135 ECONOMIC GEOLOGY 
which have thrown the limestones against shale; the ore is 
bounded on one side by the fault and passes on the other 
into limestone with such an irregular boundary that, according 
fo the miner's expression, the ore and rock have “grown 
together.” In places the ore encloses masses of limestone 
and beds of shale in their original position, and casts of fossil 
shells. These hematites are therefore due to replacement, 
but whether by ascending solutions connected with the late 
Carboniferous and early Permian or the Kainozoic igneous 
activity, or to descending solutions of post-Triassic age, has 
been long debated. Pebbles of haematite in the breccias at 
the local base of the Permain have been regarded as evidence 
that the ores are earlier than Middle Permian. The evidence 
is however in favour of the view that the ores were derived 
from the Permian and Triassic red rocks by water which 
percolated downward and deposited the iron in the lime- 
stones, the hematite in the breccias being replaced pebbles. 
Bilbao—The iron mines of Bilbao in Northern Spain are 
famous for their high-grade hematite, which being low in 
sulphur and phosphorus was especially adapted for the 
Bessemer process, and was largely used in British iron-works. 
The mines were opened about 1865; their development 
was delayed by the Carlist War from 1872 to 1876, after 
which active mining began. The output increased rapidly 
and attained its maximum of nearly six million tons in 1899 ; 
there has since been a steady decline, interrupted by the 
War, until 1925 when the output was two million tons. The 
price of the ore before the War averaged about 11s. a ton. 
It rose during the War to 18s. 6d., but has fallen since to 
about 8s. About a third of the present output is spathic 
iron which is less profitable, as it has to be calcined from 
carbonate into oxide at a cost of about 3s. a ton, and calcined 
ore has a lower price than natural oxide, The cost of mining 
has moreover increased with depth, and with the narrowing 
of the ore. 
The Bilbao iron mines occur (Fig. 41) in an anticline of 
Lower Cretaceous beds, which trends approximately from 
W.N.W to ES.E. Its arch had been faulted parallel to 
the strike, and across it from N.N.W. to S.5.E. The faults 
have dropped blocks of limestone against the underlying 
sandstones, and the ore occurs in the down-faulted blocks.