MINERALS IN METAMOEPHIC ROCKS. 283 



Speculations have been hazarded at various times in 

 regard to the origin or source of the crystals of apatite 

 (phosphate of lime,) of graphite (plumbago,) garnets, 

 and various other minerals, which are met with in so 

 many countries intermingled with the metamorphic or 

 crystalline limestones. But the origin of all these is now 

 easily intelligible. It is certain that this crystalline 

 character is the result of the action of heat long con- 

 tinued. But the assumption of this crystalline character 

 implies a power of movement of the particles among 

 each other, which, in fact, is seen in many cases in 

 unmelted bodies — as in the annealing of glass and 

 metals, and in the tempering and converting of iron or 

 bronze — to take place where they are kept for a pro- 

 longed period at an elevated temperature. It is certain, 

 also, that particles of a like kind have a special attraction 

 for each other — a tendency to draw towards one another 

 and cohere, when circumstances are such as to admit of 

 their moving among themselves, or among the particles 

 of other matter with which they may be mixed. And, 

 thirdly, it is certain that, when several substances which 

 incline to unite with each other are present in a mixture 

 in which circumstances admit of a movement among the 

 particles, they often unite to form definite chemical 

 compounds, exhibiting, more or less, well-defined crys- 

 talline forms. 



Now it is known that stratified limestones, when 

 deposited, are rarely free from admixtures of earthy 

 matter, which contain the constituents of garnet, chon- 

 drodite, hornblende, &c. When these limestones are 

 subsequently exposed to the long-continued action of 

 heat, the particles of the rocky mass arrange themselves 

 in crystalline forms, while the earthy matters unite to 

 form the simple minerals (garnet, &c.,) which can be most 

 readily produced out of the substances of which they 

 consist, in the proportions in which they are actually 



