338 Tlie Paragenctic Relations of Minerals. 



near the mica slate and gneiss at Munzig (Saxony), and chias- 

 tolite occurs in the clay-slate at Gefrees (Bavaria), close to 

 granite. 



At Trcuen (Saxony), the so-called " Fruchtschiefer" occurs, 

 surrounding a spheroidal mass of granite, and a perfectly 

 similar-shaped black amphibole, with garnets, occurs at 

 Airolo (Switzerland), so that it may be inferred that the in- 

 distinct and decomposed crystals in the " Fruchtschiefer" 

 were formerly amphibole. The same kind of slate occurs at 

 Schneeberg, near the granite ; and in the mines at that place 

 it has been found that the clay-slate is harder and more sili- 

 ceous near the granite, this contact phenomenon even extend- 

 ing into the slate to a distance of 800 feet from the granite. 

 This fact is alone sufficient to shew to what a distance the 

 atoms of relatively recent rock may be transferred into an 

 older one w r ith which it comes in contact. The horn-slate, 

 which forms a kind of mantle round the granite of the 

 Brocken, affords almost precisely the same evidence. These 

 altered, and in part essentially hardened slates, do not always 

 present evident porphyritic inclosures, but they probably exist 

 as microscopic particles, or the slate has been otherwise che- 

 mically altered. 



It is probable that the alterations which rocks have suf- 

 fered under the influence of more recently-formed rocks rarely 

 consisted in merely mechanical modifications of the molecu- 

 lar aggregation ; they appear rather to have been far more 

 deeply seated, to have been more or less chemical. In this 

 point of view, the theory put forward by Von Buch, that, 

 under certain circumstances, dolomite may have originated 

 from limestone by the action of melaphyr, comes within the 

 bounds of possibility, and, under one condition, acquires much 

 probability. 



The mica-slate of Scharfenstein (Erzgebirge) is remark- 

 ably altered in contact with the porphyry which traverses it, 

 being converted into gneiss for short distances. In this in- 

 stance felsite has been transferred from the porphyry into the 

 mica-slate, which might, indeed, be termed porphyritic, were 

 it not customary to call a schistose mixture of felsite, quartz, 

 and mica, gneiss. 



