THE GEOLOGY OF THE N^OETHEEN ETBAI. 



the metamorphosed blue clay which, between Kina and Kosair, is 

 bare on the crest ; rising from under sandstone under limestone on 

 the west and sinking on the east under sandstone under limestone ; 

 nor is there any apparent difference in the sandstone on either side 

 of the crest, though the samples have not yet been examined micro- 

 scopically. The ' cataract ' rock sinks at Abu Tiur, and it seems 

 very probable that it reappears in the Jebel Patirah quarries in 

 latitude 26° 50' JS^. To the north of the east-and-west depression 

 of Kina-Kosair the ridge rises from 1400 to 1800 feet, and the 

 masses along the western portion of it are uniformly metamorphic 

 sandstone. Nor does any igneous rock appear till the porphyry 

 quarries of Mr. Brindley are reached in latitude 27° 20' K., where 

 are three parallel lines of upthrust, counting from the west : Um 

 Sidra and Jebel Dukhan (Mons Porphyritis), Jebel Aish, and Jebel 

 Zeit close to the sea, where petroleum pools are found in what is 

 apparently a deposit of immense thickness of Miocene limestone. 



§ 5. Points to be examined. 



It is interesting to compare the sections west of the Red Sea with 

 the geological map by Walther of the Sinai peninsula. A section 

 drawn from Walther' s map between the 28th and 29th parallels 

 would give first the fringe of recent coral, then the ' stock-granit ' 

 which corresponds to the ' cataract ' rock ; over this comes the 

 Nubian Sandstone, and over this the limestones. 



Before quitting the general geological survey mention must be 

 made of two points on which imperfect geological knowledge debars 

 the Author from expressing an opinion, but which seem to be of 

 considerable interest to geologists. 



The first question relates to the great Eocene sea drawn by that 

 eminent geologist Prof. Hull. It seems a most interesting matter 

 for future travellers to examine whether the Eocene sea did not 

 stretch across to Arabia. Eor, unless the observations are mistaken, 

 the ridge has in latitude 23° N. been thrust through the sandstone 

 which spreads both east and west of the watershed. 



Another question of interest is what is the history of the blue 

 clay which lies under the sandstone, and of which, so far as the 

 Author has been able to ascertain, no mention has been made pre- 

 viously. 



A Pluvial epoch is thought necessary by some to account for the 

 erosion of the valleys, but from an examination of the valleys the 

 Author inclines to the opinion of Prof. Schweinfurth that they do 

 not require for their erosion more rain than falls to-day. 



§ 6. The Emeeald Mines. 



The Emerald Mines cover some forty square miles of valley and 

 mountain, and much resemble a large rabbit-warren. Erom count- 

 less holes in every dark hillside pour streams of silvery, powdered 



