C. B. Horicood (^ A. Wade— The Old Granites of Africa. 455 



IV. — The Old Granites of thk Transvaal and of South and 

 Central Africa, by Cuthbert Baring Horwood, A.R.S.M., Assoc. 

 M.Inst.C.E., F.G.S., with a Petrographical Description of the 

 Orange Grove Occurrence, by Arthur Wade, B.Sc. (Lond.), 

 A.R.C.S., F.G.S. 



Introduction. 



JUST over three years ago I described ^ the field relations, chemical 

 composition, and petrographical characteristics of the Old, and also 

 of the Newer, Eed Granite of the Transvaal. As then, so now, I cannot 

 do better than introduce this subject with the words of the late 

 Charles Darwin- — " Granite to the geologist is classic ground; from 

 its widespread limits, and its beautiful and compact texture, few rocks 

 have been more anciently recognized. Granite has given rise, perhaps, 

 to more discussion concerning its origin than any other formation. 

 AVe generally see it constituting the fundamental rock ; and, however 

 formed, we know it is the deepest layer in the crust of this globe to 

 which man has penetrated ". South and Central Africa furnish no 

 exception. Their granites, which are of astonishingly widespread 

 occurrence, of varying and often characteristic texture, have been. 

 classic ground to geologists, as the numerous papers concerning them 

 and their mode of origin and relative geological age well testify. 



In this paper, which is divided into two parts, I propose in the 

 first portion to give a few general notes on and to describe the Old 

 Granites of the Transvaal and South and Central Africa; to review 

 and summarize the knowledge which has gradually accumulated during 

 the last decade concerning these Old Granites and associated gneisses, 

 and from this, together with my own work in the field, to draw certain 

 conclusions and deductions, the chief one being that in South and Central 

 Africa there exists a fundamental or primary granite-gneiss formation, 

 which has not up to the present been generally recognized. 



In the second portion Mr. Arthur "Wade, from a study of my slides, 

 has given a very detailed petrographical description of the Old Granite 

 occurring at Orange Grove, on the southern edge of the Johannesburg- 

 Pretoria granite boss, about 3 miles north-east of Johannesburg. 

 He has also added notes on its position in the New Quantitive 

 Classification lately proposed and adopted by eminent American 

 petrologists. Although this occurrence is not the most typical, yet 

 it is sufficiently so, and we have selected it as being that which is 

 best known in "the Transvaal on account of its proximit)^ to the Rand. 



We had hoped to compare the chemical composition and petro- 

 graphical features of these Old Granites with those of the old grey 

 soda granite which forms the nucleus of the Cordilleras of the Andes 

 of South America, but we were unable to find any record of an 

 analysis or any petrographical description of this latter rock. 



Part I. Description of the Old Granites. 

 The Old Granites are of a dull-grey colour, and are frequently 

 characterized by the presence of porphyritic felspars, which are often 



' The Witicatemrand and Associated Beds, by C. B. Horwood, June, 1905 

 (Esson & Perkins, Johannesburg"), pp. 64-84. 



* Journal of Researches, by Charles Darwin (Ward, Lock, & Co.), p. 276. 



