would involve an exposure of 50 or more centuries. 



On the other hand, Delesse states :' " In the Egyptian Museum of 

 the Louvre, the feet and the head of the colossal statue of Amenophis 

 III, as well as a large number of sculptures, which, under the per- 

 petually pure sky of Egypt, have not experienced any alteration 

 during the greater part of the time, have even preserved the most 

 perfect polish after nearl}" 4000 years." 



From all these observations at Syene, the following conclusions 

 may be drawn : — 



(1). The predominant destructive process has not been external, 

 such as disintegration by the heat of the sun, attrition by sand 

 whirled by the wind, etc.; here, as elsewhere, these have plaj^ed a 

 secondary part. A certain degree of polish has been produced on the 

 surface of ledges by sand-attrition, by occasional heavy rains, and 

 by the mud-laden waters of the Nile up to the limit of its flood-line. 



(2). The main process has been one of Internal decay, most 

 eflBcient along the joint- and bedding-planes of the granite, even to 

 the lowest depths now observable, and producing long columnar 

 masses. The chemical decay and disintegration have also seriouslv 

 attacked the irregular planes of contraction and eaten them out into 

 an irregular network of fissures, which mark the latent lines of 

 weakness throughout the materia], and divide it into angular blocks. 



(3). The gradual decomposition of the ferruginous silicates over 

 the surface of the ledges (biotite, hornblende, and the feldspars) has 

 left their feebly soluble bases, in this arid climate, as a polished 

 black crust of iron and manganese oxides. The other more soluble 

 and finer products of decay have been removed by occasional rains 

 and constant action of the wind. 



(4). The outer forms assumed by the cliffs largely indicate their 

 variation in materials and in their resistance to decomposition and 

 erosion : the projecting masses consist of the more compact kinds of 

 granite and porphyry, and even thin projecting seams and nodules 

 of quartz : the hollows and fissures, of softer granite and of inter- 

 calated seams of hornblende-schist. A considerable internal expan- 

 sion of material is shown by the general scaling of the surface and 



' Delesse, loc. cit., 490. 



