
232 STRUCTURAL AND FIELD GEOLOGY 

case oxidation is prevented, and the iron is then thrown down 
as acarbonate. The pisolitic limonite forming in the shallow 
waters of many existing lakes, and the earthy bog iron-ore 
so frequently present in swampy land, are good examples of 
this class of ore-formations. Bedded iron-ores (both oxides 
and carbonates) are met with in many geological systems— 
ranging from post-Tertiary to Palzozoic horizons. While 
many of these are of freshwater or brackish-water origin, 
others are marine. As examples may be cited the iron-ore 
of Rio Tinto, in the province of Huelva, Spain—a deposit of 
Recent age; the Mesozoic limonites and earthy carbonates 
of the Lias, the Great Odlite, the Wealden, and the Lower 
Greensand, in England; and the clay-ironstones which are so 
abundantly developed in the Carboniferous system of this 
and other countries (Fig. 82). Most of the ironstones last 

Fic. 82.—SEAMS AND NODULES OF CLAY-IRONSTONE IN 
CARBONIFEROUS SHALES. 
referred to appear to have been formed by direct precipitation 
in lakes and lagoons. In the case of the nodular concretions 
met with in the same series of strata, we have examples of 
the subsequent concentration or aggregation of ferruginous 
matter, originally diffused through the beds in which such 
nodules occur. 
Manganese ores (pyrolusite, psilomelane, wad) are not so 
abundantly met with as iron-ores. They occur, however, under 
similar conditions amongst sedimentary rocks of all ages, some- 
times as concretionary nodules, at other times in layers and 
beds, which are not infrequently pisolitic. 
(2) Clastic Ore-Formations.—These are alluvial deposits, 
derived from the disintegration of metalliferous rocks and 
ore-bodies of various origin, and are known to mining men as 
Placers (Fig. 83). The metals obtained from such deposits 
are chiefly gold, platinum, and tin. The beds vary much in 
