70 Dr Colquhoun on the Argillaceous Ore of Iron. 



phuret of iron. Massive pyrites occurred abundantly in the 

 neighbouring beds, but that which existed in the mineral was 

 in a state of such extreme division, that it could not be obser- 

 ved even with the aid of a magnifying glass. 

 • 5. The last distinct variety of the ore may be made to in- 

 clude all the minute varieties of external form, which are the 

 result of the contact of some organized body, as a fish, a shell, 

 a vegetable. All these are not uncommonly met with. M. 

 Berthier has given a particular description of a specimen of 

 this kind, which aflPected the form of trunks of trees. It was 

 found in a district (departement du Cantal) belonging to an 

 alluvial formation. * 



It is worthy of remark that in every specimen of this ore, 

 the partial decomposition of the carbonated protoxide of iron, 

 (a process of frequent occurrence,) has the effect of introdu- 

 cing into the composition of the mineral a mixture of the per- 

 oxide of iron or of the hydrated peroxide, according to cir- 

 cumstances. The hydrate thus formed is occasionally met 

 with, prevailing over a considerable tract of country. It is 

 always found to retain the form of the original carbonate, 

 whether that had existed in the shape of bands, nodules, or 

 otherwise. In general it possesses most of the characters of 

 common iron ochre : but, as the transformation of the carbo- 

 nated protoxide into the hydrated peroxide does not seem to 

 occasion any sensible diminution of the bulk of the ore, it is 

 of lighter specific gravity than the genuine hoematites.-f* The 

 infiltration of water, acting in a slow and imperceptible man- 

 ner, seems to be the cause which produces this decomposed 

 form of the ore. 



There are many localities, however, where the hydrated 

 peroxide of iron is never observed, but where, on the contrary, 

 it is an extremely frequent occurrence to find a portion of a 

 stratum converted into the simple peroxide. We possess the 

 strongest reason for believing that the decomposition of the 

 ore, on all such occasions, has been effected through the agen- 

 cy of heat. Whenever the stratum of ore is observed to have 



* Journal des Mines, xxvii. 477. 



•f- Berthier, Sur les mineratix de fer, aj^les mines douces. Ann. des 

 Mines, ix. 82.5. 



