422 ARRAN. GEOLOGY. PITCHSTONE. 



lamellae of a darker colour which appear on the surface 

 like stripes. 



5. Simple, with distinct alternate laminae of black, and 

 of dark or of bottle green. 



6. Dark green and porphyritic. The imbedded grains 

 are however not crystallized, but spheroidal; and, when 

 large, present a crystalline nucleus, which gradually blends 

 with the pitchstone by means of an intermediate enamel : 

 when small, the enamel alone is found. In the most 

 distinct specimens, these grains can scarcely be recognised 

 for felspar, except by the change they undergo on weather- 

 ing. This is by far the most common appearance of the 

 pitchstone porphyries of Arran; and the resemblance to 

 that of Glamich, formerly described, is so striking, that it 

 is unnecessary to remind the reader of it. 



7. The same with a thin schistose fracture. 



8. Dark bottle green, also commonly enumerated 

 among the pitchstone porphyries, but the grains are 

 rounded and consist of quartz. They are white in the 

 centre, but are surrounded by a brown crust, which has 

 the aspect of chalcedony and appears to be a transition 

 from quartz to pitchstone. This substance will be pre- 

 sently shown to occur on a much larger scale. 



9. The same, with this variation ; that the grains are 

 disposed in thin layers alternating with equally thin laminae 

 of the pitchstone ; being further often so condensed and 

 united as to form granular continuous laminae. 



10. Pale greyish green, granular-conchoidal, and por- 

 phyritic in appearance ; but the grains consist also of 

 quartz. The quartz becomes gradually excluded in this 

 case, and thus many additional varieties are produced. 



11. Dark bottle green, containing grains of felspar so 

 transparent as to be scarcely distinguishable on a fresh 

 fracture. They are only discovered to be abundant after 

 the rock is weathered; when they are seen, white, and 

 thickly sprinkled over the surface. 





