Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 133 



Art. XYIII. — On the Genesis of Riebeckite and Riebeckite 

 Rocks y* by G. M. Murgoci, Bucharest. 



Recent investigations have shown that riebeckite rocks are 

 not uncommon; new occurrences are being discovered, and in 

 old localities alkali rocks described as containing black or blue 

 hornblende are often identified as really containing riebeckite. 

 These rocks are attracting special attention, because of the 

 presence of this rare sodium-iron amphibole, and because some 

 of them are the most acid of alkali rocks, rising to 78 per cent 

 in Si0 2 (according to the analyses of Butureanu, Ludwig, 

 etc.) and to 10 per cent in ]STa 2 and K 2 0. 



I have discovered these interesting rocks in Dobrogea at 

 Jacobdeal and Piatra rosief (at the mouths of the Danube), 

 and was able to study in situ their geological characters and 

 relations to the enclosing rocks. I have also compared them 

 in the laboratory with similar rocks from other localities. A 

 resume of the facts in Dobrogea is as follows : 



In the Paleozoic formations composed of quartzites, sand- 

 stones and conglomerates, calcareous and argillaceous shales 

 and crystalline limestone, the following rocks are found as 

 intrusive masses : various kinds of granites, microgranites, 

 quartz and orthoclase porphyries, diorites and olivine gabbros, 

 etc., which in general occupy the anticlines of the sedimentary 

 formations. All the foregoing rocks and the Triassic sand- 

 stone and limestone are penetrated by dikes of microgranites, 

 porphyries, diorites, pearl and porphyritic diabases, etc. It 

 has been proved satisfactorily that there have been two epochs 

 of volcanic activity : Paleozoic (pre-Permian) and Triassic. 

 Sometimes the mesocratic rocks of the two series are very 

 similar and could easily be confused one with another, if the 

 field relations are not correctly interpreted. 



The rocks of the first volcanic epoch, usually alkali rocks 

 (Mrazec), show a consanguinity obvious in the field and con- 

 firmed by investigation in the laboratory, Dobrogea being in 

 this respect a very interesting petrographical province. -Rie- 

 beckite rocks, however, are confined to the hills of the western 

 Dobrogea in two of the anticlines of the slightly metamor- 

 phosed argillaceous shales and sandstones. The anticlines have 

 a northwest-southeast strike and in the region of Carjelari run 



* Preliminary communication read before the Geological Society in Phil- 

 adelphia, December, 1904. 



f Gr. M. Murgoci, Eidicari geologice in N. Dobrogei, Bull. Soc. Ingineri- 

 loc de mine, 1898, Bucuresti. 



Among the specimens that I collected in the quarries of Jacobdeal, Prof. 

 L. Mrazec recognized riebeckite granite and described it in : Note prelim- 

 inaire sur un granite a riebsckite and aegirine des environs de Turcoaia 

 (Dobrogea). Ibidem. 1899. 



