04

ECONOMIC GEOLOGY

replaced the shale, and has mineralized the underlying sandstone
 and occurs in the faults which are probably Kainozoic.
Hence the ore has clearly been deposited or redeposited after
the formation of the shale. Beyschlag attributed the source
of the ore to the underlying lavas ; but they contain no copper
and the ore does not appear due to extraneous solutions.
Bedded copper ore is generally associated with red sandstones
 laid down under arid conditions; and the Mansfeld
ore may be due to alluvial grains derived from the Harz
Mountains and washed into the sandstones, and there dissolved
 and redeposited in the shale.
The ores in the Triassic sandstones of Alderley Edge in
Cheshire! are probably also derived from detrital grains.
The ore is sandstone with the grains cemented by carbonates
of copper and lead. The patches of ore often rest on clay ;
they may pass gradually into the sandstone or be sharply
separated from it. Some of the ore lies in faults, which it
entered from the sandstone. There is no evidence that the
copper was introduced from fissures. The ore was probably
 due to the solution of alluvial grains scattered in the
sandstones and their redeposition where the solution was
kept stagnant by underlying clay. This field between 1857
and 1877 yielded 158,000 tons of ore with an average of 2-1
per cent. of copper, which is similar to the usual grade at
Mansfeld.
The copper fields of Michigan, on the southern side of Lake
Superior, are in pre-Palzozoic sandstones and conglomerates
 interbedded with diabase lavas and volcanic ash belonging
 to the Keweenawan System, which is equivalent
to the British Torridonian. The copper is mostly native.
In places it acts as a cement to the pebbles, replacing the
matrix of the conglomerate and sometimes the pebbles and
boulders also.2 In the volcanic rocks it fills vesicular cavities
and fissures, and is often associated with zeolites. The ore
forms shoots of which the most important. the Calumet and

LCf. Dewey, Geol. Surv. Gt. Brit., Spec. Rep. Min, Res., xxx, 1925,
who, p. 15, adopts the alluvial origin of the ores.
# The replacement nature of the ore was early suggested by Pumpelly,
Proc. Amer. Acad., xiii, 1877, p. 253; for the geology of the field cf.
R. D. Irving, U.S.G.S. Mon., No. 5, 1883; A. C. Lane, Michigan
G.S. Publ, 6, 1911, for formation of the ores, vol. i, PP. 41-4.