138 Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 



aegirite granite carries much fluorspar ; iu the pure riebeckite 

 granite, on the other hand, it is almost entirely wanting. 

 Brogger* mentions the occurrence of much fluorspar and 

 gegirite on the salbands of the groruclites of Omholtsaeter, 

 which are apophyses of the soda-granite of Kongsberg and in 

 relation with the genuine pegmatitic dikes and akmite granite 

 of Rundemyr. According to Rosenbuschf in this massif there 

 occurs also riebeckite granite. 



Brogger;); described also a peripheral transformation of bar- 

 kevikite into gegirite and lepidomelane with a rich accompani- 

 ment of fluorspar, and he explains this change as due to pneu- 

 matolitic action at the end of the consolidation of the hydato- 

 pyrogeneous mass or immediately after it. This may be 

 possible in the pegmatitic dikes of Christiania, where Brogger 

 has proved four phases *of pneumatolitic action, gegirite being 

 formed in the third phase. If the rock contains both riebeck- 

 ite and gegirite, often grown together with fluorspars and car- 

 bonates, I believe the process to have occurred in another 

 manner, riebeckite and gegirite being primary, fluorspar and 

 carbonates also. It is well known what complicated relations 

 of zonal and other infcergrowths there are between the pyrox- 

 enes and amphiboles when they occur together, especially in 

 gegirite-riebeckite rocks ; some penologists have considered the 

 gegirite as a transformation product of riebeckite, and others 

 have taken the riebeckite for a secondary product of gegirite. 

 Among other examples there might be mentioned the one 

 furnished by Cross§ in his description of amphiboles from 

 Silver Cliff, Col., and another by Boggild (loc. cit.), who has 

 found in the pegmatitic schlieren of Narsarsuk arfvedsonite 

 covered by secondary gegirite, and in the same rock riebeckite 

 with a core of gegirite. Most penologists state, however, that 

 riebeckite and gegirite are primary in their rocks. Without deny- 

 ing the later transformation of riebeckite. into gegirite, a fact 

 easy to imagine considering that their composition is similar and 

 that gegirite seems to be the more stable form, I believe, how- 

 ever, that in general riebeckite and gegirite are both primary 

 in rocks, and if transformations have taken place, they must 

 have occurred before the consolidation of the magma. 



The genesis at the same time of these two minerals of almost 

 identical composition is a very interesting phenomenon and 

 deserves to be taken for a moment into consideration : Steen- 



* W. C. Brogger, ibidem, p. 190. 



f H. Rosenbusch, Massige Gesteine, II ed., p. 59. 



\ W. C. Brogger, Die Mineralien der Syenit-pegmatitgange von Norvegien. 

 Zeitschrift der Krystallographie und Mineralogie, 16, 1890. The first part is 

 devoted to the rocks of that region. 



§ Whitman Cross, Note on some secondary minerals of the amphibole 

 and pyroxene groups. This Journal, xxxix, 1890. 



