138 ECONOMIC GEOLOGY 
is a common primary vein mineral and often replaces sand- 
stone, as shown by the * whinny boles * or nodules of siderite 
in Glasgow quarries. Where however siderite occurs as a 
hydrothermal product it is usually associated with copper, 
lead, and zinc. Thus in the hydrothermal siderite lodes of 
Czecho-Slovakia and the Siegerland (cf. p. 134) quartz is 
the chief veinstone, pyrite and chalcopyrite are frequent, 
while blende, galena, tetrahedrite, bornite, various nickel 
and cobalt minerals, and stibnite also occur. A little copper 
has been found at Bilbao, but the amount is small and the 
ordinary hydrothermal minerals are absent. The nearest 
igneous rocks are too remote to have taken part in the ore for- 
mation, especially as there is no ore beside them. 
The Bilbao ores are therefore probably due to the perco- 
lation of descending waters along the fractures beside the 
down-faulted crown of the Bilbao anticline. Solutions passed 
through the shattered rocks, leached out their iron, and en- 
tering the limestone altered it to carbonate of iron and pro- 
duced siliceous ores where the solutions entered sandstones 
and sandy shales. Subsequently the upper part of the ore 
was altered into hzmatite. 
Lake Superior—The iron fields that have proved of the 
highest industrial importance are beside Lake Superior, and 
have supplied most of the ore to the iron works of Penn- 
sylvania.! They still hold 2500 million tons of ore. The 
chief fields on the southern side of Lake Superior are those 
of Marquette, Menominee, and Gogebic; on the north- 
western side are the Mesabi and Vermilion fields whence 
lower-grade iron ores, on the same geological horizon, extend 
eastward into Canada. The ore is mostly hematite associated 
with jaspers and chert; some of the ore was a ferruginous 
carbonate with oolitic grains and greenalite, which from its 
analogy with glauconite, was doubtless of marine origin 
(Fig. 42). The Iron Formation belongs to the Keewatin 
Series, of which the typical rock is a basic pillow-lava. This 
series rests on gneisses and coarse schists with intrusive 
granites, and is covered unconformably by sedimentary 
rocks and conglomerates. The whole area of the iron fields 
was probably buried under the red sandstones and associated 
L U.8.G.S., Monographs Nos, 19, 28, 36, 43, 45, 46, 52, 1802-1911.