ARRAN. GEOLOGY. PITCHSTONE. 421 



tions, most of which are, in different respects, interest- 

 ing; while many of them illustrate circumstances in the 

 history of pitchstone that have either been overlooked 

 or misapprehended. For these reasons the leading cha- 

 racters of the most remarkable varieties will be described ; 

 by which method the mineralogist will become more 

 perfectly acquainted with this substance as it occurs in 

 Arran, than by any minute description of two or three 

 individuals. Those which I have selected for notice are 

 the following : 



1 . Uniformly small conchoidal in the fracture, and bottle 

 green.* 



2. The same, with minute grains of quartz interspersed. 



3. Concretionary, simple, and of a bottle green colour; 

 presenting great variety in the mode of breaking, or in the 

 forms of the fragments. The following are r the most 

 remarkable : 



Spheroids or ellipsoids of irregular forms, varying from 

 one to three inches in length, and from half an inch to two 

 inches in breadth. 



Columns of an irregular shape, some sides being flat 

 and others round, from one sixth of an inch to one inch 

 and a half in diameter, and reaching to five inches in 

 length. 



Prisms, square, triangular, or polygonal. 



Lamellar concretions, straight or curved. 



Columns, separating into joints with concave and convex 

 surfaces. 



These several structures pass into each other ; and the 

 lamellar, which are most frequent, are sometimes procured 

 in fragments a foot long and six inches wide ; their thick- 

 ness varying from half an inch to an inch. This variety 

 occurs only about Corygills. 



4. Simple, of a bottle green colour, alternating with 



* Leek green of some. The colour is nearly that of bottle glass, 



