ROCKS. 259 



Rxure likewise modify the direction in which the different par- 

 ticles arrange themselves in the act of crystallization, and also 

 afiect the form of the crystal.* Even when a body is not in 

 a fluid condition, the smallest particles may undergo certain 

 relations in their various modes of arrangement, which are 

 manifested by the different action on light. t The phenome- 

 na presented by devitrification, and by the formation of steel 

 by cementatron and casting — the transition of the fibrous into 

 the granular tissue of the iron, from the action of heat,$ and 

 probably, also, by regular and long-continued concussions — 

 likewise throw a considerable degree of light on the geological 

 process of metamorphism. Heat may even simultaneously in- 

 duce opposite actions in crystalline bodies ; for the admirable 

 experiments of Mitscherlich have established 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 

 Plutonic erupted rocks, into a bluish-black, glistening roofing 

 slate. Here the planes of stratification are intersected by an- 

 other system of divisional stratification, almost at right angles 

 with the former, II and thus indicating an action subsequent to 

 the alteration. The penetration of silica causes the argilla- 

 ceous schist to be traversed by quartz, transforming it, in part, 

 into whetstone and silicious schist ; the latter sometimes con- 

 taming carbon, and being then capable of producing galvanic 

 efiects on the nerves. The highest degree of silicification of 

 schist is that observed in ribbon jasper, a material highly val- 

 uable in the arts, IF and which is produced in the Oural Mount- 



* Oa the dimorphism of sulphur, see Mitscherlich, Lehrhuch der 

 Chemie, § 55-63. 



t On gypsum as a uniaxal crystal, and on the sulphate of magnesia, 

 and the oxyds of zinc and nickel, see Mitscherlich, in Poggend., An7ia' 

 len, bd. xi., s. 328. 



X Coste, Versuche am Creusot uber das hruchig werden des Stabeisens. 

 Elie de Beaumont, Mem. O6ol., t. ii., p. 411. 



§ Mitscherlich, Ueber die Ausdehnung der Krystallisirten Korper durch 

 die Wdrmelehre, in Poggend., Annalen, bd. x., s. 151. 



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

 Geologic de la France, p. 41 ; Credner, Geognosie Tkuringens nnd dea 

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

 B. 5 und 9. 



1[ The silica is not merely colored by peroxyd of iron, but is accom- 

 panied by clay, lime, and potash. Rose, Rcise, bd. ii.. s. 187. On the 

 tbrn>ation of jafper b) the action of dioritic pojphyry, augite, and by 



