168 DOMAIN n. SILICEOUS. 



obstinately. After very long exposure it becomes 

 covered with a thin crust, which has usually a 

 greyish-white colour, but a reddish crust in iron- 

 shot varieties. 



" 4. Like basalt it forms single conical hills ; 

 but they are not so regular, and are more marked 

 with cliffs and irregular rocky forms. 



" 5. Excepting small traces of iron-pyrites, and 

 iron-sand, it contains no ores. 



" 6. It appears from observations I have made 

 in the islands of Arran and Lamlash, in Dumfries- 

 shire, and on the porphyry-slate hills near Edin- 

 burgh and Haddington, that this rock passes, on 

 the one hand, into compact felspar and clay-stone, 

 and, on the other, into pitch-stone and basalt. 



" 7. It occurs abundantly in the islands of Arran 

 and Lamlash, in the frith of Clyde ; also in smaller 

 quantities in the upper part of Dumfries-shire, 

 and in the county of Selkirk. Braid hills, and 

 part of the Pentland hills, near Edinburgh; the 

 Girleton hills afc Haddington, and, according to 

 my pupil Dr. Ogilvy, North Berwick Law, and 

 Traprain or Dumpender Law, in the same county, 

 are composed of this rock. I suspect that the 

 porphyry of Cumberland, which probably occurs 

 among transition mountains, is also porphyry- 

 slate. It occurs in great abundance in Bohemia ; 

 also, but in less quantity, in Lusatia; in the prin- 



