LEON HARD ON PORPHYRY. O 



phyry. Rocks of a totally different character were confounded with 

 it. Thus we find in a memoir on the Lead-mines of Viconago, not 

 far from the Lake of Lugano, that the mica-slate mountain which 

 contains the ore rests on granite or gneiss ; the summit consisting of 

 the "roche argileuse, parsemee des grains calcaires a laquelle les 

 AUemands ont donne le nom de Feldspath-Porphyr." Soon after- 

 wards Gerhard endeavoured to show that all Porphyry, usually sub- 

 divided into Clay-porphyry, Hornstone-porphyry, and Felspar-por- 

 phyry, invariably had Felsite for its basis. 



The term " Felsite" was first used by Kirwan in his * Mine- 

 ralogy*,' to denote a substance described by Wiedemannf as " com- 

 pact felspar." Klaproth observes, "I understand under the name 

 of Felsite that mineral, hitherto called * compact felspar,' which, 

 when united with greenish hornblende, forms, with a crystalline 

 structure. Greenstone, and, with a slaty structure, Greenstone-slate|." 



Gerhard says of Felsite, that it seldom forms mountain-masses, but 

 is united with other rocks, and that in combination with hornblende, 

 felspar, quartz, and mica it forms the various species of porphyry. 

 He distinguishes: — 1. Earthy felsite, forming the basis of all the 

 so-called clay -porphyries (amongst these Gerhard includes Werner's 

 clay-stone) ; 2. Splint-felsite, forming the basis of all hornstone- 

 and felspar-porphyry; and 3. Laminar felsite or Labrador-stone. 

 Adopting the clay-stone and clay-porphyry phraseology of the Ger- 

 mans, the French formed their Porphyre argilleux and P. terreux, the 

 English their " Clay-porphyry" and " Clay-stone," and the Italians 

 their Porfido argilloso. Brongniart considered the clay-stone as a 

 peculiar rock and named it Argilophyre^ subdividing it according to 

 the colours. Haiiy called it Felspath compacte porphyrique decom- 

 pose. Jasche described the porphyry of the district of Elbingerode 

 on the Hartz as Feldspath-gestein, 



Considerable progress in the knowledge of porphyry was made by 

 d'Aubuisson de Voisins. This active follower of Werner called at- 

 tention, amongst other things, to the analogies between granite and 

 porphyry ; and to the transition of these two rocks into one another, 

 already observed by Saussure, Dolomieu, and others. It was d'Au- 

 buisson also who maintained that the basis of porphyry was only a 

 compact granite. He proposed that it should be called " Eurite." 



After giving d'Aubuisson' s description of eurite, the author conti- 

 nues, p. 6 : — On account of the red colour of the rock which pre- 

 vails in many districts {e.g. Thuringia, Silesia, and the Tyrol), it re- 

 ceived the peculiar name of "Red Porphyry," an expression afterwards 

 used by L. von Buch in another sense to distinguish it from " Black 

 Porphyry" or Melaphyre. 



Leopold V. Buch was also the first to attribute a greater import- 

 ance to the presence of quartz. He directed attention to the pecu- 

 liar significance of the presence of this mineral in decomposed, wea- 



* Elements of Mineralogy, vol. i. p. 439. 

 t Bergm. Journal, 1791, p. 345. 



X See his Chemical Enquiries into the Felsite of Siebenlehn : Klaproth, Chem. 

 Abhandlungen, v. p. 259. 



B 2 



