124 HOLLAND: CHARNOCK1TE SERIES. 



CHAPTER II. 

 Distinguishing features of the Series. 



The unaltered varieties of this series present such a remark- 

 able and unmistakeable individuality in macros- 

 ConSa me U mber S ? f the C0 P ic characters that they are easily distin- 

 guished in the field from the other crystalline 

 rocks with which they are associated. On account of the striking 

 nature of the characters which give the different varieties of the 

 group such an unmistakeable family likeness — "consanguinity," to 

 use Iddings' expressive term — the peculiar characters distinguishing 

 one variety from another are so well masked, that the forms con- 

 taining sufficient free quartz and siliceous minerals to raise the 

 silica percentage to that of the granites might very easily be con- 

 fused with the varieties whose mineral composition agrees with that 

 of typical norites. 



The leading features in hand specimen of the common, that is 



the unaltered and medium-grained types, are 

 Macroscopic characters. 



the blue-grey to dark-green colour, the sub- 

 conchoidal fracture, and the absolutely fresh condition of the rock. 

 Examination of the coarse-grained types with the naked eye, or of 

 the fine-grained ones with the lens, shows that quartz when present 

 is almost invariably blue in colour, like that of the well-known Rum- 

 burg granite (granitite) ; the felspars present a similar blue or blue- 

 grey colour, and, but for their cleavage faces, might easily be 

 mistaken for quartz. These minerals, therefore, which generally 

 give the lighter colours to our ordinary acid rocks, are almost as dark 

 in the Madras " pyroxene-granulites " as the associated ferro- 

 magnesian silicates ; and this circumstance, together with the fact 

 that opaque iron-ores are equally abundant in all types, are the 

 principal causes which give the acid and basic members of the series 

 such a similarity of appearance in hand-specimen. 



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