18 



THE GEOLOGIST. 



Phlogophite mica occurs in reddish-brown laminae, which invests 

 in a very remarkable manner the augite of Monroe, and which are 

 disposed almost perpendicularly to its faces. 



The mica which is formed in hornblende, augite, iolite, pinite, 

 chlorophyllite, presents sometimes a determined direction, and its 

 laminae are parallel. 



The felspar of the syenite of Norway, as we have seen, envelopes 

 natrolite, which in its turn envelopes a kernel of this same felspar. 

 Fluor envelopes pyrite concentrically. 



Symmetrical relations may again be well observed in macle* (an- 

 dalusite), which has symmetrically grouped the schist which it 

 envelopes. It appears also in certain crystals of hyalin quartz, 

 which contain small grains of quartz, which are crystalline and very 

 distinct ; these latter are grouped parallel to the faces, either of the 

 regular hexagonal prism, or of the pyramid which surmounts it. 

 This, for example, M. Des Cloizeaux has observed in the quartz of 

 Brazil. He has also shown that Iceland spar contains isolated grains 

 of calcite, which are generally grouped parallel to the faces of the 

 metastatic or to those of the primitive rhombohedron. When pyrite 

 is disseminated in microscopic grains in spathic carbonate of lime, it 

 also groups itself, following the same plane ; and it is the same with 

 the chlorite (ripidolite) which is enveloped by the dolomite of Tra- 

 verselle. 



But the symmetrical arrangement may be still better characterized 

 than in the preceding examples ; and then it occurs at once in the 

 two minerals, either by relation to a centre, or by relation to axes, 

 according as one or the other case occurs ; it is hence central or 

 axial. 



Central Symmetrical Arrangement. — Metalliferous lodes sometimes 

 show a well marked central symmetrical arrangement. Thus at La 

 Chevrette, in Dauphine, spathic iron envelopes quartz, and both pre- 

 sent crystals symmetrically arranged towards a centre, from which 

 result a radiated structure. According to M. Burat, it is the same 

 with towanite, blende, and galena which are enveloped by the fibrous 

 and radiated augite of the mines of Tuscany. 



Rocks which have a globulous structure also especially afford us 

 particularly clear examples of envelopment with a central symmetri- 

 cal arrangement. In the Rapakivif of Finland and in certain por- 

 phyries, the oligoclase envelopes the orthoclase, around which it 

 forms a regular aureole. In the pyromeride % of Corsica, the globules 

 are composed of felspar crystallized in needles, which start from the 

 circumference or the centre, and which follow the direction of the 

 radii. In orbicular diorite the felspar envelopes the hornblende, and 

 ihv laminse of the two minerals are symmetrically arranged towards 



* Chiastolite. 



t For this rook see Gotta—" Gesteinslehre" p. 123. — H. C. S. 

 X Ibid, p. 102. — H. C. S. 



