240 Dr Colquboun on the Argillaceous Ore of Iron. 



the ore is found to be incorporated by an intimate mechanical 

 union, with so many of the minerals whose strata lie in juxta- 

 position, or in near contact with it, that it is often difficult to 

 mark the proper mineralogical territory of the ironstone. It 

 is found to exist intermixed with carbonate of lime, with argil- 

 laceous schist, witb coal, and with sandstone in every variety 

 of proportion ; and as it is not in a state of crystalline aggre- 

 gation, it of necessity assumes the external characters of those 

 extraneous substances, precisely according to the extent of their 

 intermixture. Thus, it is not only destitute of the most decid- 

 ed and certain characteristic of minerals, a crystalline figure, 

 but the remaining external characters, such as colour, hard- 

 ness, fracture, texture, and specific gravity are subject to the 

 greatest irregularity and variation. It is in such a case per- 

 haps impossible to draw any better line of distinction, separa- 

 ting what may be justly considered an argillaceous iron ore, 

 from what may more properly be regarded as a ferruginous 

 limestone, or clay, or coal, than just to take those specimens in 

 which the iron ore is the predominating ingredient to form the 

 one class, and those in which it forms a less prominent consti- 

 tuent to form the second. Tl)is principle of discrimination 

 seems the more appropriate in a metallurgic treatise, since it 

 will certainly embrace all the ironstones which are an object of 

 interest for the metal which they yield. But even after adopt- 

 ing it as marking out the precise range of the ore within which 

 the following classification will be confined, the more minute di- 

 vision of the field into the several sub-species and varieties that 

 occupy it, will still prove somewhat embarrassing from the 

 causes which we have just mentioned. We shall endeavour, 

 in the first place, to state the general average of external cha- 

 racter which they exhibit, and to mark the extremes within 

 which the variation of each character is limited ; and, in the 

 next place, to group the different ores into such subdivisions as 

 may be suggested by their leading chemical variations. The 

 genuiiie mineralogical character of the sub-species of the carbo- 

 nate of protoxide of iron, the argillaceous carbonate of iron, is 

 the following :— 



The predominating colour of the ore is grey, of various 

 shades, as light ash-grey y bluish-grey^ blackish-grey, yellowish^ 



