ORES OF IRON 
137 
The ores are mainly in a coral limestone, which is from 
150-300 feet thick. Its age corresponds to the Lower 
Greensand. It is overlain by shales and sandstones, some 
of which contain pyrites and abundant nodules of clay 
ironstone which beside the Abandanado Mine have been 
altered into iron oxide. The main ore is brown hematite, 
and its banded botryoidal stalactitic structure shows that it 
ls of secondary formation. It rests on a massive carbonate 
of iron which passes into the oxide and often includes lumps 
of limestone. 
As these ores are secondary limonite, as they narrow below 
and rest on hummocks and pinnacles of limestone, it is 
natural to explain them by the replacement of limestone, 
NW. 
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Sh. 
Fie. 41.—THE BILBAO ANTICLINE. 
Section across the Bilbao anticline; the iron mine at H, between faults 
FF. The beds consist of Cretaceous shales containing, CLL, clay 
Ironstones ; Sh,, shale: L. limestone: the lowest bed is sandstone. 
and to a smaller extent of sandstone and shale, by solutions 
that obtained their iron from the once extensive overlying 
beds. The view was therefore adopted (as by F. D. Adams, 
Fourn. Canad. Min. Inst, 1901, iv, p. 202; Lindgren, 
Mineral Deposits, 1913, p. 310) that the spathic ore results 
from the descent of an iron solution which altered the lime- 
stone into siderite. 
According to an alternative view (e.g. Beyschlag, Vogt, 
and Krusch, Ore Deposits, 1916, ii, p. 833; R. W. van der 
Veen, Econ. Geol., xvii, 1922, p. 602; R. M. de Rotaeche, 
Minas de Bilbao, 1926, p. 157), the iron is of hydrothermal 
origin, came up along the faults, and was deposited as 
siderite by the alteration of the limestone and the replace- 
ment of sandstone and shale; and this primary carbonate 
Ore was altered to oxide by descending water. Siderite