E. H. Davison—Primary Zones of Cornish Lodes. 507 
THE Sinver-LEAD Zone. 
The number of instances in which tin- or copper-bearing lodes 
pass upwards into lodes carrying lead and silver is too small to 
admit of any generalization. Silver and lead occurred with tin and 
copper in the Engine Lode, South Tresavean, silver with arsenic 
in the South Entral Lode, Dolcoath, and horn silver in the lode at 
North Dolcoath. Most of the lead-silver lodes have a north and 
sonth direction, and are of distinctly later date than the tin-copper 
lodes. 
THE CoprperR ZONE. 
This zone is characterized by the predominance of copper minerals 
with mispickel as a frequently associated mineral. The copper 
occurs mainly as chalcopyrite, which seems to be the only original 
sulphide, the others present, such as hbornite, erubescite, cuprite, 
native copper, and the oxidized compounds having been formed 
by its secondary alteration. Pyrite and some of the mispickel 
seem also to be of secondary origin. 
Portreath. Porkellis. 
~ 
/. 
be pee 
ore e-tin OT in ae 
mons Shel ° i} 
te ee seo XY 
Tin Yom iS ¢ 
Fia. 1.—Sketch section to show relation of zones to granite surface. 
The chief gangue minerals are quartz, chlorite, fluorspar, and 
muscovite; tourmaline is not usually an important component, 
but occurs locally in one or two districts. 
The copper zone is in nearly every case restricted to the clay 
slate; its lower limit in the Camborne-Redruth district hes at an 
average depth of 100 fathoms, with a maximum of about 200 fathoms. 
As it is traced northwards away from the Carn Menellis granite 
mass, the base of the zone falls steadily (Fig. 1). 
The position of the zone in other districts can be seen from the 
following table :— 
Depth of Base of Copper | Position of area, relative 
District. Zone in fathoms. to granite. 
Average. | Maximum. 
Camborne and Redruth 100 200 On flank. of granite mass. 
Gwennap . . : 150 plus 300 Away from granite mass. 
Wendron . : : No Copper Zone. Heart of granite mass. 
Crowan 5 : ‘ 50 Flank of granite mass. 
Gwinear . 3 . 150 200 Away from granite mass 
but in area of contact 
altered slate. 
St: Day) : : 60 Flank of granite mass. 
