174 - GEOGNOSY. [Boor II. 
importance to deserve a place in the enumeration of carbonaceous 
rocks. Its mineralogical characters have already (p. 63) been 
given. It occurs in distinct lenticular beds, and also diffused in 
minute scales, through slates, schists, and limestones of the older 
geological formations, as in Cumberland, Scotland, Canada, and 
Bohemia. It is likewise found occasionally as the result of the 
alteration of a coal seam by intrusive basalt, as at New Cummock in 
Ayrshire. 
(5.) Ferruginous. 
The decomposition of vegetable matter in marshy places and 
shallow lakes gives rise to certain organic acids, which, together with 
the carbonic acid so generally also present, decompose the ferru- 
ginous minerals of rocks and carry away soluble salts of iron. 
Exposure to the air leads to the rapid decomposition and oxidation 
of those solutions, which consequently give rise to precipitates, 


LN 
consisting partly of insoluble basic salts and partly of the hydrated — 
ferric oxide. These precipitates mingled with clay, sand, or 
other mechanical impurity, and also with dead and decaying 
organisms, form deposits of iron-ore. Operations of this kind 
appear to have been in progress from a remote geological antiquity. 
Hence ironstones with traces of associated organic remains belong 
to many different geological formations, and are being formed 
still.* | 
Bog Iron-Ore (Lake ore, minérai des marais, Sumpferz).—A dark 
brown to black earthy but sometimes compact mixture of hydrated 
peroxide of iron, phosphate of iron, and hydrated oxide of manganese, 
frequently with clay, sand, and organic matter. An ordinary 
specimen yielded, peroxide of iron, 62°59 ; oxide of manganese, 8°52; 
sand, 11:37; phosphoric acid, 1°50; sulphurie acid, traces; water 
and organic matter, 16°02=100:00. Bog iron-ore may either be 
formed wm situ from still water, or may be laid down by currents in 
lakes. Of the former mode of formation, a familiar illustration is 
furnished by the “moor-band pan” or hard ferruginous crust, which 
in boggy places and on some ill-drained land forms at the bottom of 
the soil on the top of a stiff and tolerably impervious subsoil. 
Abundant bog-iron or lake-ore is obtained from the bottoms of 
Jakes in Norway and Sweden. It forms everywhere on the shallower 
slopes near banks of reeds, where there is no strong current of water, 
occurring in granular concretions that vary from the size of grains of 
coarse gunpowder up to nodules 6 inches in diameter, and forming 
layers 10 to 200 ee long, 5 to 15 yards broad, and 8 to 30 inches 
thick. These deposits are worked during winter by inserting 
perforated iron shovels through holes cut in the ice; and so rapidly | 
do they accumulate, that instances are known where, after haying 
been completely removed, the ore at the end of twenty-six years was 
1 See Senft, op. cit. p. 168; also postea, Book III. Part II. Section iii. 
