CLASSIFICATION OF THE ROCKS. 123 



indeterminate felsitic product surrounded by radiate fringes of mica- 

 ceous and hornblendic minerals, now far gone in the processes of 

 chloritization (fig. 1). If these two rock masses, biotite-gneiss and the 

 charnockite series, merely existed as adjacent formations it is possible 

 that one of them might suffer dynamo-metamorphism without alter- 

 ation of its neighbour ; but when thin tongues of the two are so com* 

 pletely dove-tailed, it is difficult to see how the charnockite could 

 escape the metamorphism which has been so evidently disastrous 

 to the tongues of the gneiss.. The most straightforward inference 

 is that the charnockite attained its present position after the crushing 

 of the gneiss, that in fact it has trespassed across the foliation planes 

 of the latter. This implies that the charnockite has behaved after 

 the fashion of an igneous rock, and that it is younger than this parti- 

 cular biotite-gneiss. 



But there are two facts, however, which detract from the simpli- 

 city of this conclusion and introduce an element of doubt : one is 

 the absence of chilled selvages at the junction of the charnockite 

 with the gneiss, and the other is the presence in the charnockite of 

 indistinct dark patches parallel to the dark patches in the gneiss. 

 These two facts show that the charnockite tongues have not been 

 intruded into fissures in the gneiss after the fashion of a simpie 

 igneous injection ; but rather that the i( trespass " was more of the 

 nature of corrosion, imperfect pseudomorphs of the old structures 

 being preserved by the corroding charnockite magma. Here, because 

 we have no exact parallel among our simple igneous intrusions, 

 we approach the dangerous ground of speculation. But it must 

 always be kept in mind that the phenomena displayed when an 

 igneous magma attacks a rock already highly heated at great depths 

 are not likely to be analogous in all respects to the phenomena 

 presented when an igneous rock is injected into a cold formation. 



Basic varieties. 

 Basic varieties, besides occurring as small autoliths in the 



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