[O00 
ECONOMIC GEOLOGY 
limestone. The ores in the field of Alston Moor are mainly 
in a zone about 600 feet thick; in Derbyshire they are 
worked to the depth of 1100 feet, and many of the upper 
veins are rich in fluorite. 
The primary lodes in Flintshire are along fault fissures 
in Carboniferous Limestone, and trend from N.W. to S.E.; 
they contain galena, blende, and about 15 oz. of silver to 
the ton. Some cross-courses contain secondary ore which 
contains no zinc, and is poor in silver. In the Isle of Man the 
Laxey Lode has been worked through the Carboniferous 
Limestone into slate and granite, to the depth of 1900 feet. 
The lead lodes in the Ordovician sandstones, grits, and 
graywackes are often more regular than those in limestone, 
as the fractures are simple and straight. The lodes of Lead- 
hills and Wanlockhead in Scotland (J. Mitchell, Ming. Mag., 
xxi, 1919, pp. 10-20) are confined to the Lowther Grits, as 
the fissures in the slates were closed by the collapse of the 
walls. The lodes are usually about 2 feet thick, but widen 
up to 18 feet. The ore is usually in shoots. The first-formed 
minerals in the primary ore are quartz, dolomite, and calcite ; 
next pyrites and chalcopyrite; subsequently galena and 
blende. A second mineralization produced much larger 
crystals. Finally surface waters formed the rare minerals 
for which the gossan and oxidized ore of these mines are 
famous. The age of the ore formation has been attributed 
to the Caledonian; but the parallelism of the lodes to the 
faults in the Hamilton and Sanquhar coalfields and the partial 
mineralization of some Lower Devonian felsite dykes indicate 
their Permian age. 
The Shropshire lead-field at Shelve and Snailbeach was 
worked by the Romans, and has been mined to the depth of 
1650 feet; the lodes are in the sandstones of an anticline, 
and are unproductive in the shales. Barite is the chief 
veinstone in the upper part; the proportion of zinc to galena 
increases with depth. 
The lodes in Central Wales lie along faults in Ordovician 
slates and mudstones; they contain much brecciated 
rock and the walls are strongly slicken-sided. The lodes 
extend for miles trending E.N.E. with ore-shoots at intervals. 
Much of the ore is found along secondary fractures, some of 
which are horizontal flats. The lead ores in Cornwall and