230 HOLLAND : CliARNOCKlTE SERIES. 



or scattered through the rock as isolated granules. The remaining 

 constituents of the rock are characteristic for the basic members of 

 the charnockite series : green-brown hornblende, pale blue-green 

 augite, hypersthene, water-clear felspar with very undecided twin 

 bands, and iron-ores. The structure is granulitic almost invariably, 

 but the rocks are finer in grain than the average massive form of 

 the charnockite series ; the difference in grain, however, is just the 

 same in degree and kind as we should expect to find between, say, 

 a large stock of gabbro and a dyke of its corresponding diabase. 



Here then we have a rock which shows its intrusive, igneous 

 origin as plainly as any diabase dyke ever does, and yet in compo- 

 sition and structure it shows all the essential points of the charnock- 

 ite series. The instances examined are sufficiently numerous to 

 show that we are not dealing with a merely local accident in des- 

 cribing what is considered to be the chilled selvages of these dykes. 

 No one probably would be rash enough to assert that all the 

 pyroxene-granulites we know belong to one formation, one petro- 

 graphical province, but here we have rocks unquestionably igneous 

 in origin yet similar in all essential respects to the adjacent typical 

 pyroxene-granulites (charnockite series). With, therefore, as good 

 ground as we usually get in petrography it is safe to accept this as a 

 corroboration of the other evidences which point to the igneous 

 origin and intrusive behaviour of the charnockite series. 



(c) Contact metamorphism. 



The recognition of distinct contact zones amongst the old crys- 

 t L , talline rocks, will, from the nature of the case. 



Imperfection or evi- » ' 



dence - be always extremely difficult. In the first place, 



rocks already crystalline are seldom susceptible to the action of 

 an invading igneous mass, and, secondly, subsequent metamor- 

 phosing agencies would obliterate the results of contact action in the 

 oldest rocks. The difficulty of distinguishing between the results of 

 the old contact and the subsequent metamorphism will always, of 

 ( 112 ) 



