On the Formation of Pegmatite. ^71 



upon the composition, temperature and pressure prevailing 

 during the time of cooling of the magma, as well as upon 

 the alterations in these conditions,and finally upon the action 

 of special "agents mineralisateurs." As the crystallization 

 intervals of the different minerals separating out from the 

 magma frequently overlap for considerable distances, the 

 order, as every mineralogist knows, is not an absolute one, 

 but only determined within certain limits ; it is only right 

 to mention here also that the order of crystallization has 

 been determined mainly with regard to the principal min- 

 erals and in a less degree for a number of accessory minerals 

 produced by special processes, although for many of the 

 latter the period of formation is pretty sharply defined. 



For the nephcline syenite pegmatite veins of the boundary 

 zone of the Langesundfjord it was evidenced above, that the 

 order of crystallization over large areas is, within certain 

 limits, a definite one, and the same as in the corresponding 

 boundary rock of the Laugenthal which is undoubtedly 

 eruptive. Similarly, for example, for the genuine granitite 

 pegmatite veins (with black biotite) the crystallization 

 order is in all probability a definite one and corresponds to 

 that of the massive granitite. 



This circumstance is also one of the strongest arguments 

 for the eruptive genesis of the genuine pegmatitic veins ; 

 it can as little be accidental for the latter as for the rocks 

 which have certainly crystallized out from a magma, and 

 must in both cases be explained in similar ways. 



