Dr Colquhonn cm the Argillaceous Ore of Iron. 83 



this country. So great a contamination is it considered, that 

 the visible presence of iron pyrites to any amount in a coal is 

 enough to cause its immediate rejection by the iron-smelter. 

 There are some of the best coals which are almost entirely 

 free from it ; but, in many others, it is found to exist in vari- 

 ous proportions, and sometimes to prevail so largely diffused 

 in certain strata, that they cannot be applied even to the or- 

 dinary purposes of domestic fuel'. The distribution of the 

 iron-pyrites in any stratum of coal is in most cases extremely 

 irregular. Where the contamination is greatest, the pyrites 

 sometimes exists in large isolated masses, which are either pure, 

 or but slightly intermixed with carbonaceous matter. At 

 other places of the same stratum, it will almost entirely disap- 

 pear, leaving the coal nearly, if not altogether, free from its 

 presence. A similar inequality in the distribution of iron py- 

 rites may be observed on examining detached fragments of the 

 coal belonging to such strata. It is generally found to exist 

 in small disconnected portions, in thin veins, or in incrusta- 

 tions ; while the coal which is immediately contiguous and 

 circumjacent to these visible intermixtures, is absolutely un- 

 tainted with sulphur. 



The extraneous earthy matters which occur associated with 

 coal are of two kinds, the argillaceous and the stoney. The 

 first of these exists diffused through the whole constitution of 

 the coal, in a state of so minute division, and so intimate me- 

 chanical incorporation, that it is quite invisible even with the 

 aid of a magnifying glass. It appears to be universal in its 

 distribution among the coals, just like the small proportion of 

 clay which is similarly diffused among its kindred ironstones, 

 and limestones, or like the trifling amount of bituminous mat- 

 ter which occurs in those minerals and in the schists by which 

 they are accompanied. 



The stoney matter which is found in coal occurs very irre- 

 gularly distributed through it, and is visibly distinct from its 

 substance. It is most frequently found in the shape of thin 

 and semitranslucent crusts, or veins traversing the coal, pa- 

 rallel to each other, and in a direction which is perpendicular 

 to the general stratum. These again are not unfrequently in- 

 tersected at right angles by another set of similar veins, which 



