104 



may be called either glassy or pitchy) . There are, however, 

 distinctions in some of them, which may be founded on 

 geological principles, as their being chiefly combined with 

 either Lime, Magnesia, or Alumine and Alkali, which often, 

 perhaps, depends upon the kind of roek they are formed 

 in : the colouring matter being either Bitumen, Manganese, 

 or Iron, in different states of oxygenization and quantity. 



Pitch is of different textures, and has a fracture more or 

 less large or small, conchoidal, or glassy, and lighter or 

 darker in the splintery illinitions, depending on the degree 

 of heat to which it has been subjected. Pitchstone varies 

 as much j therefore, the present specimen may be called a 

 Pitchstone having a glassy appearance, with a rather small 

 conchoidal and sharp splintery fracture. As it includes 

 Feldspar, which spots it in some parts, it may be called 

 Pitchstone Porphyry. It is generally harder than glass, or 

 will scratch glass as Flint does. We are not sure that even 

 Bitumen enough may not be found hereafter, to mark its 

 analogy with Pitch very completely. 



The present specimens were obtained from Arran, where 

 the substance seems to be found in veins in a Porphyry 

 rock, and to pass into the more usual appearance of Pitch. 

 It has occasionally a basaltic aspect. 



The visible and chemical characters of Obsidian agree so 

 closely with those of Pitchstone> that they must surely be 

 considered as the same species. 



