PITCHSTONE. 269 



Veins of pitchstone vary considerably in breadth, 

 ranging from many yards to a few inches ; and where 

 basaltic veins become pitchstone during the progress 

 of ramification, they descend to the dimensions of a 

 thread. In composition, they are not always the same 

 throughout ; many different qualities sometimes occur- 

 ring in the same vein. In these cases, the most per- 

 fect, or vitreous-like parts, are found at the sides ; 

 and they present differences of colour, as well as of 

 texture and aspect. Occasionally, like those of trap, 

 they possess a laminar disposition, parallel to the vein ; 

 and, in such instances, there is sometimes a change of 

 substance where each lamina terminates ; the pitch- 

 stone passing into chalcedony and into chert. In a 

 few cases, they exhibit a prismatic tendency at right 

 angles to their course ; while that which forms the 

 Scuir of Egg is divided into columns somewhat paral- 

 lel to its sides, but so placed with regard to the planes of 

 the vein, and so entangled, that the ends of many of 

 them are extenuated by being compressed between the 

 middle portions of those adjoining. 



The internal structure of pitchstone is various. Oc- 

 casionally, it is uniform, breaking in every direction 

 alike, with a fracture more or less perfectly conchoidal, 

 or flat, or splintery, or imperfectly granular, or com- 

 pounded of two or more of all those. In other cases, 

 it presents an internal concretionary structure; the 

 concretions being irregularly spheroidal, or prismatic, 

 or cylindrical, or lamellar of different sizes, and either 

 straight or curved. In a few instances, the prismatic 

 concretions are jointed, with surfaces alternately con- 

 cave and convex ; each joint sometimes also contain- 

 ing a central atom of felspar, or of a white enamel. 

 Through the spheroidal structure it passes into Pearl- 

 stone, as I shall immediately show ; these being here 

 considered as varieties of one rock ; though for the 



