No. 7.] GENESIS OP IRON ORES. 435 



of pyrites in the decayed crystalline rocks of the Blue Ridga. The 

 similar ores found in the decayed Taconic schists of the groat 

 Appalachian valley can be shown to be due in some cases to the 

 alteration of included masses of pyrites, and in others to the al- 

 teration of similar masses of siderite, both of which species are 

 found in the unaltered Taconic rocks, as, indeed, at various other 

 horizons in the geological series. 



If we take the specific gravity of pyrites at 5.0, we snail find 

 that its complete conversion iuto a limonite of sp. gr. 4.0 would 

 be attended with a contraction of only 2.7 hundredtli.s, while if 

 the limonite have a sp. gr. of 3.6, there would be an augmentation 

 of 10.7 p. c. With siderite of sp. gr. 3.6, on the contrary, its 

 conversion into a limonite of the same density would result in a 

 contraction of 19.5 p. c, and into a limonite of sp. gr. 4.0, in 

 » contraction of 27.5 p. c. The evidences of this contraction 

 may be seen in the structure of the limonite derived from 

 siderite, which is often found a porous or spongy mass. In 

 the case, however, of nodules or blocks of very compact ore, 

 the conversion beginnino; at the outside of the mass, an external 

 layer of compact limonite is formed, and then another within 

 this, and still another, till the change is complete. The void 

 space resulting from contraction is then found between the lay- 

 ers, which are arranged like the coats of an onion, or sometimes 

 wholly at the centre, where a cavity will be formed, holding in 

 many cases, more or less clay or sand, the impurites of carbonate 

 which have been separated in the process of conversion into 

 limonite. In this way are formed the hollow masses sometimes 

 known as bomb- shell ore. Their structure will generally serve 

 to distinguish the sideritic from the pyritic limonites. 



These difi"srences were illustrated in the history of various iron- 

 ores in the Applachian valley, and it was further pointed out 

 that the pyritic limonites, other circumstances being equal, should 

 be freer from phosphorus than those derived from siderite, since 

 the native carbonates almost always contain phosphates, from 

 which pyritous deposits are comparatively free. The source of 

 limonites thus becomes a question of importance to the metallur- 

 gist. In conclusion, it was pointed out that deposits of manganese - 

 ores are, in som« cases at least, generated by the alteration in 

 situ of manganous carbonates, by a process analogous to that 

 by which limonite is produced from siderite. 



Vol. IX. BB No. 7 



