III.—Some Notes on Archæan Stratigraphy

Extract

Much has been written concerning the Archæan rocks from a petrographical point of view, but their structural relations have as a rule received little attention. I do not wish to be understood as overlooking the work that has been done in England, and especially in America, of late, and which has added so much to our knowledge of the older rock groups. 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 other quarters of the globe. In the European area there is so great a preponderance of sediments whose nature is obvious, and whose 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 and metamorphic,” whereas in an Archæan area the basis of all stratigraphical work (if I may use such a term) must be the distinction between the igneous rocks and those which 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 Archæan 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 coating through which the ‘basement rocks’ 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 for eight different sedimentary formations, which range from 5,000 to over 25,000 feet apiece in thickness. Only one of these is known to be of pre-Archæan date, and what are now admitted to be the Cape equivalents of the seven remaining groups are all officially classed as Archæan by the Cape Geological Survey, and it is certain that none are pre-Silurian. It is perhaps necessary to remind the reader that the once popular correlation of the Transvaal rocks with the Palæozoic strata of the Cape is, as was first pointed out by the writer two years ago, quite untenable, and is now thoroughly discredited.