528 PITCHSTONE. 



the same circumstances, as is further pointed out 

 in the Synopsis. 



Pitchstone presents many remarkable varie- 

 ties of structure, both on the great and small scale. 

 In Arran, it is imperfectly prismatic on a very 

 large scale, the prisms being at right angles to the 

 plane of the vein . In Egg, the prisms are smaller, 

 but capable, in many cases, of being easily de- 

 tached, and often, of very perfect forms : being, at 

 the same time, placed in various intricate direc- 

 tions with regard to the plane of the vein, and, 

 in such a manner, that the ends of the several 

 prisms are extenuated, by being compressed be- 

 tween the middle parts of those adjoining. The 

 laminar disposition of the veins, noticed in a pre- 

 ceding paragraph, is sometimes also on a scale so 

 large as to divide the vein into two or three sepa- 

 rate portions, which are most frequently marked 

 by some corresponding change of character in the 

 different laminae. 



The smaller varieties of structure, are nume- 

 rous, and often very remarkable. Among these, 

 a laminar concretionary structure is not unfre- 

 quent ; and the lamellae, which vary very much 



