48 Mac Culloch on Pifchstone. 



Occasionally, these veins like those of trap present a laminar 

 disposition, the lamina being parallel to the vein ; and, in such 

 instances, it sometimes happens that there is a change of sub- 

 stance where each lamina terminates; the pitch-stone passing 

 into chalcedony and into chert. In a few instances they present 

 a prismatic tendency at right angles to the course of the vein ; 

 and if the Scuir of Egg be also a vein, they must further be 

 conceived capable of dividing into regular columns parallel to 

 its sides. 



In respect to the internal disposition of pitch-stone as a rock, 

 it is various, presenting many remarkable peculiarities of 

 structure, both on the great and small scale. In Arran, it is 

 found 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 of being easily detached, in 

 many cases, and often of very perfect forms ; being at the 

 same time placed in various intricate directions 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 between 

 the middle parts of those adjoining. The laminar disposition 

 of the veins noticed in a preceding paragraph, sometimes also 

 occurs on a scale so large as to divide the vein into two or 

 three separate portions, which are most frequently marked by 

 some corresponding change of character in the different 

 laminae. 



The smaller varieties of structure are numerous, and 

 often very remarkable. Among these, a laminar concretionary 

 structure is not unfrequent ; and the lamellae which vary much 

 in size, are either straight or curved. In some cases they are 

 as thin as paper; in others they are thick, and further sepa- 

 rable by joints at right angles to the planes of the lamellae. 

 Thus they become divisible so as sometimes to present a minute 

 prismatic disposition. 



Occasionally these prisms present also some curved surfaces 

 together with the straight, so as to form columns on a very 

 small scale, which are further jointed, in some very rare in- 

 stances, by surfaces alternately concave and convex ; and, 



