The Banket a Study of the Auriferous Conglomerates of the Witwatersrand And the

repay even a more exhaustive study than the writer has given to it, there can be no doubt. The rock is of ancient date and has come under a variety of influences which have induced in it many mineral changes, the full elucidation of which, more especially those of a metasomatic nature, would con- stitute an important contribution to mineralogical and geochemical knowledge. It is true that the changes referred to are by no means unique, but the facilities for their investigation afforded by the banket are unrivalled. These are, first, the sim- plicity of its original mineral composition, the rock being made up principally of quartz, and second, its extensive exposure. With regard to the latter, it may be mentioned that the banket is now exposed in the direction of the dip to a vertical depth of about a mile, while hundreds of miles of mine work- ings traverse it at varying depths along the strike. The staffs of the mines are in great part com- posed of men who have had some training in the methods of petrology, and it is hoped that the publication of the present Memoir, by providing a basis for further study, will induce some members of this army of potential investigators to enter the field of research. It will scarcely be gainsaid that a thorough investigation of the banket and associ- ated rocks is of value to the gold-mining industry, or that an intelligent interest on the part of the mine officials in the rocks with which they deal will add to efficiency. The present work is based largely on the follow- ing papers contributed by the writer to the Transac- tions of the Geological Society of South Africa Notes on the Auriferous Conglomerates of the Witwatersrand, vol. x., 1907. Further Notes on the Auriferous Conglomerates of the Witwatersrand, with a Discussion of the Origin of the Gold, vol. xii., 1909. The Replacement of Quartz by Carbon in Rand Banket, vol. xiii., 1910. The Problem of the Rand Banket, Presidential Address, 1911. Notes on the Pebbles of the Rand Banket, vol. xv., 1912. Note on the Origin of the Iridosmine in the Banket, vol. xv., 1912. Note on Diamonds in the Banket, vol. xvi., 1913. Metasomatism in Banket, vol. xvii., 1914. Due acknowledgment is made in the text of any indebtedness to the work of others. With regard to the vexed question of the origin PREFACE vii of the gold in the banket, the writer was inclined during the earlier stages of his research to favour the infiltration theory. However, as the work pro- ceeded, he was gradually led, by the difficulties that appeared in the way of accepting that hypothesis, to the belief that a more satisfactory solution of the problem was provided by the placer theory. This belief has been greatly strengthened by the recent stratigraphical work of Dr E. T. Mellor. Never- theless, as it is necessary on this theory to assume that the detrital gold has undergone solution and reprecipitation, it has been thought advisable to include the descriptions of the gold in the chapter dealing with the authigenic constituents of the banket... --This text refers to the Paperback edition.