Dr Colquhoun 07i the Argillaceotcs Ore of Iron. 229 



spar. Its rank in a mineralogical system must be along with 

 the native crystallized carbonate of protoxide of iron. It is 

 characterized however by too many peculiarities, both chemi- 

 cally and geologically, to be classed otherwise than as a dis- 

 tinct sub-species. The geological characteristics of the sub- 

 species we shall defer examining, according to the order which 

 has just been laid down, until the mineralogical history sh^l 

 have been disposed of. ^'/^. . 



That we may be able to appreciate what are the properties 

 which truly distinguish the argillaceous carbonate as a sepa- 

 rate sub-species, it will be necessary to keep in view the cha- 

 racters of the general species to which it belongs. These are, 

 in the type of the species, a regular crystallized form, reduci- 

 ble to an obtuse rhomboid, transparency, and absence of co- 

 lour. The less pure specimens are translucent and sometimes 

 opaque, and their colour varies from pale yellow to deep 

 brown ; but even when occurring in mountainous masses, it 

 almost invariably possesses traces, more or less decisive, of 

 crystallization. 



The most palpable peculiarity of the argillaceous carbonate 

 of iron is that it never occurs crystallized, and never exhibits 

 even the most remote indication of a crystalline structure. 



Another important characteristic is to be observed in attend- 

 ing to those substances which are found to enter into its com- 

 position, and which are of a nature completely foreign to the 

 carbonate of iron. These substances are very various, both 

 in their kind, and in their proportions ; but they may be di- 

 vided into two general classes, the earthy or argillaceous, and 

 the carbonaceous or bituminous. We shall consider each of 

 them separately. 



In respect to the first, the argillaceous constituent, it is be- 

 lieved that no specimen of our ore has yet been discovered, 

 which did not contain a notable quantity of earthy matter. 

 The average amount of this ingredient ranges from 10 to SO 

 per cent. ; though an ore containing so little impurity as the 

 first mentioned quantity is rarely met with, while, on the other 

 hand, it may sometimes be found in a much larger proportion , 

 occasionally amounting to 25 or 30 per cent. But whenever it 

 goes beyond 12 or 15 per cent., the iron smelter cannot use it 



