BY H. I. JENSEN. 535 



Ch. iii. Occurrence of Foyaitic Rocks. 

 A. Europe. 



1. Christiania Gebiet, Norway.* — In this district we have one 

 of the largest alkaline areas in the world, and thanks to the 

 work of Brogger it is the most thoroughly examined. There is 

 a complete series of intrusive rocks ranging from the most acid 

 to the most basic. The oldest masses consist of proterobase and 

 diabase; next in age are the granites and granitites; then come 

 the nordmarkites, and still later the more alkaline series com- 

 prising the rhombenporphyr, laurdalite and laurvigite, and lastly 

 gabbrodi abase, augite, porphyrite, mandelstein, melaphyre, and 

 tuffs. 



Whether there is any definite relationship, pointing to a 

 common origin, between the calcic groups (such as gabbro and 

 granite) and the alkaline (such as nordmarkite, laurvigite, etc.) 

 seems to be extremely doubtful. It has been shown that some 

 rocks, such as proterobase, camptonite, and essexite, belong 

 sometimes to the one and sometimes to the other group. 



But in the study of the different members of the alkaline 

 group of the region Brogger has found a complete series com- 

 prising proterobase, camptonite, farrisite, foyaite, ditroite, laur- 

 dalite, rhombenporphyry, soda-minette, tinguaite, nordmarkite, 

 solvsbergite, grorudite, lindoite, bostonite, etc., some being 

 coarse-grained and occurring in large masses, others fine-grained 

 or porphyritic, occurring in small lamprophyric and aplitic dykes 

 (BrSgger, op. cit. Vols. i. and iii.). 



The sequence has also been determined, and it is shown that 

 the earlier masses were little differentiated and had the composi- 

 tion of laurdalite, whilst the more differentiated acid and basic 

 dykes were intruded later, often into a plastic mass which had 

 not yet assumed the solid condition. 



* " Die Eruptivgesteine des Christiania Gebiets." Vols, i., ii. and iii. By 

 Dr. Brogger. 



