332 The Paragenetic Relations of Minerals. 



the Eifel present abundant examples of this mode of occur- 

 rence, and the curvature in the surrounding layers of rock 

 may be easily recognised. Sometimes the pyrites has been 

 completely removed, its former existence being indicated only 

 by hexahedral cavities, and then the fibrous character of the 

 quartz is more prominent. It is probable that iron pyrites 

 is very extensively disseminated throughout some schistose 

 rocks, for the stagnant water in slate quarries frequently 

 contains sulphate of iron. 



If. then, it is satisfactorily proved that the porphyritic 

 formation of iron pyrites has taken place subsequently to the 

 formation of the strata in which it occurs, there appears to 

 be no reason why we should not consider other crystalline 

 minerals present in the same rocks, and sometimes in imme- 

 diate proximity to the pyrites, to have been formed in a simi- 

 lar manner. In some of the most ancient slates octohedrons 

 of magnetic iron occur together with the pyrites, and even 

 the granular rocks of eruptive and plutonic origin present 

 analogous facts. In the granite of Saubersdorf (Voigtland) 

 the iron pyrites is partially decomposed, and converted into 

 crystallised specular iron, the granite itself being somewhat 

 disintegrated. In the syenite of Zschitschewig, titanite is 

 imbedded together with pyrites. It is, however, inconsistent 

 with chemical principles to suppose that bisulphuret of iron 

 existed in a melted eruptive mass ; in melting processes only 

 mono- and sub-sulphurets are formed, and this renders it still 

 more probable that the pyrites is of more recent formation 

 than its matrix, likewise proving either that these rocks 

 were not formed at any very high temperature, or had cooled 

 considerably when the formation of pyrites took place. 



Bunsen's obsJtrations in Iceland would appear to prove 

 that the formation of iron pyrites in clay may sometimes be 

 owing to other causes than those above mentioned. The prin- 

 cipal gases associated with the exhalations of vapour in that 

 island, are sulphuretted hydrogen and sulphurous acid, their 

 mutual decomposition giving rise to the sublimation of im- 

 mense masses of sulphur. When the sulphuretted hydrogen 

 is in excess, the oxide of iron is converted, under the in- 



