Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 143 



added that in general this mineral does not occur in genuine 

 schists. The well known forellengranulite (orthogneiss) of 

 Gloggnitz,* the riebeckite granulite of Alter Pedroso in Por- 

 tugal^ are surely eruptive rocks,;}; and their characters are 

 quite similar to those summarized above. Lacroix§ mentions 

 genuine schists with riebeckite from Corsica, from the Alps of 

 Savoy, from Bulgar Dagh in the Taurus, etc., which occur asso- 

 ciated with glaucophane schists. It is to be noted that the rie- 

 beckite of these rocks occurs as needles and fibers, radially 

 spheruiitic or lenticular, and it has always been compared with 

 the tourmaline of luxullianite. In some rocks, especially non- 

 crystalline ones, needles and fibers of riebeckite(?) are found in 

 such relation to large crystals of riebeckite or segirite, that they 

 look like secondary products and are quite similar to those 

 described by Cross, Lane, White and Washington. Cross has 

 identified the blue amphiboleof Silver Cliff with the crocidolite 

 of Laeroix | ; perhaps it is the case that all these kinds of blue 

 amphiboles should be referred to crocidolite.^ Crocidolite 

 seems to be different from riebeckite both in occurrence and 

 composition ; its genesis in riebeckite rocks seems to show that 

 it should there be primary, indicating circumstances which Wein- 

 schenk presupposes in piezocrystallization ; and many characters 

 of crocidolite are in accordance with this hypothesis, as S. 

 Franchi has deduced for the glaucophane (and crocidolite) 

 schists. 



From the above mentioned characters of riebeckite and of 

 the rocks in which it occurs, we may, to some extent, deduce 

 the circumstances under which these rocks have been formed, 

 especially those from Dobrogea. 



The magma which has given rise to the riebeckite rocks 

 ascends from an alkaline magma basin, from which it is derived 

 by a process of magmatic differentiation. The molten mass, 



* H. Keyserling, Der Gloggnitzer Forellenstein : Tschermak's Mineral, 

 petrogr. Mittheilungen, xxii, 1903. 



f Y. de Souza Brandao, Ueber einen portugesischen Alkali gran ulit. Cen- 

 tralblatt fiir Mineralogie, Geologie nnd Paleontologie, 1902, p. 50. 



% Dr. Teall and Fleet have found, last summer, in Wales, a riebeckite gneiss 

 which in mineralogical and petrographical characters seems a granite gneiss. 



£ A. Laeroix, Mineralogie de la France, i, p. 697. 



|| With the blue amphibole described by W. Cross, C. Palache has identi- 

 fied another blue amphibole, crossite, from an albite schist of Berkeley, 

 Bui. of Geol. Depart. Calif. Univ., i, 1894. I may observe, however, fol- 

 lowing the comment of Lane and my own determinations, that this cannot 

 be done ; the crossite of Berkeley is a blue amphibole (glaucophane) which 

 has the plane of the optic axes perpendicular to (010), b : c=16°. But I have 

 found the amphibole of Cross in a syenite from Spanish Peak, Cal., and it 

 looks like crocidolite. 



^[ S. Franchi, Prof. Louderback. etc., have found crocidolite quartzite in 

 the area of glaucophane schists which are very similar to the riebeckite 

 schists described by Laeroix. 



