Murgoci — Genesis of Riebeckite and Riebeckite Rocks. 135 



(Madagascar, Corsica, Colorado), Pelikan (Socotra, etc.), Har- 

 ker (I. of Skye, etc.), and many interesting remarks in Rosen- 

 busch's writings, where he discusses am pinhole and pyroxene 

 granites, etc. The very interesting example is given by Lacroix* 

 in the rhyolite of Somalis, where schlieren of microgranite 

 were observed in the rbyolitic mass. 



I have recently compared my riebeckite rocks with those 

 from Scotland, Wales and Massachusetts, and have been able 

 to extend and generalize the conclusions of Lacroixf deduced 

 from observations on different granites with riebeckite and 

 segirite, and I would include also microgranitic and porphy- 

 ritic types from granite and quartz syenite series. Summariz- 

 ing the observations, we may emphasize the characters which 

 reveal to us the genesis of the riebeckite rocks as follows : 



1. The massives with riebeckite rocks are characterized by a 

 great variety of types rich in soda as schlieren or as dikes. 

 In such massives there is very frequently a tendency towards 

 a pegmatitic or miarolitic structure in some of the schlieren, 

 and a fluidal or microgranitic one in others. Schlieren with a 

 protoclastic structure may be observed in the noncrystalline 

 types and also in porphyritic ones. 



2. Variations occur not only in the structure, but also much 

 more in the constituents, especially in the dark-colored ones, 

 which are, however, almost always amphibole, or pyroxene, or 

 both, often grown together or zonal. The amphibole in the 

 most acid rocks is of the arfvedsonite-riebeckite group, in the 

 relatively basic ones it is of the kataforite-barkevikite group ; 

 the pyroxene is segirite or gegirite-augite. ^Egirite nearly 

 always accompanies riebeckite ; but while segirite occurs often 

 as well developed, more or less idiomorphic, crystals, the large 

 patches of riebeckite have, as is well known, a spongy, 

 poikilitic structure with a well marked allotriomorphic devel- 

 opment. The character of its occurrence, in even granular 

 rocks and in porphyries, shows that riebeckite has been formed 

 continually during the whole time of the consolidation of the 

 magma. It is found as microlites, in small and minute prisms 

 and needles or fibers, included in other constituents such as 

 feldspar, quartz, etc.; as large dark-blue crystals grown together 

 with other minerals, such as segirite, zircon, and pyrochlore ; 

 as patches cementing feldspar and even quartz ; as poikilitic 

 shreds and beads in the groundmass of the porphyritic rocks ; 

 further it is found in miarolitic cavities, in pneumatogeneous 

 inclusions, in the cracks and druses of the rocks filled by 

 pegmatitic masses, etc. 



* A. Lacroix, Les Rhyolites a aegirine and' riebeckite de Somalis. C. R., 

 Ac. Sc Paris, cxxviii, 1899. 



f A. Lacroix, Materiaux pour la Mineralogie de Madagascar. Nouvelles 

 Archives du Museum d'Hist. Natur. IV e s, 1902, p. 164, etc. See also Les 

 travaux de M. A. Lacroix, 1903. 



