Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 145 



tion of the magmas ; its occurrence in cavities and its para- 

 genesis with zircon, pyrochlore, fluorspar, sulphides, etc.; its 

 presence as large crystals in pegmatites in immediate relation 

 with veins of fluorspar or cryolite, and its absence in non- 

 eruptive rocks, are many facts which support the view as to its 

 origin presented in this paper. 



The mineralizers which aid to produce it are not rich in 

 water and sulphur vapors, but are characterized by an abund- 

 ance of Zr, which has played a part in riebeckite granite' 

 similar to that of Sn in cassiterite granites. Zircon also forms 

 throughout the whole period of consolidation. One can cor- 

 relate : tourmaline — Sn ; riebeckite — Zr and katoforite (or 

 another soda amphibole) — Ti. 



The magma has been fairly rich in A1 2 3 , Na 2 0, and K 2 0, 

 but a large quantity of soda and iron have been brought in by 

 mineralizers and the genesis of riebeckite facilitated. The 

 occurrence of masses and small areas of hsematite and limon- 

 ite, around or across the inclusions and schlieren, does not 

 come from secondary alteration, but from areas which at the 

 moment of consolidation were still more or less impregnated 

 with iron compounds and water vapors. 



Doubtless new upwellings of magma and of mineralizers 

 have caused some transformations in the minerals already 

 formed in the riebeckite and segirite, but needles and fibers 

 of crocidolite could be formed in eruptive rocks, as well as in 

 metamorphic schists, as a phenomenon of piezocrystallization, 

 which is quite in accord with the process which I have here 

 tried to sketch. 



Am. Jour. Sci. — Fourth Series, Vol. XX, No. 116. — August, 1905. 

 JO 



