90 The Paragenetic Relations of Minerals. 



(Russian Finland), and all other localities, without any ex- 

 ception, furnish evidence of the more recent formation of the 

 quartz. 



The same remarks apply to beryl, fragments of it occurring 

 imbedded in quartz (America and Siberia), while it appears 

 of anterior formation to the quartz with which it is associated 

 in druses. When definite crystals of topaz and quartz are 

 associated together in druses, as in Siberia and Saxony, the 

 latter always appears as the more recently-formed mineral ; 

 but there is in the collection of Prince Lobkowitz a large 

 crystal of quartz from Capo di Villa Rica (Brazil), contain- 

 ing an imbedded fragment of topaz. 



Near Krageroe (Norway), fragments of amphibole occur 

 imbedded in a reddish white felsite, which is probably peg- 

 matolite. 



Wherever wolframite and scheelspar are associated, the 

 latter must be regarded as a product of the decomposition of 

 the former, which is always the older mineral. When they 

 form twins, the wolframite is never in a perfect state at the 

 points of contact. However, a large mass of scheelite from 

 Schlaggenwald (Bohemia), contains imbedded fragments of 

 ferro-wolframite crystals, the edges of which are somewhat 

 broken. This mineral has recently been met with in a simi- 

 lar manner in iron pyrites at Schneeberg ; while, in other 

 places, iron pyrites appears as the more recent mineral, being 

 implanted upon the wolframites. 



The following table comprises the most important instances 

 of porphyritic structure, either in simple minerals or in 

 those rocks which, like basalt clay-slate, appear to the naked 

 eye to be homogeneous. Among the sedimentary rocks are 

 some which, like astrite-slate, talc slate, &c, cannot be re- 

 garded as such in their present state : — 



I. Porphyritic Minerals which do not occur as rocks : — 



Substances imbedded. 

 Calcite, . . . Mica. 



Apatite, 

 Chlorite, . 

 Lepidomelan, 

 Sanidino, . 



Cryptolite. 

 Garnet, scheelite. 

 Black amphibole. 



Apatite, hauyne, nosean, nepheline, meion- 

 ite, melanite, zirkon, titanite, ilmenite. 



