142 Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 



The pneumatolitie elements Lave been in the magma and 

 reacting rather during the time of the consolidation of these 

 riebeckite rocks than later (as I have attempted to show above). 

 The study of the inclusions of riebeckite granite give further 

 support for this statement. Such inclusions have been men- 

 tioned by JBrogger, Washington, and White, but do not seem 

 to have aroused much interest I have found many inclusions 

 in the granite of Jacobdeal and Piatra rosie, and have observed 

 some in the Quincy granite. They may be classified as 

 follows : 



Homogeneous inclusions, that is aggregates of riebeckite 

 which are often fibrous (crocidolite ?), with spongy quartz, 

 zircon and hematite ; the riebeckite forms large prisms but is 

 not well developed. 



Pneumatogeneous inclusions, that is, those formed by miner- 

 alizing vapors which are porous or hollow, with a great deal of 

 fluorspar, galena, pyrites, pyrotite, mispikel, hematite and 

 many earthy looking minerals which are certainly alteration 

 products of other minerals ; riebeckite and augite-segirite are 

 rare, feldspar is more frequent and there is very little quartz.* 



Polygenous and enalogenous inclusions formed by varying 

 combinations of processes, of variable size and composition ; 

 in such inclusions occur large crystals of orthoclase and albite, 

 pyroxenes, amphiboles (but no riebeckite), astrophillite, mica, 

 etc. Through the assimilation of these inclusions local varia- 

 tions in the composition and structure of the granite arise, 

 and rocks are formed (endopligenons inclusions) of the types 

 of nordmarkite, akerite, grorudite, paisanite and even quartz- 

 pulaskite and solvsbergite. The occurrence of these different 

 rocks as inclusions is very striking in the many quarries of 

 Dobrogea and also in specimens from Quincy, Massf. In 

 many of these rocks riebeckite is replaced by katoforite or 

 barkevikite, and this fact can be explained, in my belief, by 

 variation in the composition of the magma nnder definite limits, 

 while pressure and mineralizers remain the same as in the 

 main mass. 



Lacroix has described from Ampasibitika contact rocks of a 

 riebeckite granite (perhaps in part inclusions) and states that 

 they contain the same minerals, riebeckite — sometimes in pseu- 

 domorphic forms — segirite, fluorspar, spinel and zircon, which 

 occur in the granite. The contact of Dobrogea does not show 

 marked metamorphism ; there are epidote, pyroxene and 

 amphibole hornfelses, but without riebeckite, and it may be 



* G. Murgoci, Minerale din Dobrogea. Publicatiunile Soc. Naturalistilor, 

 2, 1902. Bucharest. 



f I may further remark that the analyses of an enclosure in granite of 

 Pigeon Hill, by Washington (loc. cit.), does not differ at all from the akerite 

 analyzed by the same investigator. Journal of Geology, 1898-99. 



