402 
THE GEOLOGIST. 
maiite the quartz supports the crystals of ortlioclaso, and, on the con- 
trary, impresses itself on them.* 
Mr. Gr. Rose lias pointed out that in granite, the silver- white mica, 
which is very aluminous, generally envelopes the blackish or pinch- 
beck brown, which is fcrro-maguesian. On the other hand, I have 
established a reciprocal envelopment of the former in the granite of 
Cornwall, which is employed in buildings in London ; its micas 
form, in fact, agglomerations in which it is the ferro-magnesian mica 
which, on the contrary, envelopes the aluminous mica (Hgn. 3). 
Fig. 3. — Ferro-magnesian Mica enveloping Aluminous Mica. 
Finally, reciprocal envelopment is also observed in the same rock, 
and, what is more, at the same point. For, according to Mr. 
Scheerer, the orthoclase felspar of the syenite of Norway envelopes 
the mesotype (spreustein), which, in its turn, envelopes a kernel of 
this same felspar ; so that a zone of mesotype is confined between 
two zones of felspar. 
Mr. Blum has shown that the garnet of Pittigliano, in Italy, con- 
tains in its cavities crystals of idocrase and garnet, which penetrate 
and envelope each other mutually. According to Messrs. Seytfert 
and Sochting, it is the same with rutile and hematite in the valley of 
Tavetsch. 
The following are the principal examples of reciprocal envelop- 
ment : — Pyrite and fluor, galena and fluor, fluor and quartz, rutile 
and hematite, magnetite and asbestus, magnetite and talc, magnetite 
and chlorite, franklinite and willemite, chrj'soberyl and quartz, 
corundum and diaspore, qu^artz and emerald, quartz and garnet, 
quartz and topaz, quartz and baryte, quartz and s?heelite, quartz and 
calcite, quartz and chalybite, augite and hornblende, emerald and 
topaz, garnet and idocrase, garnet and gypsum, garnet and calcite, 
epidote and scapolite, aluminous mica and ferro-magnesian mica, 
mica and andalusite, mica and kyanite, mica and tourmaline, felspar 
and tourmaline, felspar and mesotype, felspar and calcite, andalusite 
and kyanite, bastite and serpentine. 
To resume, reciprocal envelopment is observed among all the 
* Bulletin de la Socicte Geologique, 2nd seiic, t. x., p. 568. 
