238 Correspondence — Professor T. G. Bonney. 



is north and south, whilst to the south of the hill the strike is a few 

 degrees on either side of east and west. 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. 

 On the crystalline mass of Jebel Raft, and apparently overlain by 

 the Nubian Sandstone, is a very coarse conglomerate containing 

 fragments of crystalline rock, which appears to be older than 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 ; whilst 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 Eaft 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 pi-esent, when the rainfall was heavier. 



Miss Eaisin gives accounts of both massive and schistose crystal- 

 line rocks, and also of sedimentary rocks. The massive crystallines 

 occur around Murrat and Jebel Eaft, and these two areas are 

 separated by the southern mass of schists. Another area of massive 

 crystallines is formed beyond these schists, extending to the head of 

 Wady Allaki. The crystalline rocks described include gneiss, horn- 

 blendite, gabbro, diabase, quartz-diorite, granite, felsite, certain 

 schists, and sediments. 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 mud and desert-sand and the formation of a weathered 

 coating. The crystalline schists do not present a very modern facies, 

 and might be late Archaean or early Palgeozoic. The massive 

 crystallines may belong to different epochs. 



Miss Aston gives two tables, one of which shows the actual 

 amounts of substances found in the wells of Murrat, Bir Tilat Abda, 

 and Bir-Ab-Anaga, whilst the second shows their approximate 

 constitution. 



ooS/iaiESiPon^rzDiEiisrGiE. 



FOEEIGN BOULDERS IN THE CHALK. 

 Sre, — Mr. Martin in your last number suggests that the granite 

 boulders from the chalk-pit at Betchworth, described by Mr. 

 Stebbing, may have come from the south-west. Permit me to say 

 that I know the granites of this part of England, especially that 

 of Hey Tor, fairly well, and for reasons which I think will be 

 obvious to any petrologist who reads my description, consider this 

 place of origin most improbable. Before publishing his speculation 

 Mr, Martin would have done well also to consider the following 



