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XVIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from vol. xliii. p. 463.] 



April 28th, 1897.— Dr. Henry Hicks, F.B.S., President, in the Chair. 



r PHE following communications were read : — 

 -*- 1. '.Note on a Portion of the Nubian Desert South-east of 

 Korosko.' By Capt. H. G. Lyons, B.E., F.G.8. With Notes on the 

 Petrology by Miss C. A. Raisin, B.Sc. And Water-Analyses by 

 Miss E. Aston, B.Sc. 



A general description of the physical structure of the area, which 

 consists mainly of Nubian Sandstone and crystalline rocks, is given. 

 Corniug from Korosko to the Murrat Wells, the crystalline rocks are 

 first met with on the east side of Jebel Raft. At Wadi dur Nabadi 

 are ancient gold-workings, indicated by M. de Bellefonds on his map 

 of the Etbai. The crystalline rocks are both massive and schistose. 

 The strike of the latter to the east of Jebel Baft is north and south, 

 while to the south of the hill the strike is a few degrees on either 

 side of east and west. On the crystalline mass of Jebel Baft, and 

 apparently overlain by tbe Nubian Sandstone, is a very coarse 

 conglomerate containing fragments of crystalline rock, which 

 appears to be older tban the Nubian Sandstone. 



The Nubian Sandstone has little or no dip, and shows very slight 

 variation in composition. The water-supply of the Nubian Desert 

 is directly dependent on the rainfall, which is very irregular. The 

 wells are sunk in the detritus of the valleys, and contain a large 

 amount of mineral matter in solution, which renders them almost 

 undrinkable ; while the second source of supply — the rain-water 

 reservoirs — are deep holes in the ravines which intersect the 

 crystalline hills. These holes must be attributed to water-action ; 

 and in the reservoir of Medina in Jebel Baft the spherical stones 

 which assisted in forming the pothole still occur. The author 

 believes that these ravines and reservoirs were formed at an earlier 

 period than the present, when the rainfall was heavier. 



Miss Baisin gives accounts of both massive and schistose crystal- 

 line rocks, and also of sedimentary rocks. The crystalline rocks 

 described include gneiss, hornblendite, gabbro, often much altered 

 and resembling some of the Alpine gabbros, some allied rocks 

 containing lustre • mottled hornblende, other forms of diabase, 

 quartz-diorite, granite, felsites, certain schistose and a few dis- 

 tinctly fragmental rocks. None of the igneous rocks can be stated 

 with certainty to have originated as a lava-flow. Many of them 

 have undergone much alteration since their consolidation, and the 

 results of this are described. There is clear evidence in many cases 

 of erosion by desert-sand and the formation of a weathered coating. 

 The schistose rocks do not present a very modern facies, and might be 

 late Archaean or early Palaeozuie. The massive crystallines may 



T063S 



