334 The Paragenetie Relations of Minerals. 



which serves as a matrix, occurs implanted upon it in 

 druses. 



In the Zillerthal (Tyrol), pistacite occurs imbedded in 

 mica (astrite) slate, and in the same locality fine specimens 

 of green epidote are found upon ripidolite. Chondrodite oc- 

 curs in many places in saccharoid limestone and calcite ; but 

 in the druses in the masses ejected from Vesuvius, it is im- 

 planted upon calcite. Zeilanite likewise occurs imbedded in 

 calcite, and in the above druses upon it. Yellow titanite oc- 

 curs imbedded in chlorite at the Zillerthal (Tyrol), and at 

 St Gothard, implanted upon it. Brown titanite occurs im- 

 bedded in hornblende slate at the Stubeithal (Tyrol), and in 

 the well-known druses of common hornblende from Arendal 

 (Norway), it is implanted upon this mineral. 



The porphyritic occurrence of iron pyrites in copper 

 pyrites is remarkably frequent, especially when the latter is in 

 large masses. Sometimes the iron pyrites is quite porous, and 

 only partially occupies the cavity in the copper pyrites. When 

 these minerals are associated in druses, the copper pyrites 

 always appears as the oldest of the two, Pseudomorphous 

 iron pyrites, after copper pyrites, presents an apparent ex- 

 ception to this rule, but it must be remembered that pseudo- 

 morphs are in some sort abnormal products. There are, 

 however, some real exceptions, though few in number. 



Copper glance is in many instances followed by erubescite 

 and iron pyrites, a fact which perhaps admits of explanation 

 upon chemical' principles. If, for example, a solution eon- 

 aining oxide of copper and peroxide of iron, is acted upon 

 by sulphuretted hydrogen, sulphuret of copper is first formed, 

 while the peroxide of iron must be reduced to protoxide be- 

 fore a bisulphuret can be formed ; and during this time it is 

 readily conceivable that compounds of the sulphurets of iron 

 and copper may be formed. 



The most direct illustrations of this third proposition are 

 furnished by those instances in which minerals occur im- 

 bedded in a rock, and at the same time implanted upon it in 

 veins, sometimes even filling the fissures. 



Thin plates and lamina? of talc occur imbedded separately 

 in the chlorite slate of the Zillerthal (Tyrol) ; and this talc 



