18 
THE GEOLOGIST. 
Phlogophite mica occurs in reddisli-brown laminae, which invests 
in a very remarkable manner the augite of Monroe, and which are 
disposed almost pei'pendiciilarly 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 
natrohte, which in its turn envelopes a kernel of this same felspar. 
Fluor envelopes pyrite concentrically. 
Symmetrical relations may again be well observed in made* (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 sm^mounts 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 gTouped parallel to the faces of the 
metastatic or to those of the primitive rhombohedron. When pyrite 
is disseminated in microscopic grains in spatliic 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 an^angement 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, 
accordiiag as one or the other case occurs ; it is hence central or 
axial. 
Central Symmeirical 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 pyromeridej of Corsica, the globules 
are composed of felspar crystalHzed 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 
the laminae of the two minerals are symmetrically arranged towards 
* Chiastolite. 
t For this rock see Gotta— " Gesteinslehre," p. 123.— H. C. S. 
X Ibid, p. 102.— H. C. S. 
