210 Walcot Gibson — Palceoxoic Hocks of South Africa. 



its waves and currents might exert some mechanical force.^ Fine 

 sediment would accumulate, and percolatu)g water would deposit 

 carbonate of lime. Secondary silica also occurs as in a flattened 

 geode (about 3 inches long), lined with bluish mammillated chalce- 

 dony (resembling beekite), and in the opaline silica cementing the 

 concretion in the section shown in Fig. 1. 



III. Summary. 

 This district belongs to the extended volcanic area associated with 

 the line of the Great Eift Valley. The abrupt descent of the bed in 

 the Eed Sea to a central channel may mark a part of that long 

 depression, the form having been modified by later river action. 

 The rocks of Perim probably correspond in age with the Aden 

 basalts (the later volcanic period so named by Dr. Blanford), and 

 they were doubtless emitted from some crater to the west of that at 

 Aden. This ejected the material forming the pumiceous tuff of 

 Perim, followed by the more abundant basaltic lavas. During 

 depression and subsequent elevations of the land, the sea helped to 

 carve out the straits of Babelmandeb and the gullies and rifts of the 

 island. Coral reefs grew in the surrounding waters, and finally the 

 raised beaches were formed, which record in Perim an uplift of at 

 least 12 feet. The outline and depth of the harbour lend no support 

 to the view that it marks an ancient crater, and the lavas and ashy 

 beds which form the cliffs around it have generally a roughly 

 horizontal direction. The indentation north-west of the harbour 

 seems the end or continuation of a valley (containing the quarry of 

 Fig. 1), extending from a raised beach here towards the raised 

 beach near Balfe Point. Thus the present form of the land is mainly 

 due to the usual forces of denudation and to oscillations of level, 

 acting on an island built up of volcanic masses, within a shallow 

 coral-bearing sea. 



IV. — On the Correlation op the Paljsozoic Kooks of 



South Africa. 



By Walcot Gibson, B.Sc, F.G.S. 



(Continued from the April Number, p. 165.) 



THE ancient rocks of Prieska are extensively developed in 

 Griqualaud West, where they build up the elevated Campbell 

 Eand plateau and the lofty ranges of the Langebergen Mountains 

 situated to the west of the Orange Eiver Colony. The order of 

 succession, which is much disguised by faulting and folding, has been 

 described in great detail by Mr. G. W. Stow,^ whose descriptions 

 render it evident" that the rocks of these regions resemble the older 

 strata which extend over such vast areas in the Transvaal. 



Several attempts have been made to classify the ancient rocks of 

 the Transvaal, the results being nearly as numerous as the observers. 

 In his latest classification of the rocks older than the Beaufort 



^ One example is described where the boulders appear as if piled by the sea, 

 forming a kind of j island in the middle of the Birkhud Eaised Beach to the north of 

 Perim. 



2 Q.J.G.S., Yol. XXX (1874), pp. 581 680. 



