2GO COSMOS. 



ains by the contact and eruption of augitic porphyry (at Orsk), 

 of dtoritic porphyry (at Aufschkul), or of a mass of hyper 

 sthenio, rock conglomerated into spherical masses (at Bogos- 

 lowsk). At Monte Serrato, in the island of Elba, according 

 to Frederic Hoffman, and in Tuscany, according to Alexander 

 Brongniart, it is formed by contact with euphotide and ser- 

 pentine. 



The contact and Plutonic action of granite have sometimes 

 made argillaceous schist granular, as was observed by Gustav 

 Hose and myself in the Altai Mountains (within the fortress 

 of Buchtarminsk),* and have transformed it into a mass re- 

 sembling granite, consisting of a mixture of feldspar and mica, 

 in which larger laminae of the latter were again imbedded.* 

 Most geognosists adhere, with Leopold von Buch, to the well- 

 known hypothesis " that all the gneiss in the silurian strata of 

 the transition formation, between the Icy Sea and the Gulf of 

 Finland, has been produced by the metamorphic action ol 

 granite. J In the Alps, at St. Gothard, calcareous marl i& 

 likewise changed from granite into mica slate, and then trans- 

 formed into gneiss." Similar phenomena of the formation of 

 gneiss and mica slate through granite present themselves in 

 the oolitic group of the Tarantaise, in which belemnites are 



persthene rock, see Rose, bd. ii., a. 169, 187, uud 192. See, also, bd. 

 i., s. 427, where there is a drawing of the porphyry spheres between 

 which jasper occurs, in the calcareous gray wacke of Bogoslowsk, being 

 produced by the Plutonic influence ot the augitic rock ; bd. ii., s. 545 ; 

 and likewise Humboldt, Asie Centrale, t. i., p. 486. 



* Rose, Reise nach dem Ural, bd. i., s. 586-588. 



t In respect to the volcanic origin of mica, it is important to notice 

 that crystals of mica are found in the basalt of the Bohemian Mittelge- 

 birge, in the lava that in 1822 was ejected from Vesuvius (Monticelli, 

 Storia del Vesuvio negli Anni 1821 e 1822, $ 99), and in fragments of 

 argillaceous slate imbedded in scoriaceous basalt at Hohenfels, not far 

 from Gerolstein, in the Eifel (see Mitscherlich, in Leonhard, Basalt- 

 Gebilde, B. 244). On the formation of feldspar in argillaceous schist, 

 through contact with porphyry, occurring between Urval and Po'iet 

 (Forez), see Dufr6noy, in Geol. de la France, t. i., p. 137. It is proba- 

 bly to a similar contact that certain schists near Paimpol, in Brittany, 

 with whose appearance I was much struck, while making a geological 

 pedestrian tour through that interesting country with Professor Kunth, 

 owe their amygdaloid and cellular character, t. i., p. 234. 



\ Leopold von Buch, in the Abhandlungen der Akad. der IVissen- 

 tchaft zu Berlin, aus dem Jahr 1842, s. 63, and in the Jahrbuchern fur 

 Wistenschaftliche Kritik Jahrg. 1840, s. 196. 



$ Elie de Beaumont, in the Annales dc* Sciences Naturellet, t. xv., p. 

 362-372. " In approaching the primitive masses of Mont Rosa, and the 

 mountains situated to the west of Coni, we perceive that the secondary 

 strata gradually lose the characters inherent in their mode of deposition. 

 Frequently assuming a character apparently arising from a perfectly 



