352 NOTES ON THE GEOLOGY 



cnty miles northeast of Akabah, which stands at the head of the 

 eastern branch of the Red Sea, is the remarkable gorge where 

 once stood the capital of Edom, now called Petra. And from 

 the descriptions given us by those who have visited it, we must 

 Consider it highly probable, that the rock forming the gorge is 

 the new red sandstone ; though no organic relics have been de- 

 scribed in it. This formation probably also extends southerly 

 far towards Sinai, and northerly an unknown distance. 



It would not be sti'ange if the syenite of Mount Sinai should 

 be found to extend continuously beneath the Red Sea, to the fa- 

 mous quarries of red granite at Syene, in Upper Egypt. In re- 

 gard to the latter rock (No. 107) it may perhaps appear presump- 

 tuous in me to suggest the suspicion, that some of it ought to be 

 regarded as gneiss. This thought first occun-ed to me, on ex- 

 amining a similar rock in southeastern Massachusetts ; which I 

 at first supposed to be granite, but afterwards became satisfied 

 was granitic gneiss. This suggestion, however, is not of much 

 importance : for often there is not a more difficult point in geol- 

 ogy, than to cbaw the line between granite and gneiss. Yet I 

 find that the more I examine, the more disposed I am to reduce 

 the limits of granite, and enlarge those of gneiss. 



We have all heard much of the deserts of Arabia, and of the 

 moving sand-hills there. No. 553 is a specimen of these sands : 

 and, on examination with the glass, I was surprised to find them 

 to consist of fragments of genuine yellow quartz, not to be dis- 

 tinguished by the eye from topaz. The gi-ains are very much 

 rounded. Was this done by water, or by wind, or by both ? 

 This specimen was presented by the Rev. Justin Perkins of Per- 

 sia ; to whom it was given by a Mahommedan pilgrim, on his 

 way from Mecca, or Medina, to Persia. He stated, that the car- 

 avan was a fortnight in crossing these sands. 



My specimens from Egypt are too few, perhaps, to throw any 

 light upon the geology of that country. The silicified wood. No. 

 896, is the most interesting. This is from the extensive depos- 

 it near Cairo. " For miles," says Dr. Robertson, " the surface 

 is covered with fragments, from the size of lliis specimen to 

 many feet in length." The texture of the siliceous matter is 



