248 GEOLOGICAL DESCRIPTION OF THE EARTIl's CRUST. 



yield products very dissimilar in appearance, a fact of (lie 

 highest importance in the study of eruptive rocks, and the 

 transformations which they occasion ; as, for example, car- 

 bonate of lime, melted nnder high pressure, does not part 

 with its carbonic acid, but becomes when cooled granular 

 limestone or saccharoidal marble when the operation is 

 performed by the dry method, while in the humid pro- 

 cess calcareous spar is produced with a less, and arrago- 

 nite with a greater, degree of heat. ( 262 ) The mode of 

 aggregation of the particles which unite in the act of crys- 

 tallization, and consequently the form of the crystal itself, are 

 also modified by differences of temperature ; ( 263 ) and even 

 where the body has not been in a state of fluidity, the parti- 

 cles, under particular circumstances, may undergo a new ar- 

 rangement, manifested by different optical properties. ( 264 ) 

 The phsenomena presented by devitrification, by the produc- 

 tion of steel by casting or cementation, by the passage from 

 the fibrous to the granular texture of iron, occasioned by in- 

 creased temperature, ( 265 ) and possibly by the influence of 

 the long-continued repetition of slight concussions, may 

 elucidate the geological study of metamorphism. Heat some- 

 times elicits opposite effects in crystalline bodies ; for Mit- 

 scherlicfr's beautiful experiments have established the fact, 

 that without altering its condition of aggregation, calcareous 

 spar, under certain conditions of temperature, expands in 

 one of its axial directions while it contracts in the other. ( 266 ) 

 Passing from these general considerations to particular 

 examples, we may mention the case of schist converted by 

 the vicinity of plutonic rocks into roofing slate of a deep 

 blue colour and glistening appearance; the planes of 

 stratification are intersected by other divisional planes, often 



