402 



THE GEOLOGIST. 



matite tlie quartz supports the crystals of ortlioclase, and, on tlie con- 

 trary, impresses itself on them.* 



Mr. Gr. Rose has pointed out that in granite, the silver- white mica, 

 which is very aluminous, generally envelopes the blackish or pinch- 

 beck brown, which is feiTO-magnesian. On the other hand, I have 

 estabhshed 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 (lign. 3). 



Fig. 3. — Ferro-magnesian Mica enveloping Aluminous Mca. 



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. Seyfiert 

 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 mllemite, chrysoberyl and quartz, 

 corundum and diaspore, quartz 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 Societe Geologique, 2nd serie, t. x., p. 568. 



