260 



tinned concussions likewise throw a considerable degree of 

 light on the geological process of metamorphism. Heat may 

 even simultaneously induce opposite actions in crystalline 

 bodies, for the admirable experiments of Mitscherlich have 

 3Stablished the fact* that calcareous spar, without altering 

 its condition of aggregation, expands in the direction of one 

 of its axes and contracts in the other. 



If we pass from these general considerations to individual 

 examples, we find that schist is converted, by the vicinity of 

 plutonice rupted rocks, into a bluish-black glistening roofing- 

 slate. Here the planes of stratification are intersected by 

 another system of divisional stratification, almost at right 

 angles with the former,f and thus indicating an action sub- 

 sequent to the alteration. The penetration of silica causes 

 the argillaceous schist to be traversed by quartz, trans- 

 forming it in part into whetstone and silicious schist ; the 

 latter sometimes containing carbon, and being then capable 

 of producing galvanic effects on the nerves. The highest 

 degree of silicification of schist is that observed in ribbon 

 jasper, a material highly valuable in the arts,! and which is 

 produced in the Oural mountains by the contact and eruption 

 of augitic porphyry (at Orsk); of dioritic porphyry (at Aulsch- 

 kul) ; or of a mass of hypersthenic rock, conglomerated into 

 spherical masses (at Bogoslowsk) ; at Monte Serrate, in the 

 Island of Elba, according to Frederick Hoffman, and in Tus- 

 cany, according to Alexander Brongniart, it is formed by 

 contact with euphotide and serpentine. 



The contact and plutonic action of granite have sometimes 



* Mitscherlich, Ueber die Ausdelmung der krystattisirtcn Korper 

 durch die Wdrme., in Poggend. Annalen, bd. x. s. 151, 



t On the double system of divisional planes, see Elie de Beaumont, 

 Gtologie de la France, p. 41 ; Credner, Geognosie Thilringens und den 

 Harzes, s. 40; and Romer, Das RheAnisclie Uebergangsgebirge, 1844, 

 s. 5 und 9. 



J The silica is not merely coloured by peroxide of iron, but is accom- 

 panied by clay, lime, arid potash ; Rose, Reise, bd. ii. s. 187. On 

 the formation of jasper by the action of dioritic porphyry, augite, and 

 hypersthene rock, see Rose, bd. ii. s. 169, 187, und 192. See also bd. i. 

 B 427. where there is a drawing of the porphyry spheres between which 

 jasper occurs, in the calcareous greywaeke of Bogoslowsk, being pro- 

 duced by the plutonic influence of the augitic rock; bd. ii. s. 545 arid 

 likuuseHumboldt, A tie Centrale, t. i. p. 486. 



