526 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



III. The Segregation Veins. 



By O. A. Sussmilch and H. Stanley Jevons. 



17. Distribution of the Segregation Veins in the Mass. 



In petrological literature the term segregation vein has 

 a generally understood but not very precise signification. 

 It will be used here to mean all those rock masses whicli 

 are distinct in composition from the main rock of the 

 intrusion, tending to be more acid, and which in their origin 

 are intimately connected with the later stages of cooling 

 and crystallisation of the magma, whether occurring in 

 sheets or irregular shapes, within or without the main 

 mass. The term denotes the rocks commonly known as 

 aplites and pegmatites, the former finer-grained, the latter 

 coarser-grained than the main rock ; but we extend it 

 to rocks of this character in the widest sense — whether 

 their grainsize differ much or little from that of the main 

 mass, and whatever the composition of the latter. 



In the segregation veins at Prospect there occur both 

 aplitic and pegmatitic rocks, and these only according to 

 our definitions which will be explained later; but these 

 distinct rocks are not generally confined to different veins, 

 but are closely intermingled or associated in the same vein. 

 In tracing the distribution of the segregation veins through 

 the mass, therefore, no account need be taken of the rocks 

 composing them. The only general feature we have 

 observed regarding the distribution of the different types of 

 rock is that the smaller veins (those up to about 10 cm. in 

 thickness) appear to be more frequently composed of the 

 aplite only, than of either the pegmatite only, or both rocks 

 mixed. Both constituents of the segregation veins appear 

 at a distance lighter in color than the main rock, whether 

 fresh or weathered, the aplite rather more so than the 

 pegmatite, and there is thus little difficulty in finding and 

 following them over any available area of rock. The veins 

 vary much in thickness, from 1 to 120 cm. 



