ORIGIN OF RIEBECKITE ROCKS 575 



The fifth paper was 



SOME CRYSTALLINE ROCKS OF SAN GABRIEL MOUNTAINS, CALIFORNIA 

 BY RALPH ARNOLD AND A. M. STRONG 



The paper was discussed by E. H. Kraus and H. S. Washington. It 

 is printed as pages 183-204 of this volume. 



The sixth paper was 



SUGGESTION AS TO THE ORIGIN OF RIEBECKITE ROCKS 

 BY G. M. MURGOCI * 



Riebeckite rocks have been found by the author in the Dobrogea, and have been 

 studied in situ in relation to the rocks around them, especially the granites, dio- 

 rites, porphyries, and diabases, and in the laboratory, where they have been com- 

 pared with riebeckite rocks from Scotland, Wales, Rhenish Prussia, and Quincy, 

 Massachusetts. In the Dobrogea they form large patches and undulating zones, 

 so called " Schlieren," mixed at random with similar rocks without riebeckite, but 

 very acid and very poor in black constituents. In all riebeckite rocks a spongy, 

 so to speak, a pseudopoikilitic texture, has, as is well known, been observed. 



Riebeckite has been formed continually during the whole time of the consolida- 

 tion of the rock. It is sometimes found as microlites included in other constitu- 

 ents, and as patches cementing feldspars, and even quartz ; also in microlitic cavi- 

 ties or in cracks of minerals and rocks filled by pegmatitic masses. Zircon 

 (Lacroix found up to T. 5 per cent), which accompanies riebeckite, has been formed 

 also during the entire period of consolidation of the rock. Some riebeckite under- 

 goes reactions immediately after its formation, such as yielding pseudomorphs of 

 zircon and quartz after it, chloritization, variation in its composition at the surface 

 or in patches, etcetera. 



The study of the inclusions of riebeckite granite, which are found in great num- 

 bers in the granite of Dobrogea, throws light on many questions. There are homo- 

 genetic, pneumatogenetic, and polygenetic inclusions, the first-named being separa- 

 tions of riebeckite, with spongy quartz, zircon, titanite, hematite, pyrochlore, 

 etcetera, as large crystals. The pneumatogenetic inclusions are hollow, with a great 

 deal of fluorspar, zircon, titanite, galena, pyrites, pyrrhotite, mispickel, hematite, 

 etcetera. In the polygenetic inclusions large crystals of orthoclase and acid plagio- 

 clase, pyroxenes (augite, aegerite), amphibole, hornblende (common hornblende* 

 arfvedsonite), black mica, chlorite, and uralite. 



The same inclusions have been observed by the author in the granite of Quincy, 

 Massachusetts. 



The magma which has given rise to the riebeckite rocks develops from a reservoir 

 by a process of magmatic differentiation, caused by the abundance of mineralizers, 

 which also keep it acid. On account of the impermeability of the layers of argil- 

 laceous shales between which it has been introduced, the mineralizers are not lost ; 

 they continue to act on the magma and to combine witb it. 



The presence of fluorspar, zircon, and sulphides as constituents of riebeckite 

 rocks, the existence of pneumatogenetic and polygenetic inclusions, and the contact 



* Introduced by J. F. Kemp and E. O. Hovey. 



