36 



The walls of Alexandria which defended its in- 

 dustry and riches, still defend its ruins,"* yet the stones 

 of which they are constructed, are not decomposed. 



never fails to corrode and to decompose substances where felspar 

 is a constituent. Examples may be adduced of marble, after con- 

 tinual exposure to air and moisture during two thousand years, 

 still retaining the original polish upon its surface unaltered ; but 

 granite, under similar circumstances, has not only undergone 

 alterations, but, in certain cases, has crumbled and fallen into the 

 form of gravel." Vol. III. page 170. 



In my remarks on the decomposition of granite, I have expressly 

 admitted that, in certain cases, it is liable to disintegration, so 

 much so that, in time, the whole mass will crumble and fall into 

 sand and gravel 5 and of such, most probably, were the instances 

 which he mentions as being seen among " the ruins of Alexandria, 

 Troas, and over all the district of Troas in general." Ibid. 



\ have also admitted that, (whether in granite or porphyry,) if 

 potash forms a constituent of felspar, as it does in some instances, 

 it is apt to promote the disintegration and, perhaps, decomposition 

 of the felspar. But the far greater part of the felspar that enters 

 into the composition of granite, porphyry, &c. is not of this des- 

 cription, and particularly that which forms a constituent part of the 

 granite, qf which Pompey's pillar and the obelisks are composed. 

 Hence we are led to infer, that the slight change which the felspar, 

 in Cleopatra's Needles, had undergone in so many ages, (and 

 slight it must have been indeed, since only its red colour had 

 faded,) must have depended, in an eminent degree, on local cir- 

 cumstances. 



It is a well known fact, that monuments of art composed of 

 these materials and placed upon the borders, or in the vicinity of 

 the ocean or a sea, arid exposed to the saline vapours that are al- 



* Baron de Tott, volume II, part II, page 36. 



