Dr Colquhoun on the Argillaceous Ore of Iron. 241 



rrrey. Frequently, also, it is red^ brozmiish-red, and brown ; and 

 when this is the case it probably contains a certain quantity of 

 the peroxide of iron. The presence of bituminous matter 

 darkens the shade of all these colours ; and when this substance 

 exists to any considerable extent, it often deepens the hue into 

 a perfect black. 



The texture is most commonly compact^ but sometimes, also, 

 it is tabular or slaty. Even the nodular varieties of the ore, 

 although they occasionally consist of concentric layers, are in 

 general quite compact and uniform in their composition. 



The fracture in the large is compact^ and more or less un- 

 even ; sometimes, also, imperfect conchoidal or slaty. In the small 

 it is generally even ^fine-grained, earthy ; but sometimes coarse- 

 grained, and slightly uneven. It frequently happens that the 

 nature of the fracture differs in different parts of the same spe- 

 cimen, a variation which is probably caused by a different state 

 of mechanical intermixture with other bodies. 



The aspect of xhQjractured surface is didl and earthy. The 

 fragments are rather blunt edged, and have no determinate 

 form. 



The ore varies much in hardness : being sometimes hard and 

 tough, and difficultly frangible ; at other times, soft, and easily 

 reduced to powder. In general, it is easily scratched by the 

 Jcnife. It feels meagre to the touch, and sometimes adheres 

 slightly to the tongue. Its specific gravity varies from 2.8 or 

 2.9 to 3.4 or 3.5. 



A frequent distribution of foreign bodies occurs in the mass 

 of the ore, consisting partly of organic, and partly of inorganic 

 substances. Of the former, the most common are shells, gene- 

 rally of the bivalve description, and impressions of vegetables. ' 

 Impressions of fishes are also similarly discovered, though these 

 remains of organized nature are much more rarely met with. 

 Of the latter, the most frequent are carbonate of lime and iron 

 pyrites, which are sometimes disseminated through the ore in 

 small pieces, and at other times penetrate it in the form of 

 slender veins. Sulphate of lime, blende, galena, and copper 

 pyrites are also occasionally observed : but these latter bodies, 

 especially the metallic sulphurets, are of infrequent occurrence. 

 When the ore is calcined, its grey colour changes to red. 



