04 
ECONOMIC GEOLOGY 
replaced the shale, and has mineralized the underlying sand- 
stone 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 sand- 
stones 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 dis- 
solved 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 prob- 
ably 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 conglomer- 
ates interbedded with diabase lavas and volcanic ash be- 
longing 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.