74 Dr Colquhoun on the Ar^lUaceous Ore of Iron. 



the most part also make their appearance in schist, but they are 

 occasionally found imbedded in other minerals, such as indu- 

 rated clay, limestone and coal. * A continuous stratum of the 

 ore is termed a hand of ironstone. 



The bands of ironstone are found to vary in, thickness from 

 half an inch to sixteen inches, but they most frequently run 

 from six to eight inches thick. 



The balls or nodules have generally a flattened form, and it 

 is invariably observed that the long diameter lies in a direction 

 strictly parallel to the stratum in which they are imbedded. 

 They present themselves of every size from a few inches to four 

 or five feet in horizontal diameter, and rather less than hali* 

 these dimensions vertically. Their weight varies from a few 

 ounces to upwards of a ton. Those of most general occurrence 

 seldom exceed twelve inches in their long diameter. The ex- 

 ternal surface of the balls is smooth, and its predominant co- 

 lour inclines to red. In structure, they are compact, except 

 in those rare cases where they are found to consist of concen- 

 tric lamellae, and which occurs when the exterior portion of the 

 carbonate of iron has begun to undergo decomposition. Upon 

 being fractured, they exhibit nearly the same appearances as 

 the band ironstone. 



There is a very singular peculiarity which is occasionally 

 observed in these balls of ironstone, and which belongs in an 

 eminent degree to some individual stratifications of them, al- 

 though it cannot be said to be possessed universally by all the 

 nodules situated in any stratum. A considerable quantity of 

 a black-coloured bituminous substance is found to be inclosed 

 within the nodule. This bitumen is soft and slightly elastic. 

 It is destitute of taste, and is inodorous when cold, but emits 

 a weak bituminous odour upon being slightly heated. It burns 

 without leaving any earthy residue, according to an experiment 

 of Mr Mushet. •(• Many balls have been lately excavated in 

 the coal field around Glasgow which contained on examination 



• I am not aware that balls of ironstone have been met with in Scotland 

 imbedded in coal ; but an instance of this kind, occurring at Anzin in 

 France, is related by MM. Clere and Tournelle in the Annates des Mines, iv. 

 345. 



t Philosophical Magazine, iii. 47* 



