144 Mtcrgoci — Genesis of Riebeckite and Riebeckite Rocks. 



isolated perhaps in an anticline or in a laccolith, is maintained 

 for a long time as a mother liquor in a state of hydrothermal 

 fusion in which there swim crystals already formed, or in pro- 

 cess of formation. On account of the impermeability to vapor 

 of the beds of shale, quartzite, etc., between which it has been 

 introduced, the mineralizers cannot escape, they continue to 

 act on the magma and to be gradually assimilated. The pres- 

 ence of fluorspar, zircon, titanite, and sulphides as constituents 

 of the riebeckite rocks, the occurrence of pneumatogeneous 

 and polygeneous inclusions and of schlieren with characteristic 

 minerals confirm this view. 



The chief factors in the formation of riebeckite rocks are 

 the pressure and definite mineralizers ; with the variation of 

 these tw r o factors and of the composition of the magma, the pro- 

 ducts of crystallization are also changed. Only under high' 

 pressure due to tectonic movements and to the persistent reten- 

 tion of mineralizing vapors, and with a large quantity of the 

 latter present, could riebeckite be formed ; if one of these 

 factors varied, especially the pressure, segirite would then occur 

 in addition to riebeckite. Of course the pressure, the mineral- 

 izers and the composition of the magma usually differ from 

 one point to another, as may be seen from the quantitative and 

 qualitative variation of the mineral elements of the rocks. 

 Especially had the assimilation of inclusions of neighboring 

 rocks provoked such variations of chemical and physical con- 

 ditions, that riebeckite could no longer be formed. 



In addition to the chemical, a mechanical action was present ; 

 new upwellings of fluid magma and of mineralizers cause 

 streams and vortices in the consolidating molten mass. These 

 influence the crystallization and aid in the formation of 

 schlieren ; in the more quiet parts a pegmatitic or a granular 

 structure is produced ; there in the streams and more agitated 

 areas a fluidal or a protoclastic structure originates ; the rapid 

 sinking of the temperature, the loss of mineralizing vapors, 

 or lowering of pressure in other parts, determines a porphyritic 

 structure with the two periods of consolidation more or less 

 well pronounced of the mineral constituents.* 



Riebeckite forms only in the relatively most acid magmas, 

 and especially under the influence of mineralizers : its compo- 

 sition, content of fluorine, long period of crystallization from 

 the beginning up to the end of and even after the consolida- 



* In respect to the conception of the role of the mineralizers and inclu- 

 sions, their influence at the time of consolidation of the magma, the form- 

 ing of schlieren and of the structure, I incline toward the views of the 

 French penologists. None the less, in the ideas of Prof. E. Weinschenk I 

 have found many points a propos to these views ; the short and clear chap- 

 ters on these questions in his book, Gesteinskunde I, 1902, excite my hearti- 

 est admiration. 



