260 COSMOS 



the Andes.* In descending toward the South Sea, fit/in Cax- 

 amarca toward Guangamarca, I have observed vast masses 

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

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

 Can these beds have been transformed from sandstone, as 

 Elie de Beaumont conjectures in the case of the quartz strata 

 on the Col de la Poissonniere, east of Brianpon ?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 laminsB of 

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

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

 at 08 lat, on the European slope of the Uralian Mountains, 

 bear a geognostic relation to the black carboniferous dolomite 

 of AdolfTskoi$ 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, we 

 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 of 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 arid 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 ./Etna. IT The veins that 



* Humboldt, Essai Oeogn. sur le Gisement det Roches, p. 93 ; Asit 

 Centrale, t. iii., p. 532. 



t Elie de Beaumont, in the Annales des Sciences NaturcUes, t. xv., ]> 

 3G2 ; Murchison, Silurian System, p. 286. 



t Rose, Rcise nach dem Ural, bd. i., s. 364 und 367. 



$ Leop. von Bucb, Bricfe, s. 109-129. See, also, Elie de Beaumont 

 On the Contact of Granile with the Beds of the Jura, in the Mm. Ge~ol. 

 t. ii., p. 408. || Hoffman, Reise, s. 30 und 37. 



*$ Oa the chemical process i:i the formation of .specular iron, see Gay 



