The Paragenetic Relations of Minerals. 327 



a vitreous mass resulting from igneous fusion, and there is 

 other evidence which strongly favours the opinion that it 

 originated in this way. The opaline structure is equally 

 peculiar, and more frequent, in minerals, than the vitreous. 

 It would appear that those bodies which have an opaline 

 structure, frequently present indications of having been 

 originally in a plastic state ; indeed, opal itself has actually 

 been found so. 



"Without taking into account those minerals which may 

 have been formed from solutions, there is an abundance of 

 facts which fairly admit of the inference that the substance 

 of a very great number of minerals possessed at some period 

 of their existence a certain degree of internal mobility, being 

 either liquid, viscous, or plastic, although opinions differ as 

 to the precise nature of this former state.* 



Fuchs is of opinion, that entire masses of rock have been 

 in this state, and that the metamorphism of rocks is essen- 

 tially connected with such a softening. The curvature in 

 the axes of crystals, the partial fracture, contortion and 

 separation of crystalline minerals as it were into slices, as in 

 the garnets, in some schistose rocks, are favourable to this 

 view. Some quartzose rocks consist of angular fragments 

 cemented together by amorphous quartz, and it is not im- 

 probable that the fragments have been formed by the con- 

 traction of gelatinous or plastic silica, and that the cement 

 has been subsequently introduced in a plastic state. 



Moreover, crystalline substances may, without losing their 

 solidity, experience an alteration of structure, as in the con- 

 version of arragonite into calcite, and there is in many in- 

 stances a remarkable fact of paragenesis connected with the 

 calcite, which has originated in this way. Crystals of celes- 

 iine are found upon it, which would appear to indicate that 

 sulphuric acid has had some share in the change. Stro- 

 meyer's investigations have shewn, that arragonite generally 

 contains strontia ; and as celestine is always accompanied by 



* In a physical point of view, it is probable that the molecular structure of 

 bodies depends mainly upon the conditions under which solidification takes 

 place, and any inferences which may be drawn from the structure of minerals, 

 would appear to refer rather to those conditions than to the former state of the 

 masses from which they were formed. 



