The Paragenetic Relations of Minerals. 95 



tamorphic rocks, has recently gained ground, although with- 

 out being in itself very definite ; for, while some geologists 

 assume that erupted rocks originated from the agglutination, 

 and even fusion, of sedimentary deposits, others consider that 

 erupted masses have effected the metamorphism of sedimen- 

 tary rocks. Taking into consideration the contact phenomena 

 so abundantly made known by Murchison, the latter appears 

 to be most probable ; but it is not applicable to all kinds of 

 gneiss, mica, and clay-slates, which are so frequently in con- 

 tact with granite, syenite, &c. ; and in those cases are 

 most probably primitive, as their mineralogical analogy with 

 those rocks is very close. 



The accessory constituents occurring in the principal rocks 

 of this group are the following : — 



Rocks. Accessory Constituents. 



Gneiss, . Oligoclase, disthene, garnet, tourmaline, dichroite 



(only in certain conditions, such, as contact 

 with granite), rutile, allanite. 



Mica-slate, . Chlorite (associated with the usual mixture of 

 quartz and phengite,* forming a quite pecu- 

 liar kind of mica-slate (between Falkenau and 

 Schellenberg, Erzgebirge), amphibole, dis- 

 thene, garnet, tourmaline, and alusite (it is re- 

 markable that this mineral is almost always ac- 

 companied by fibriolite), staurotide, magnetite, 

 allanite, iron pyrites, gadolinite, gold, graphite. 



According to Durocher, the occurrence of many crystallized 

 minerals imbedded in the gneiss and mica-slate of the Scan- 

 dinavian peninsula is limited to the granite and amphibole 

 dikes penetrating these rocks. The principal minerals con- 

 tained in these rocks, and apparently connected with the 

 phenomena of metamorphism, are amphiboles, pyroxenes, 

 garnets, epidotes, disthene, dichroite, tourmaline, beryl, topaz, 

 apatite, titanite, rutile, and graphite. He excludes gado- 

 linite and orthite, which are undoubtedly quite independent 

 of gneiss, and occur only in coarse-grained granite dikes. 



Granite, . Apatite, monazite, astrite, nepheline, petalite, 

 pegmatolite (in crystals), amazonite, oligo- 



* It is, however, very rare to find astrite and phengite associated in the same 

 rock. 



