50 Mac CuUoch on Pitchstone. 



marbled paper, and evidently resulting from some correspond- 

 ing difference of laminar structure ; the lamina most sensible 

 to atmospheric action being thus detected, although invisible 

 in the fresh rock. In a further progress to decomposition, 

 pitch-stone is at length converted into a fine clay, forming in 

 water a very tenacious paste ; and it appears to be very readily 

 acted on by the water, as may be witnessed in Arran. It does 

 not always weather to a white enamel, becoming occasionally 

 brown, and more rarely turning into a black powder. Occa- 

 sionally it exhibits the kind of bloom seen on a plum, or on 

 certain kinds of bottle-glass. 



It must here also be remarked, that notwithstanding its 

 vitreous aspect, pitch-stone often contains a great deal of loose 

 water, which is easily separated by drying, as in many other 

 rocks. In these cases it is tender, but becomes brittle after 

 the water has evaporated. 



The lustre of pitch-stone varies exceedingly, according to its 

 several states or varieties ; passing from the most perfectly 

 vitreous to one scarcely more glossy than that of the finest 

 basalt. 



The colours are various, greyish white, pale ochre-yellow, 

 brown, olive brown, olive green, dark reddish brown, dark 

 bottle-blue, and black. In Scotland the darker colours, and 

 the olive greens in particular, are predominant. 



The ordinary transitions of pitch-stone are into chalcedony, 

 chert, and semi-opal : it appears also to pass into a substance 

 not easy distinguishable from the conchoidal shining jasper 

 found among the clay strata that are entangled in trap or 

 volcanic rocks, with which, as I formerly remarked, it appears 

 to have been occasionally confounded. The transitions into 

 basalt have been already noticed. 



Such is a general sketch of its geological and mineral cha- 

 racters, as far as my limited knowledge extends. The more 

 minute particulars will be found in the synopsis of varieties. 



It is scarcely necessary to define it, further than to say, that 

 in a general way, it is distinguished from all other rocks by its 

 vitreous or by its resinous aspect; and that it differs from 



