May 



1905] 



NA TURE 



JO 



SOUTH AFRICAN GEOLOGY.' 



■]\/rR. ROGERS has produced a handbook to the 

 •^'A geology of Cape Colon}' which is sure to re- 

 main a standard treatise. New observations will be 

 recorded in future editions, as the work of his survey 

 is carried on ; but results made public as recently as 

 1904 are included in the present volume. The book 

 appears with especial appropriateness, now that the 

 visit of the British Association to South Africa has 

 been officially organised ; and the included geological 

 map, on the scale of about one inch to ninety miles, 

 gives an admirable impression of the country. In it 

 we see the huge Karroo svnclinal, occupy- 

 ing almost all the colony, and lying be- 

 tween the pre-Devonian masses that crop 

 out upon the north and the closely folded 

 rocks of the Cape system along' the south ; 

 while Mr. Rogers's introduction connects 

 the scenic feattu'es with the geological 

 structure in a manner that attracts us at 

 the outset. 



It is unfortunate that the names chosen 

 for the colonial systems of rocks are not 

 readily represented by adjectives. Hence 

 such ungrammatical expressions as " pre- 

 Cape " and " pre-Karroo " have been re- 

 ceived indelibly into literature. Even the 

 International Congress may hesitate to 

 speak of an " etagc bokkeveldien," though 

 we have, to be sure, " purbeckien " and 

 " bathonien " in Europe. This use of local 

 names is, of course, greatly to be com- 

 mended, in view of the scarcity of fossils 

 in the great majority of the series. 



The invasion of the old Malmesbury beds 

 in the west of the colony by granite is con- 

 cisely described on p. 38 ; and it is interest- 

 ing to note how gneissic structures have 

 arisen in the granite, as in so many other 

 instances, without " evidence of a great 

 amount of crushing or rearrangement of 

 its component minerals after it solidified." 

 The foliation-planes in the gneissoid 

 granite are parallel with the strike and 

 cleavage of the adjacent sedimentary rocks, 

 and the whole structure seems one of sub- 

 terranean flow. The granulites of the 

 Darling area will clearly bear comparison 

 with those that have been so much dis- 

 cussed in Saxony. The intercalation ol 

 orthoclase crystals from the granite in 

 lenticular areas between lamina of slate 

 (p. 43) reminds us, again, of the composite 

 rocks of Donegal. 



Mr. Rogers gives an interesting account 

 of the stages in the passage from the well 

 known blue crocidolite to the more siliceous 

 yellow " griqualandite " in the slates of 

 the Griquatown series. The slates them- . <""« 

 selves are converted into jasper-rocks 

 where the most altered amphibole occurs ; and the 

 crests and troughs of the folds have afforded hollows 

 in which the fibres of amphibole have crystallised across 

 from one surface to another. 



The Cape system, including the Table Mountain 

 series at its base, has been greatly contorted and 

 overfolded in the south ; but the southern edge of 

 the Karroo beds is also involved (p. 407), and the 

 great east-and-west ridges of the continental margin 

 date from somewhere about Jurassic times. Flattened 

 and striated pebbles occur in the Table Mountain 



to I he Geology of Cape Colony." By A. W. Rogers, 

 .ir of the Geological Survey of Cape Colony. Pp. 

 Lonemans, Green anil Co., 1905.) Price gs. net. 



beds, and are regarded as the first evidence of a 

 neighbouring highland on which glaciers gathered. 

 The Devonian Bokkeveld beds follow, and the still 

 higher and famous Dwyka conglomerate is, as all 

 geologists know, of Permo-Carboniferous age. It is 

 somewhat fascinating to conceive the growth of 

 glacial conditions through at least two long geo- 

 logical periods, until the flood of ice at last spread 

 southward from the Transvaal territories, and scored 

 and rounded all the preceding rock-masses down to 

 the region of the Cape itself. 



The Dwyka beds, a facies of the Kimberley-Ecca 

 series, and long regarded as volcanic tuffs, are here 



1 "An Introduction 

 M.A., F.G.S.,DireQl 

 xviii + 463. (London : 



NO. 1854, VOL. 72] 



folded quartzites of the Table Mo 

 of the great upheaval, which probably took place in early Jurassic 

 From Rogers's " Geology 01 Cape Colony." 



very adequately described, with several effective illus- 

 trations. The glacial series at Vereeniging is associ- 

 ated with beds containing the Glossopteris flora, and 

 also Sigillaria and other northern forms ; and Mr. 

 Rogers points out that the cold cannot have been 

 responsible for preventing a more frequent mingling 

 of these two well marked floras. The most novel 

 portion of the account of the reptiliferous Beaufort 

 beds of the " Karroo system " is the strong hint 

 (p. 198) that they should be regarded as Permian 

 rather than Triassic. This view, based on Ama- 

 litzky's work in Russia, would lead to a reconsider- 

 ation of the Elgin Sandstone also, and to the accept- 

 ance of a development of reptilian life in Permian 



