FORMATION OF PRIMITIVE MOUNTAINS. 343 



gins to become solid, for example, sulphur, in a round 

 vessel, a crust of sulphur is not formed upon the surface 

 of the cooled vessel, and another crust upon the surface 

 of the sulphur itself, as might be expected ; on the con- 

 trary, if a crystal be formed upon a point of the inner 

 surface of the vessel, the crystal enlarges by growing in 

 the direction of its axis, and the mass which surrounds 

 the crystal remains liquid, and sometimes cools, without 

 the molecules arranging themselves in the same manner 

 as the crystal already formed. On examining the cooled 

 mass, we observe that it shews a lamellar texture where 

 the crystal was formed, and that the mass which sur- 

 rounded it does not shew this texture in the same degree. 

 This explains how veins of large-granular granite traverse 

 a small-granular granite, as well as other phenomena of 

 the same nature. 



This observation also affords an explanation of an- 

 other phenomenon. If the half of the liquid mass has 

 become solid, and if the fluid part be poured off, we ob- 

 tain isolated crystals, which have been formed in the 

 fluid mass. If the fluid part be not pouijed off, and be 

 permitted to cool slowly, it contracts, as is the case with 

 most bodies, and the contraction produces the same ef- 

 fect as the decantation ;* small cavities will be formed, 

 and these will be traversed and covered over with dis- 

 tinct crystals. We also observe this phenomenon in the 

 geodes of primitive and volcanic mountains, in which 

 the crystals they contain are of the same minerals as those 

 of which the mountains themselves are composed. 



