^i Dr. Mac CuIvLOCh's Shub of the 



Among the rocks, for which I could not find a place in the 

 geological description without disturbing its order, pitchstone re- 

 quires to be noticed. Although not found in situ it oifers as a 

 mineral specimen some appearances which are interesting, and 

 which I shall therefore describe. It was on the hill of Glamich 

 that I found the specimens in question, and it is probable that they 

 had been detached from some veins which I was unable to trace. 

 There are two varieties, a black one very little differing from 

 that of Rum, except that it contains a few dispersed crystals of 

 glassy felspar j and an olive green one, which as it offers some 

 apparently important peculiarities hitherto unobserved, I shall 

 describe more fully. It is often of a granular combined with a 

 small conchoidal fracture, and is generally disposed in distinct 

 concretions which are either of the flat or curved lamellar form. 

 It is remarkable for containing irregular rounded cavities similar 

 to those of the amygdaloids, filled with compact grains of a grayish 

 hue. The structure of these is so singular as to be deserving of 

 notice. On breaking the smaller ones they are discovered to 

 consist of a grayish white enamel similar to that which is formed by 

 the fusion of felspar. But if we break the larger grains we can 

 distinctly see that the center is composed of glassy felspar, the 

 crystalline transparency and platy fracture of which are perfect, 

 while the surface to a certain depth is converted into the white 

 enamel I have described. I have not observed this very peculiar 

 and striking appearance in any other pitchstone which has come 

 under my notice, although there are appearances not much unlike 

 it in some of the varieties found in Arran. 



Those who conceive pitchstone, like basalt, to be of igneous 

 origin, will have little difficulty in explaining this phenomenon, 

 and will even find in it strong evidence to support that theory. 

 It is unnecessary to enter on a reasoning so obvious. 



