ARRAN. PITCHSTONE. 415 



easily broken when in a moist state, like many other 

 rocks recently removed from the earth, but acquires a 

 much greater degree of induration after some days of 

 exposure to the air. Where in contact with running 

 water, it decomposes into a tenacious clay ; a thin coating 

 of which is often found on the exposed surfaces, dimi- 

 nishing their natural lustre. 



A large mass is to be seen by the road side at the 

 corner of Brodick wood. This appears to be a wide 

 vein ; but is very short in its course and much inclined. 

 A trap vein lies near it with an angular course, such 

 as to indicate their probable junction ; but the ground 

 does not permit their contact, if it actually takes place, 

 to be traced. The colour of this pitchstone is a bottle 

 green, and its texture is porphyritic. 



It is not unusual in this rock, to find some specimens, 

 like many obsidians, presenting indications of an internal 

 structure ; consisting of laminae imperfectly parallel, and 

 generally waved and contorted. In the vein now 

 described, no such appearance can be observed in a 

 fresh fracture; but, like many of the trap family, it 

 displays, on weathering, obvious indications of that 

 which could not be conjectured by examining the 

 unaltered rock. The exposed surfaces are found beau- 

 tifully spotted with white, and marked with white curved 

 lines, following each other in the conformable manner 

 seen in certain varieties of marbled paper. The white 

 spots in this case are produced by the weathering of 

 the felspar ; and, by this indication, many pitch stones 

 are discovered to be porphyritic when the imbedded 

 crystals can scarcely otherwise be observed. As the 

 striped appearance is doubtless a result of the internal 

 structure, it must proceed from the mixture of two 

 substances, differing in hardness, or in tendency to 

 decomposition; and as these marks are often bent and 

 twisted in a variety of capricious forms, they seem to 



