Murgoci — Genesis of Riebeckite and Riebechite Rocks. 141 



eralizers have not a catalytic action only; it has been demon- 

 strated several times that they have an active part in the con- 

 solidation of magmas and in the formation of minerals. 

 Accordingly, in a magma of a definite composition containing 

 mineralizers and nnder a sufficient pressure to give rise to rie- 

 beckite, a part of the mineralizers (Fl, Na, Ti, Zr, etc.) play 

 an active role in the formation of the riebeckite, entering also 

 into its composition.* Where there is low pressure and the 

 mineralizers are not appropriate for the formation of riebeckite, 

 aegirite is formed, and the mineralizers, which it does not 

 require for its production, react on the magma and among them- 

 selves, giving rise to other characteristic minerals with the 

 form and paragenesis which we have seen above. 



8. The former presence of high pressure and abundant 

 active mineralizers can be clearly deduced from the study of 

 riebeckite rocks, as may be seen from the works of BrOgger, 

 Lacroix and others. Lacroix, in particular, concludes from the 

 presence of fluorspar, galena, zircon, and the pseudomorphic 

 changes and alterations undergone by riebeckite and segirite, 

 that emanations characterized by fluorine and zirconium were 

 active at the moment of consolidation, and that in riebeckite 

 rocks zirconium plays the part of tin in alkali rocks with tourma- 

 line. He, like Brogger, assumes a powerful manifestation of 

 post-volcanic activity, which in some cases has produced deposits 

 of cryolite, as at St. Peter's Dome, Colo., and in Greenland, or 

 marked transformations in the structure and composition of 

 the rocks, as in the rhyolite of Somalis. 



According to my researches in Dobrogea the post-volcanic 

 activity is almost wanting in massives with many schlieren and 

 much variation in the kinds of rocks. No pegmatitic dikes or 

 veins like those in Greenland or Norway have been seen in the 

 many quarries in Jacobdeal and Piatra rosie, and the contact 

 metamorphism of the neighboring rocks is very small. I may 

 add that, in this respect, the alkali granite of Macin and 

 Pricopami shows much greater contact phenomena and pneu- 

 matolitic post- volcanic activity. In the cracks of the Jacob- 

 deal granite I have found only a few spots coated with little 

 crystals of quartz, hgematite, very rarely fibers of crocidolite, 

 and beautiful dendrites of ferromanganese hydroxides like 

 those from the Quincy granite. 



* This deduction finds a certain verification in the composition of rie- 

 beckite ; unfortunately the existing analyses are very unsatisfactory. In a 

 recent conversation with Dr. Tassin, he informed me that he had found 

 fluorine in a riebeckite which he is analyzing/ Amphiboles with fluorine are 

 known; for example, see the pargasites, etc., in the table of analyses by 

 Hintze, and the hornblende from Grenville (Quebec) with 2'8 per cent Fl 

 (Harrington, B., this Journal, 1903, p. 392). Perhaps the loss in Konig's 

 (riebeckite) analyses (made, he says, with all precautions) may be fluorine. 



