266 COSMOS. 



the Andes.* In descending toward the South Sea, fiom Cax- 

 amarca toward Guangamarca, I have observed vast ra,asses 

 of quartz, from 7000 to 8000 feet in height, superposed some- 

 times on porphyry devoid of quartz, and sometimes on dioritc. 

 Can these beds have been transformed from sandstone, as 

 EHe de Beaumont conjectures in the case of the quartz strata 

 on the Col de La Poissonniere, east of Brian^on ?t In the 

 Brazils, in the diamond district of Minas Geraes and St. Paul, 

 which has recently been so accurately investigated by Clausen, 

 Plutonic action has developed in dioritic veins sometimes ordi- 

 nary mica, and sometimes specular iron in quartzose itacol- 

 umite. The diamonds of Grammagoa are imbedded in strata 

 of solid silica, and are occasionally enveloped in lamiuje of 

 mica, like the garnets found in mica slate. The diamonds 

 that occur furthest to the north, as those discovered in 1829 

 at 58'^ lat., on the European slope of the Uralian Mountains, 

 bear a geognostic relation to the black carboniferous dolomite 

 of xldolfiskoit and to augitic porphyry, although more accu- 

 rate observations are required in order fully to elucidate this 

 subject. 



Among the most remarkable phenomena of contact, v/e 

 must, finally, enumerate the formation of garnets in argilla- 

 ceous schist in contact with basalt and dolerite (as in Northum- 

 berland and the island of Anglesea), and the occurrence of a 

 vast number of beautiful and most various crystals, as garnets, 

 vesuvian, augite, and ceylanite, on the surfaces oi" contact be- 

 tween the erupted and sedimentary rock, as, for instance, on 

 the junction of the syenite of Monzon with dolomite and com- 

 pact limestone. § In the island of Elba, masses of serpentine, 

 which perhaps nowhere more clearly indicate the character of 

 erupted rocks, have occasioned the sublimation of iron glance 

 and red oxyd of iron in fissures of calcareous sandstone.il We 

 still daily find the same iron glance formed by sublimation 

 from the vapors and the walls of the fissures of open veins on 

 the margin of the crater, and in the fresh lava currents of the 

 volcanoes of Stromboli, Vesuvius, and -^tna.lF The veins that 



* Humboldt, Essai Oeogn. sur le Gisement des Roches, p. 93 ; Asie 

 Ccntrale, t. iii., p. 532. 



t Elie de Beaumont, in the Annales des Sciences Naturelles, t. xv., ]) 

 362 ; Murchison, Silurian System, p. 286. 



+ Rose, Reise nach dem Ural, bd. i., s. 364 und 367. 



<^ Leop. vou Bucb, Briefe, s. 109-129. See, also, Elie do Beaumont, 

 Oil the Contact of Granite toith the Beds of the Jura, in the 31^m. GeoL, 

 t. ii., }). 408. II Hoifman, Reise, a. 30 niid 37. 



11 On the chemical process iu the f<jrmatioa oi' bpeculur i:'oii, s(.n^ Guy 



