70 Canadian Record of Science. 



appellation " pegmatitic." In the boundary zone on 

 the Langesundfjord are many veins, truly pegmatitic, 

 which are less coarse-grained than the surrounding 

 laurvikite, e. g. the hiortdahlite-bearing vein of Lan- 

 godden, Ober-Aro. It is the habitus as a whole which 

 determines this: the relatively irregular nature of min- 

 eral composition and structure in the different parts of 

 the vein, the foreign appearance of the veinstone caused by 

 the wealth in accessory pneumatolitic minerals, the in- 

 timate intergrowth with the wall rock, etc. The very in- 

 definiteness of these limitations, which must always adhere 

 to the definition of the pegmatite idea, is itself character- 

 istic and gives a completely correct expression of the actual 

 condition of things, that altogether between pegmatitic and 

 not pegmatitic rock formation in nature often no sharp 

 line can be drawn and indeed is not present. 



Structure is, as Lossen l has so aptly said " of the first 

 rank as an exponent of the geological relationships of 

 rocks ;" that the structural peculiarities of the pegmatite 

 veins are in the main such as we otherwise find in the case 

 of undoubtedly eruptive rocks only, is therefore one of the 

 strongest evidences of their eruptive origin as veins formed 

 principally through magmatic solidification. 



4. In connection with the structural peculiarities, stand 

 the age relations of the individual vein minerals, which may 

 be comprehended under the common conception "order of 

 crystallization" (Krystallisationsfolge). 



One of the principal results of the study of eruptive 

 rocks in the light of the newer petrography is that, within 

 certain limits, a regularity in the order of crystallization of 

 the minerals of these rocks can be observed ; researches by 

 Rosenbusch, Michel-Levy, Iddings and numerous other in- 

 vestigators have determined pretty certainly the leading 

 features of this regularity in the order of crystallization for 

 a large number of rock-types. The sequence is dependent 



1 K. A. Lossen : " Uber die Anforderungen dcr Geologie an die petrographische 

 Systematik," Jahrb. d. kgl. preuss. geol. Landesanstalt f. d. J. 1883, Berlin, 1884, 

 p. 512. 



