2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XXIV. 



INTRODUCTION TO THE THEORY OE IGNEOUS UOOKS. 

 Classification of the Igneous Rocks. 



In this report the prevailing classification of igneous rocks, as compiled and 

 elaborated by Rosenbusch, has been followed. Once again a prolonged study of 

 large igneous areas has proved the value of his division of eruptives into three 

 principal classes : the plutonic, the effusive, and the dike rocks. The distinction 

 is obviously fundamental to the geologist, for he must never lose sight of the 

 fact that the structural relations of igneous bodies indicate earth history as 

 truly as do the series of stratified rocks with their contained fossils. 



Likewise the petrologist, who is primarily interested in the origin of rocks 

 and in the processes by which they have assumed their known compositions and 

 structures, should regard this time-honoured, threefold division as essential. 

 The general contrasts of texture and structure among the three classes are too 

 patent to need emphasis. A fact less conspicuous but certainly worthy of 

 distinct recognition in arranging a classification, is illustrated in the following 

 table of chemical averages. It is there seen that the leading effusive types are 

 steadily contrasted in a chemical way with the corresponding plutonics. The 

 former are slightly but distinctly richer in silica and alkalies, and poorer in 

 iron oxides, magnesia, and lime than the respective plutonic rocks. This relation 

 is not fortuitous but is almost certainly a result of some kind of magmatie 

 differentiation. A classification which obscures such principal indications of 

 origin must be ranked as imperfect. 



Most of the ' diaschistic ' dike rocks have no direct equivalents among the 

 plutonics or the effusives, and these chemical units are found nowhere else than 

 in dikes or, more rarely, in sheets or other small injected bodies. True it is that 

 the ' aschistic ' dikes are essentially like corresponding plutonic rocks in chemi- 

 cal composition, but the petrogenist must give much more weight to the 

 diaschistic division. Understood in this way, Rosenbusch's separation . of the 

 dike rocks from the other two classes seems to be a prime necessity to the 

 investigator in the genesis of rocks. 



Rosenbusch's principles of subdivision in each of the three great < classes 

 fulfil the requirements of the field geologist. A quantitative estimate of the actual 

 mineralogical composition checked, ideally, by microscopic and chemical 

 analysis, is the only natural basis of classification for the man in the field. 

 According to the mode in which the minerals of rocks are assembled we have, 

 thus, what has been called a Mode classification. Variable as igneous rocks 

 may be, their modes represent a quite limited number of types, each of which 



25a— vol. iii— 44J 677 



