SUMMARY. 249-1 



form of more or less wide reaction zones, in which the peculiarities 

 of the charnockite series on the one hand and of its neighbour on 

 the other are found to be intermingled. But it is not difficult, 

 nevertheless, to distinguish between such apparent passage forms, 

 which separate dissimilar igneous types, from real transitions, which 

 join genetically related, adjacent, igneous masses. Marginal inter- 

 penetrations and wide zones of contact products may increase the 

 difficulties of delineating the boundaries of large crystalline masses 

 on large- scale maps, but such border difficulties do ;not detract from 

 the individuality of the main-mass as a geological unit. Near the 

 junction, for instance, between the charnockite series and the great 

 gneissose granite of the Baramahal division of Salem, there might 

 be local difficulties in drawing a sharp boundary line, but, by all the 

 points which constitute rock individuality, the contrasts between 

 these two formations are unmistakeably marked. 



The average composition, and by far the most prevalent type, 

 of the charnockite series has an intermediate silica percentage 

 (see p. 146), and the occurrences of anything approaching large 

 masses of acid or basic types are comparatively rare, whilst, 

 pyroxenites never form more than narrow bands or small lenses. 

 Although these four distinct types belong to separate rock groups, 

 if regarded from the purely lithological point of view, there is no 

 doubt about their consanguinity ; and the writer would consequently 

 refer to the charnockite series as another instance which shows 

 that, from a geological survey standpoint, the recognition of petro- 

 graphical provinces is a much more natural system of classification 

 than the customary subdivision of rocks according to silica per- 

 centage, which is true only for hand-specimens and of value only 

 in the laboratory, but possibly still a convenient system for im- 

 parting lithological information to elementary students. 



IC2 ( I--3* > 



