Dr Colquhoun on the Argillaceous Ore of Iron. 71 



undergone this alteration, it is very generally found to Lave 

 come into the vicinity of whinstone. And the individual cha- 

 racters of this rock, as well as its geological relations to the 

 strata in which it occasionally makes its appearance, conspire 

 to prove that it has been forced up from beneath in a state of 

 igneous liquefaction, among the minerals which compose the 

 coal formations at some period subsequent to their consolida- 

 tion, and that it has, of course, induced more or less alteration 

 upon them, in proportion to their comparative liability to suf- 

 fer decomposition from the application of heat. The colour 

 of the ironstone thus altered depends both on the extent to 

 which the decomposition had proceeded, and on the nature of 

 the extraneous ingredients which it happened to contain : it 

 varies from red to purple and reddish black. 



The five subdivisions above described seem to mark the 

 leading varieties which occur in the ironstone. The next 

 point of view in which we have proposed to regard it is its ge- 

 ological character. In this part of the subject we shall confine 

 ourselves to giving a rapid and general delineation of the most 

 prominent features of its history. 



One of the most striking geological peculiarities of the ar- 

 gillaceous carbonate of iron, is its almost universal occurrence 

 in the immediate vicinity of a stratum of coal. How import- 

 ant this circumstance is to the manufacture of iron, we shall no- 

 tice afterwards in speaking of the smelting of the ore, but at 

 present we shall only remark that so invariable a connexion 

 appears to subsist between the two minerals, that the discovery 

 of the one in any situation, is the surest proof which can be 

 obtained of the near vicinity of the other. In those immense 

 isolated basins, the independent coal formations, which are 

 found scattered throughout Britain, and many parts of the 

 continent of Europe, the argillaceous carbonate of iron is al- 

 ways found to constitute a component mineral. It belongs 

 therefore to that portion of the crust of the earth which has 

 been distinguished by the appellation of secondary. There 

 are some continental mineralogists, indeed, who state that it 

 has also been found occasionally associated with minerals of 

 the transition series ; but if the fact be so, at least no instance 

 of it appears to have yet been met with in this country. 



