The Paragenetic Relations of Minerals. 97 



differences, it is possible that their modes of formation were 

 analogous. 



It can scarcely be doubted that these minerals have crystal- 

 lised out from the mass of the rocks ; they have even formed 

 druses in which a definite succession of species may be re- 

 cognised. It is moreover probable that they were originally 

 in a viscous state, and that the drusy cavities were formed in 

 consequence of the contraction on cooling and crystallising. 



These masses of minerals are in every respect connected 

 with porphyritic rocks and those nodular masses which may 

 be regarded as the result of contraction. There are, how- 

 ever, such masses of minerals which from their magnitude 

 cannot be examined on all sides, — in Scandinavia for in- 

 stance ; — and it is a question of some difficulty whether these 

 are not plutonic injections. 



In the schistose rocks, masses of crystallised minerals 

 sometimes present a similarity to beds, and perhaps many of 

 the deposits of minerals which are regarded as beds of small 

 extent are in reality the result of a segregation of chemical 

 constituents subsequent to the formation of the true strata. 

 The facts which have been observed in connection with these 

 masses of minerals, afford additional evidence in favour of the 

 view already expressed, that in the formation of rocks me- 

 chanical accumulation has been followed at some period by a 

 chemical re-adjustment of the constituent molecules, giving 

 rise to those physical and mineralogical characters which 

 they now present. 



Another mode of occurrence of minerals, connected, as re- 

 gards their origin, with the porphyritic structure, is pre- 

 sented by the so-called divergent zones — accumulations of 

 minerals which are so situated as to intersect at an acute 

 angle the planes of stratification of the schistose rocks. Some 

 of the most remarkable mineral deposits of Scandinavia be- 

 long to this class. Their origin is obscure, but Professor 

 Breithaupt is of opinion that they were formerly lodes, which, 

 together with the rocks in which they are situated, have suf- 

 fered metamorphism,* thereby losing their sharp lines of de- 

 marcation, — the saalbands. 



* With regard to the metamorphism of rocks there appear to be good grounds 

 VOL. LV. NO. CIX. — JULY 1853, G 



