552 C. B. Eorioood ^ A. Wade— The Old Granites of Africa. 



A most interesting description of a rock very similar to that of 

 Orange Grove will be found in the Memoirs of the Geological Survey 

 of the United Kingdom, Silurian Rocks of Britain, 1899, vol. i, 

 Scotland, ch. xxvi. 



It will be seen that in all these cases the low percentage of alumina 

 presents a close comparison with the Orange Grove rock, but some 

 varieties of the rocks furnishing the means given in Columns III and 

 IV approximate more closely to it in composition. A granite from 

 Teufelsmauer, near Krems, mentioned by Zirkel in his Handhuch, 

 has a similar low percentage of alumina accompanied by a high silica 

 percentage. 



On the whole the analysis bears out the observations made by 

 means of the microscope. The abundance of sphene, however, in the 

 rocks would lead one to expect some small quantity of oxide of 

 titanium. The rock has one or two other peculiarities besides its low 

 percentage of alumina. It is very acid, and yet has rather higher 

 percentages of lime and iron than are usual in granites. The 

 abundance of epidote accounts for the lime, and this, together with the 

 presence of secondary albite, is somewhat significant. It suggests that 

 oligoclase was originally one of the felspathic constituents, and this 

 suggestion finds support in the presence of oligoclase in the more 

 basic fresher varieties of this Old Grey Granite of the Transvaal as 

 found at Rietfontein and Driefontein. 



The peculiarities in the composition of the rock lead one to examine 

 its position with regard to the classification of igneous rocks by 

 quantitative methods lately proposed by distinguished American 

 petrologists. Whilst not entirely accepting this American system 

 and its elaborate nomenclature, the author feels bound to recommend 

 it to the notice of all interested in petrology, as a step in the right 

 direction ; a foundation for a more perfect system of rock classification, 

 which will enable us to comprehend more clearly their relationships 

 and the laws which govern the origin of rocks. For further details 

 with regard to the system and the methods and nomenclature used in 

 the following calculations, the works on the subject published by the 

 originators are recommended.' One or two words only can be spared 

 here. Starting from the chemical composition of the rock as obtained 

 by an actual analysis, the molecular proportions of the various oxides 

 present in the rock, are first worked out. From these the ' norm ' or 

 standard mineral composition of the rock, as contrasted with the 

 ' mode ' or actual mineral composition, is obtained by allotting the 

 various oxides, in molecular proportions to form certain ' normal ' 

 minerals. The allotment is performed according to certain fixed 

 rules. Now by reason of the fixed set of minerals so obtained we 

 are able to compare all rocks with a great amount of accuracy. These 

 minerals are grouped as ' Salic ' or ' Ferric ' according as thej' are 

 acid or basic in composition. The final classification is based primarily 

 upon the ratios between these two groups of minerals, and, secondly, 

 between the minerals of the ' norm '. 



' Quantitative Classification of Igneous Mocks, by Cross, Iddings, Pirsson, aud 

 "Washington, 1903 (University of Chicago Press). 



