518 EGG. GEOLOGY. 



neighbourhood are to be seen the two well known veins 

 of pitchstone, lying within a few yards of each other. 

 Both of these traverse the surrounding rocks in a posi- 

 tion nearly vertical. They are both exceedingly unequal 

 in thickness in different parts of their courses; varying 

 from the breadth of three feet or more, to that of an inch 

 or less. They are more than once interrupted and broken 

 off, as well as shifted, during their progress ; and these 

 changes are most visible in the westernmost. There is 

 no union of the veins and the surrounding rock at the 

 planes of contact. The easternmost is the largest, and 

 is split into two parts by an intruding mass of rock which 

 has the aspect and characters of chert ; being of a light 

 grey colour, extremely hard, sharp in the fragments, and 

 sonorous : it contains besides a few brilliant crystals of 

 glassy felspar. It possesses no apparent analogy either 

 to the vein or the surrounding rock ; in which respect 

 it is however not singular, as veins of porphyry have 

 in general, in other cases, as little reference to the sub- 

 stances which they traverse. This vein of the pitchstone 

 has in some places a weathered surface, presenting a scori- 

 form and cavernous aspect much resembling that of 

 certain basalts. It is of an olive green colour in the 

 centre, where it is thickest, gradually becoming of a dark 

 blue and ultimately black at the edges. The outer parts 

 are laminar, so as to admit of being detached in the form 

 of large fissile plates, and I must add that the vitreous 

 aspect is here most perfect, diminishing gradually in pro- 

 ceeding towards the middle of the vein. The black parts 

 become in the first stage of weathering covered with a 

 black powder, and can scarcely at first sight be distin- 

 guished from the most brilliant varieties of coal. The 

 blue portions sometimes present a mealy aspect, like that 

 of the bloom on the surface of a plum, and in this case 

 resemble strongly a blue glass occasionally seen amongst 

 that of which common bottles are made. Small crystals 

 of glassy felspar are sparingly dispersed through this pitch- 



