DESCRIPTION OF THE CHIEF TYPES. 145 



much genetic relatives of the charnockites as the common acid " con- 

 temporaneous veins" which in granites have always been regarded as 

 the results of the final consolidation of the magma which gave rise to 

 the main masses of the rock they run through. No. 9/659 is an ex- 

 ample of such a rock occurring as veins in the typical charnockite near 

 St. Thomas' Mount. These veins run in various directions ; but more 

 generally coincide with the linear arrangement of the constituents of 

 the charnockite. Their constituents are arranged parallel to the 

 sides of the veins; but they show no sign of crushing, and this linear 

 arrangement of the constituents must, as in the case of the charnoc- 

 kite of the same mass, be referred to the results of pressure during 

 consolidation. 



Whilst on the weathered surfaces it is perfectly easy to distinguish 

 these coarse-grained veins from the fine-grained charnockite through 

 which they cut, it is almost impossible on a freshly fractured surface 

 to indicate the junction between the two, the crystals interlocking to 

 form one rock. The blue-grey colour of the fresh charnockite is 

 precisely that of the quartz-felspar veins which run through it. 

 Sections across the junction examined under the microscope show 

 no abrupt line between the veins and the main mass of charnoc- 

 kite. They differ from the normal charnockite merely in the complete 

 suppression of the hypersthene and concomitant increase in the size 

 of the two remaining constituents, quartz and felspar, which present 

 precisely the same microscopic peculiarities as the constituents of 

 the normal charnockite. 



The veins cannot thus be regarded as subsequent and distinct 

 intrusions with a separate origin; they are evidently related to the 

 charnockite, and present all the features which characterise the 

 so-called u contemporaneous" veins of ordinary granites; they would 

 be classed by Reyer as hysterogenetic schlieren forming the last 

 phase in the consolidation of the magma from which the associated 

 hypersthene-bearing rocks had previously separated. Larger masses 

 of rock having a similar mineral composition are exposed within the 

 immediate neighbourhood, and in the absence of evidence to the 

 contrary should be regarded also as members of this series. 



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