100 The Paragenctic Relations of Minerals. 



white, with the various minerals distributed irregularly 

 throughout. Druses are sometimes met with, and in those in- 

 stances definite successions of mineral species are recog- 

 nisable. Sometimes relations of age may be detected in the 

 groups of minerals surrounded by limestone. Upon the whole, 

 however, it would appear as though the minerals had been 

 formed almost simultaneously. 



It may be inferred from these circumstances, that the entire 

 space occupied by such a mass was at once filled with a 

 chaotic mixture, from which the various minerals gradually 

 separated. The conjecture is therefore natural that the lime 

 and magnesia were not erupted as carbonates, but as a caustic, 

 and probably pasty mass, which, acting upon the adjoining 

 siliceous rocks, gave rise to the formation of new silicates, 

 aluminates, titanites, together with such other minerals as 

 fluor spar, apatite, anhydrous iron ores, magnetic pyrites, 

 and graphite. With regard to these masses it is impossible 

 to assume that the formation of these minerals has taken 

 place by a gradual infiltration of solutions. Many of the 

 minerals have suffered subsequent alteration, — amphiboles, 

 pyroxenes, &c, have been converted into serpentine and 

 other ophitic substances. 



Professor Breithaupt regards as untenable the view put 

 forward by Mr Dana* of the formation of these primitive 

 limestones from coral beds by a metamorphic action of hot 

 sea-water. The circumstance which appears to be most 

 strongly opposed to that view is, that in Europe at least the 

 primitive limestone occurs in rocks which are much older 

 than the coralline limestones. In many instances, moreover, 

 their eruptive origin cannot be doubted, and this cannot be 

 reconciled with their formation from coral beds. 



Quartz occurs in these limestones but very rarely, a circum- 

 stance which appears to follow naturally from the presence 

 of lime and magnesia with which it was capable of combining. 



While the interpretation of the last two classes of pheno- 

 mena presented very serious difficulties, those which now come 

 under consideration — vesicular cavities, and the minerals they 

 contain — are far more intelligible. 



Silliman\s Journal 844, p. 135. 



