PORPHYRt. 



district of Leadhills, which is situated in transi- 

 tion rocks, I observed several beds of felspar, 

 and some of cla}' stone-porphyry. In the great 

 tract of transition country which extends from 

 New Galloway to Dumfries, there are beds of 

 transition porphyry that alternate with grey-wackCj 

 grey-waeke-slate, and transition greenstone. At 

 Fassney Burn in East- Lothian, transition porphy- 

 ry is associated with transition granite and syenite. 

 These porphyries also occur in the transition rocks 

 that occur so abundantly to the north of the Frith 

 of Forth. 



The fiodtz-porphyr'm, like those of the transi- 

 tion class, have a basis of felspar, or of claystone 5 

 and contain, besides crystals of common felspar, al- 

 so crystals of glassy felspar and of common quartz, 

 sometimes almost passing into rock-crystal. They 

 occur in beds, often of great magnitude, that al- 

 ternate with the first or old red sandstone, or in 

 vast mountain masses resting on that rock. " They 

 sometimes also appear in veins of great width, 

 Seventy feet and upwards, and of great extent^ 

 that traverse this sandstone. 



Floetz clay-porphyry occurs abundantly in the 

 Island of Arran, in the form of mountain masses, 

 beds, and veins, connected with the old red sand- 

 stone of that island. Floetz felspar -porphyry oc- 

 curs resting upon sandstone, in the islands of 

 Raasay and Skye, and in the Ochil and Pentland 

 Hills. Floetz pitchstone-porphyry, and floetz pitch- 

 stone occur in beds alternating with cJay-porphy- 



