ON PORPHYRY. 



In the third volume of my System of Mine- 

 ralogy, and in my Mineralogical Travels, I have 

 shewn, that several of Werner's Primitive por- 

 phyries, occur in this country, and shall now prove, 

 that we also possess Transition and Flcetz porphy- 

 ries. 



The transition porphyries have either a claystone 

 or felspar basis, and, besides crystals of felspar, 

 they sometimes also contain grains and crystals of 

 quartz, scales of mica, and crystals of hornblende. 

 They occur in beds which are from two to twelve 

 or fifteen feet in thickness, of great extent, and 

 alternate with grey-wacke and transition clay- 

 slate. I first noticed these rocks in the year 1804,; 

 during my examination of the county of Dum- 

 fries, but hesitated introducing them into the sys- 

 tem as porphyries, until I should have an oppor- 

 tunity of examining them more particularly, anci 

 in other parts of Scotland where I might find them 

 still on a greater scale, and more distinctly charac- 

 terized. Last summer, during my stay in Dum- 

 friesshire and Galloway, I paid particular atten- 

 tion to these rocks, and convinced myself that 

 they are true Porphyries, and belong to the Tran- 

 sition class. 



In the tract of mountainous country which ex- 

 tends from Langholm by Eskdalemuir and Whani- 

 phry, the district where I first noticed these porphy- 

 ries, the beds are but a few feet thick, and cannot be 

 traced far owing to the cover of soil and grass. 

 They are contained in grey-wacke. In the mining 



