THE GEOLOGY OE THE NOETHEEN ETBAI. 581 



mica. The hills are some 1200 feet in relative height, and formed 

 of talcose schist, tortuously bedded, with quartz and limestone -veins 

 and masses of tuff. The principal old shafts are on the south, while 

 the modern borings of 1820 are on the north. There are perhaps 

 500 to 800 shafts, and buildings still represent the various periods 

 of working from e.g. 20, if not earlier, to a.d, 1526, when the 

 emerald mines of South America outstripped those of Egypt in 

 producing the gem. 



Discussion. 



The Chaieman (Prof. Jitdd) expressed his regret at the absence 

 of the Author, and invited discussion on the paper. 



Prof. Hull observed that he felt great reluctance to criticize 

 adversely a paper in the absence of its Author, but there were 

 several points in Mr. Ployer's communication he could not agree 

 with, the chief of which was the view of the Author that the Nubian 

 Sandstone had been converted by a process of metamorphism into 

 granite, owing to its contact with porphyry and with the igneous rocks 

 of the ridge bordering the Eed Sea coast. Nearly all observers 

 were of opinion, in which he (Prof. Hull) concurred, that the igneous 

 and granitoid rocks of this ridge were of immense geological anti- 

 quity, in all probability of Archaean age, and as the Nubian Sand- 

 stone was of Cretaceous age there could be no possible effect pro- 

 duced on the latter by contact with the former. The breccia and 

 conglomerate found at the base of the Nubian Sandstone were in 

 reality a littoral deposit — derived from the waste of the Archaean 

 rocks. 



Mr. rioyer's 'blue clay' the speaker regarded as probably decom- 

 posed schist of the Archaean series, and he could have wished that 

 the Author had given more precise information concerning the 

 occurrence of the emeralds than was given in his paper. 



In conclusion, the speaker did not see any reason for abandoning 

 the view which he had arrived at from his observations in Arabia 

 Petraea and Southern Palestine, namely, that the formation of the 

 remarkable dry valleys in this part of the world could only be 

 accounted for on the supposition that at a former period the rainfall 

 and resulting streams had been greatly in excess of those of the 

 present day, and this Pluvial period he believed to have represented 

 the Glacial period of more northern latitudes. 



Prof. Le Neve Postek complained of the absence of specimens, 

 and said that it was impossible to discuss the paper without them. 

 He regretted that a paper on emerald mines contained so little in- 

 formation about the mode of occurrence of the mineral. 



Mr. EuDLEE explained that, although no specimens were exhibited 

 in illustration of the paper, he had had an opportunity, some time 

 ago, of seeing the specimens brought from the Emerald Mines by 

 Mr. Eloyer. The rock representing the matrix of the emerald 

 appeared to be a biotite-schist, more or less talcose. Mr. Eloyer 

 had obtained two crystals of emerald, which were attached to quartz, 



