328 The Para genetic Relations of Minerals. 



calcite or limestone, and is always of more recent formation 

 than these, it has possibly originated in all cases from stron- 

 tiferous arragonite. The sulphuric acid may have been 

 derived from the decomposition of iron pyrites. 



The association of minerals does not always indicate their 

 simultaneous formation. The granular rocks appear to 

 afford the best evidence of this ; for while, in some cases, 

 minerals implanted upon each other differ very little in date, 

 in others a very long interval may have elapsed between 

 their respective formations. The simultaneous formation of 

 different mineral species is indicated by their regular twin 

 growth, by juxtaposition, as in disthene and staurolite from 

 St Gothard ; dolomite, ripidolite, and a green amphibole, 

 (Pfitschthal, Tyrol) perhaps also in graphic granite, although 

 in the druses, of the three constituents pegmatolite appears 

 to be oldest, mica most recent, and quartz intermediate. 

 When minerals present mutual impressions, such as Fournet 

 has observed in the garnet and mica near Lyons, this circum- 

 stance is evidence of their simultaneous formation. In most 

 cases, the date of minerals which present a twin-growth by 

 superposition, is not precisely the same ; thus the hexago- 

 nites haplotypicus is rather older than the rutile with which 

 it is compounded (Tavetschthal, Switzerland). The same is 

 the case with the regular twin-crystals of iron pyrites, and 

 spear pyrites (Litmitz, Bohemia) ; the twin crystals of chlorite 

 upon and with magnetite (Fahlun, Sweden). It ought like- 

 wise to be considered as a universal rule of superposition, at 

 least with regard to varieties of the same species, that the 

 non-crystalline, or least crystalline variety is followed by the 

 crystalline ; instances of this are furnished by compact and 

 fibrous brown iron ore ; hornstone and quartz crystals ; 

 allochroite and garnets. 



Among the more uniform mixed rocks a certain geognostic 

 relation is presented by the minerals constituting those which 

 possess agranular, schistose, or porphyritic structure. With 

 regard to the phenomena of paragcnesis, these rocks may 

 be divided into two classes : — 



I. Those consisting of silicates, one of which is almost 

 always a felsite, rarely replaced by nepheline. 



