EXCURSION IV. 103 



varieties of one substance a homogeneous felspathic rock, 

 with occasionally bits of imbedded quartz, and consisting of 

 about 75 per cent, of silica, 15 of alumina, and 10 of potash, 

 soda, magnesia, arid oxides of iron and manganese, with a 

 trace of lime. All these felspathic traps or felstones abound 

 in Arran. 



Pitchstone, which figures so largely in the geology of Arran, 

 is found in several varieties in our " Museum ; " and when, 

 containing specks of white or red felspar, forms pitchstone 

 porphyry. Its composition is almost the same as that already 

 given for felstone, being from 62 to 74 per cent, of silica, 11 

 to 17 of alumina, with small quantities of potash, soda, and 

 lime, and oxides of iron and manganese. Its peculiar 

 character is derived from its rapid cooling. We have to 

 mention, lastly, a highly interesting rock found here, but 

 never met with by us in situ in Arran ; only a few blocks of 

 it are seen in the fence. It is intermediate between basalt 

 and pitchstone, and links them together as products of fire 

 generated at different stages of the cooling process. It is 

 hard and tough, and almost homogeneous like basalt, but 

 without its shining points, while it has the vitreous aspect 

 and colour of a dark-blue pitchstone. It would be extremely 

 interesting to find this rock in situ, and to mark its rela- 

 tions, whether occurring alone, with basalt, with pitchstone, 

 or whether all three are together. Veins of it must exist 

 somewhere to the N. or N.W. 



53. Our walk homeward will be varied by passing down 

 from the top of the hill into Birk Glen, along the old road. 

 Having crossed the burn, we come on a great pitchstone 

 vein, lying a little diagonally to the path. It is of a green 

 colour, lamellar structure, and is in parts porphyritic. The 

 breadth is about thirty feet, and range between west and 

 south-west. Thin white films form on the exposed surfaces, 

 and when constantly moist decompose into a tenacious clay, 

 coating the surface. It extends into the bed of the burn, and 

 is there of the same width ; but the contact with the sand- 



