380 TRANSACTIONS OF SECTION C. 



a province. Noting that nearly identical assemblages of rocks sometimes occur 

 in widely separate provinces and at different geological periods, we have some 

 reason for expecting that the provincial parent-magmas may ultimately be reduced 

 to a limited number of types. Whether these types will be sufficiently definite 

 to serve as a basis of classification it is too early to say. 



For the sake of argument, I have taken chemical composition as the criterion. 

 It is certain, however, that a rock-magma consists not of free oxides but mainly 

 of silicate-compounds, and the variation produced by magmatic differentiation is 

 a variation in the relative proportions of such compounds. The characteristics 

 common to a set of cognate rock-types will, therefore, be more properly expressed 

 in mineralogical than in chemical terms. If, to fix ideas, we take as representa- 

 tive of a province its principal plutonic series, we shall often find that some 

 particular mineral or some special association of minerals stands out as a dis 

 tinctive feature. For instance, in the charnockite-norite series of Southern India 

 the characteristic ferro-magnesian mineral is hypersthene; in the granite-gabbr* 

 series of the British Tertiary it is augite ; and in the gramte-diorite series, 

 which predominates among the ' newer granites ' of the Scottish Highlands, 

 hornblende and biotite. These three sets of rocks, all of calcic facies, are easily 

 distinguishable in isolated specimens. 



Each such rock-series embraces types ranging from acid to ultrabasic. This 

 variation is ascribed to a later differentiation of the parent-magma of the pro- 

 vince, and, therefore, in an arrangement based on genetic principles, it will find 

 expression, not in the main divisions of the scheme but in the sub-divisions. 

 Here is an essential difference between an ideal petrogenetic classification and 

 the petrographical systems which are, or have been, in use. If we are content 

 to limit our study of igneous rocks to specimens in a museum, the distinction of 

 acid, neutral, basic, and ultra-basic may seem to be one of first importance. It 

 has, in fact, been employed for the primary divisions in some formal schemes, 

 e.g., in that put forward by Lowinson-Lessing. In a less crude system, like 

 that of Rosenbusch, this element disappears, but the underlying idea still 

 remains. There is a division into families, such as the granite-family and the 

 gabbro-family, but the term, in so far as it implies blood-relationship, is a 

 misnomer. The augite-granite of Mull is evidently more closely related to its 

 associated gabbro than it is, say, to the biotite-granite of Peterhead or the 

 hypersthene-granite of Madras. 



The differentiation which evolves a varied series of plutonic rocks from a 

 common parent-magma is clearly not of the same kind as that which gave rise to 

 the parent-magma itself. It appears that the external mechanical element is here 

 a less important factor, and the variation set up is, therefore, more closely in 

 accordance with the uninterrupted course of crystallisation. This is clearly 

 indicated when we compare the order of intrusion of the several rocks of the 

 series with the order of crystallisation of their constituent minerals. The 

 history of the series is in a sense epitomised in the history of each individual 

 type, corresponding in both cases to continued fall of temperature and pro- 

 gressive change in the composition of the residual magma. In a large number 

 of rocks, more particularly those of complex constitution, the order of crystal- 

 lisation follows Rosenbusch's empirical law of decreasing basicity, and the plu- 

 tonic intrusions then begin with the most basic type and end with the most acid. 

 I mention this only to point out that, while the larger divisions of our ideal 

 classification will have a certain geographical and tectonic significance, the 

 sub-divisions will show a certain correspondence with the sequence in time of the 

 various cognate rock-types. 



To pursue the subject further would serve no useful purpose. It is clear 

 that, if a natural — by which I mean a genetic — classification of igneous rocks is 

 ever to become a reality, much work must first be done both in the field 

 and in the laboratory, each petrographical province being studied from the 

 definite standpoint of the evolution of its rock-types from one parent stock. 

 Such researches as those of Brogger in the Christiania province may serve as a 

 model. It would be rash to venture at present more than the most general 

 forecast of the lines which future developments may follow ; but I think it calls 

 for no less hardihood to set limits to what may ultimately be possible in this 

 direction. There are those who would have us abandon in despair all endeavour 

 to place petrography upon a genetic basis, and fall back upon a rigid arbitrary 



