ORES OF LEAD, ZINC, AND SILVER 97 
sandstone are barren in slate, which being softer may be 
crushed and not form an open ‘fissure ; ‘the conditions are 
reversed where the sandstone resisted fracture, and the slate 
is fissured. In some fields, as at Przibram in Czecho-Slovakia, 
the lodes are productive both in sandstone and slate. The 
usual veinstones are quartz, calcite, barite, and fluorite. 
Fluorite often occurs only in the upper levels, as superheated 
Steam prevents its formation. The pneumatolytic minerals 
tourmaline and cassiterite are exceptional; but boric acid 
has in places formed axinite. The deep-seated origin of 
the primary lodes is shown by their lead being of a higher 
atomic weight and specific gravity than the uranium lead of 
igneous rocks. 
Primary and secondary lead oresare often closely associated. 
In secondary ores the zinc and lead are usually separated 
Owing to their different solubilities. The secondary ores 
are often banded or radial with large crystals, whereas those 
In primary deposits are usually small and granular. The 
Primary lodes often go deep; they have been worked, for 
example, down to 1900 feet in the Isle of Man, to 1800 feet 
In Cornwall, to 1700 feet in Shropshire, to 1800 feet at Wan- 
lockhead, to 2500 feet at the Ceeur d'Alene in Idaho, to 
3000 feet in Clausthal, and to 3600 feet at Przibram. The 
lodes are usually a few feet thick, but are often widened by 
replacement of the walls. Igneous rocks are absent from 
most lead-fields, and if present, they appear to have had no 
Influence on the mineralization; in Derbyshire they are 
known ag toadstone, a corruption of the German todstein 
or deadstone, which indicates their barrenness and even 
nfavourable effect on the lodes. 
Primary lead lodes are mostly of medium geological age. 
They have been formed at lower temperatures and nearer 
the surface than lodes of copper and tin, but deeper than ores 
of mercury. Lead lodes occur in pre-Palzozoic rocks, at 
Broken Hill in Australia, the Northampton field in Western 
Australia, Northern Rhodesia, British Columbia, Eastern 
Canada, the Eastern United States, and in Scotland ; but 
the lodes may be much younger than the country rock. 
Some of the ores are associated with Kainozoic volcanic 
activity, as at Hauraki in New Zealand. The great ma- 
Jority of primary lead ores were formed during the 
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