34 ECONOMIC GEOLOGY
Sieg River, 40 miles E. of Cologne. These rocks have been
intensely folded and overthrust, and broken by a network
of fractures including vertical and horizontal faults (A.
Denckmann, Glickauf, 10 April, 1926, pp. 458-67). The
ore has been deposited in fissures as irregular swarms and
groups of lodes. Many are small, but some are long and deep.
They usually vary from 5—30 feet in width, and are sharply
separated from the country, which shows but slight replace-
ment. The fissures have been formed and re-opened at
different dates and the ores are well crustified. The first
filling was of siderite and quartz with some pyrite and
chalcopyrite; galena and blende are rare; barite and
fluorite are absent. This mineral association suggests
the formation of the lodes by infiltration from the
tountry rock, a case of lateral secretion, or by descending
solutions, which are suggested by the presence of chalcocite
(Cu,S), and ores containing cobalt, nickel, and antimony.
The Siegerland mines were extensively worked in the middle
of the nineteenth century, until the iron works on the Rhine
were supplied with cheaper ores from Lorraine: later the
ore was used in Silesia.
The iron of these ores, according to one hypothesis, is
derived from layers of spherosiderite that were deposited in
the Devonian sea ; another view considers the ores as primary
lodes due to the Devonian diabase. (For the former view
see A. Denckmann, Arch. Lag., vi, 1912; for the latter,
Bornhardt, #bid., iii, 1910, and viii, 1912; for microscopic
structure of the ores see Krusch, gbid., viii, PP. 447-83.)
RerracEMENT ORES
f. PyriTic Masses—Many valuable iron ores occur in
massive ore-bodies comparatively near the surface, so that
they are easily mined. One representative of this type in-
cludes the great masses of iron pyrites, but as they are worked
primarily for copper or sulphur, they are considered in re-
ference to copper ores (pp. 86-90).
2. Ox1pE ORE BODIES DUE TO DESCENDING SOLUTIONS—
Replacement has formed some of the most valuable iron
ores.
Cumberland—The Lake District of the N.W. of England
