274 Reports and Proceedings — 



of evolution ; but now that it has been shown that its supposed 

 affinity with the Pines is based entirely upon the resemblance of 

 its wood to that of the Araucaria, the structure of the whole stem, 

 pith, wood, and bark being taken into consideration, it is seen to 

 have more affinity with the Cycads as seen in Cijcas revolnta, than 

 with Araucaria, and it is found to occupy its natural place both in 

 the vegetable kingdom, and in the order of its appearance in geo- 

 logical time, according to the theory of evolution." 



XI. E. Jones details the circumstance of the f'urtlier examinations 

 of the Elbolton Cave, and states that continued exploration is 

 required. 



XII. A bibliography of memoirs relating to the geology of York- 

 shire for 1889-90 is appended at pages 119-122. T. R. J. 



e,e:pos,ts ji^isTJD lE'iEaoaiBEiDxisros. 



Geological Society of London. 



I.— April 27th, 1892.— Prof J. W. Judd, F.R.S., Vice-President, 

 in the Chair. — The following communications were read : — 



1. "Notes on the Geology of the Northern Etbai or Eastern 

 Desert of Egypt ; with an Account of the Emerald Mines." By 

 Ernest A. Floyer, Esq., F.G.S. 



The principal feature in the district is a long ridge of igneous 

 upthrust running N.N.W. and S.S.E., in which porphyry rises into 

 lot'ty peaks, whilst the lower parts are formed of granites and sedi- 

 mentary rocks. To the west of the watershed, sedimentary rocks 

 occur dipping slightly to the west. 



The fullovving succession of rocks in descending order is given 

 by the author: — Limestone, sandstone, clay, ' cataract '-rock (cor- 

 responding to the Stoclc-granit of Walther), and compact hard 

 granite. The sedimentary rocks are frequently metamorphosed, and 

 the author states that every stage of metamorphism is shown, from 

 sandstone to compact green granite. The blue clay shows various 

 kinds of metamorphism, and forms the pistachio-breccia containing 

 topazes, and the mica-schist, mica-slate, and talcose blue clay of the 

 mass of Zabbara containing emeralds. 



The author discusses certain theoretical questions, and considers 

 that the erosion of the valleys does not indicate the existence of a 

 greater rainfall than the present one. He concludes by giving an 

 account of the emerald mines. 



2. "The Rise and Fall of Lake Tanganyika." Bv Alex. Carson, 

 Esq., B.Sc. (Communicated by R. Kidston, Esq., F.R.S.E., F.G.S.) 



In this paper attention is called to certain recorded discrepancies 

 concerning the discharge of Tanganyika by the Lukuja. It is 

 suggested that the rise of the lake is due to the blocking-up of the 

 river by vegetation, assisted by silting during the first rains, whilst 

 the fall is produced by the destruction of the barrier formed in this 

 manner. 



