25G F. P. Menncll — ArclKvan Strafigrap/ii/. 



knowledge of the older rock grouprf But when we come to deal! 

 with formations of such high antiquity as those which chiefly 

 concern the geologist in South Central Africa, it becomes evident 

 that we can hope to derive little assistance from the observations 

 that have been made in otlier quarters of the globe. In the 

 European area there is so great a preponderance of sediments whose 

 nature is obvious, and wliose structural relations are only just 

 sufficiently complicated to be interesting, that it is far too common 

 to find the schistose rocks indiscriminately lumped together, with 

 the intrusions that invade them, as "igneous ami metamorphic," 

 whereas in un Arcluv3an area the basis of all stratigraphical work 

 (if I may use such a term) must be the distinction between the 

 igneous rocks and those wliich owe their proximate characters to 

 metamorphism, whatever their origin may be. We find, in fact, 

 that even where much time and labour has been spent over these 

 rocks they have resulted in so much controversy and confusion that 

 the geologist in a region like Tropical Africa finds it necessary 

 to discard most of his preconceived ideas and start to frame new 

 generalisations for himself. And how important it is to have clear 

 ideas on the subject will be realised when it is considered that in 

 some countries nearly all the rock groups are of Archaean age. 

 Indeed, as I have already had occasion to point out,' the European 

 area is probably unique in its vast development of sedimentary as 

 compared with igneous and metamorphic rocks. In Africa the 

 stratified formations may almost be looked upon in the same light 

 as the drift deposits of England. They merely form a superficial 

 coaling through which the ' basement rooks ' of the earth's crust 

 constantly protrude except along the coastal fringe. If we look, 

 for example, at the map accompanying Hatch & Corstorphine's 

 recently issued " Geology of South Africa," we find separate colours 

 used ibr eight diiVerent sedimentary formations, which range from 

 5,000 to over 2o,(.>00 feet apiece in thickness. Only one of these is 

 known to be of pre-Archajan date, and what are now admitted to 

 be the Cape equivalents of the seven remaining groups are all 

 officially classed as Archiean by the Cape Geological Survey, and 

 it is certain that none are pre-Silurian. It is pei'haps necessary to 

 remind the reader that the once popular correlation of the Transvaal 

 rocks with the PaL-vozoic strata of the Cape is, as was first pointed 

 out by the writer two years ago, quite untenable, and is now 

 thoroughly discredited. 



It may be that the structure of the Central African metamorphic 

 area is simpler than that of similar regions elsewhere, but it is 

 certain that more light is likely to be thrown on some of the 

 most interesting problems of physical geology by the study of its 

 formations than can be thrown upon these last by the results so far 

 obtained in other countries. This may in part be attributed to 

 the great extent and striking lithological features of some of the 

 principal rock groups, and even more perhaps to the clearness of 

 the exposures. Instead of the irritating little isolated patches 



' " Gi't'k'gy vi Soiitli Ixlunk'si;!,"" p. -':. 



