188 Dll. BENNETT, ON THE STRUCTURE OF 



be found to present a highly fibrous fracture, minute chips of 

 which exhibit at their edges distinctly dotted or porous ducts. 



On examining the Torbanehill mineral with the naked eye, 

 it is destitute of a fibrous structure, and presents a homoge- 

 neous appearance in whatever way it is fractured or cut. It 

 is tough and hard to break, when compared with coal, has a 

 dull brown streak, and is readily ground down into thin slices 

 of any degree of tenuity. Some specimens are of a dark, and 

 others of a light brown colour. The section of a dark speci- 

 men seen under a magnifying power of 200 diameters, presents, 

 first, a number of yellowish and reddish-brown transparent 

 masses, of a rounded form with an irregular outline, varying 

 in size from the 750 o^h to the a^Jgtli of an inch in diameter. 

 These are surrounded by a dark opaque substance, in which 

 they appear to be imbedded, and in which no trace of struc- 

 ture can be detected. These light and dark substances vary 

 in relative amount in different specimens of the mineral, and 

 according to the thickness of the section. In some specimens, 

 the rounded transparent masses are more widely separated by 

 the opaque substance, but in others, they are often so close, 

 that a very thin section presents a homogeneous appearance of 

 yellowish or reddish-yellow matter, resembling bees-wax, with 

 only a few irregular spots of the black matter. In some sec- 

 tions, especially of the light-brown specimens, the rounded 

 masses, as they are ground thinner, may be seen, as it were, to 

 melt into one another. In such sections, no difference what- 

 ever can be made out, whether they be made in a longitudinal 

 or in a horizontal direction. 



In some thin sections, these rounded transparent bodies can 

 be separated from one another, and be distinctly seen to 

 possess a radiated crystalline appearance, strongly reminding 

 one of the crystals of carbonate of lime which occur in urine. 

 At certain angles, also, a few of them refract light, and become 

 strongly tinted with the orange ray when polarized, — a cir- 

 cumstance perhaps dependent on the admixture of mineral 

 matter. When a section of the mineral, presenting both the 

 substances described, is held over the flame of a lamp, the 

 yellow matter evaporates in the form of thick smoke, leaving 

 the black matter unaffected, with large holes or loculi in it. It 

 must be clear from this experiment that the yellow matter is 

 some bituminous or resinous substance, easily decomposed by 

 the heat of a lamp, and that the black matter is an earthy 

 material, which resists the same amount of heat. We can 

 have no doubt, therefore, tliat an easily volatilized and highly 

 inflammable matter has concieted in the form of rounded 

 masses, and constitutes the light-coloured portion of the mi- 



