506 #. H. Davison—Primary Zones of Cornish Lodes. 
around the Carn Menellis granite mass, from Camborne and Redruth 
on the north to Falmouth and Helston on the south, it appears 
that the boundaries of the various zones can be determined within 
reasonable limits. I felt some hesitation at trusting to old records 
of depth which were sometimes measured on the underlie of the 
lode and sometimes vertically, but on determining the average depth 
below surface at which predominant copper gives place to copper 
with tin, from old and recent records, in the neighbouring parishes 
of Redruth and Camborne taken separately (the two areas having 
a similar geological character), I found that the two results differed 
by less than 15 fathoms, which shows that the estimates were 
sufficiently accurate to justify the conclusions drawn. 
The general character of the zoning as regards tin and copper has 
been described by MacAlister,! but wolfram and arsenic are also 
to be considered, as they are important lode constituents. The 
general occurrence of the minerals is as follows :— 
1. A zone situated some distance above the granite surface in the 
clay slate and often extending beyond the metamorphic aureole 
in which the metals silver, lead, and zinc are predominant. These 
metals arrived later than the tin or copper and often filled fissures 
belonging to a later period of earth movement, but they are also 
occasionally found in the uppermost parts of the tin-copper lodes, 
and this was probably still more often the case before the lodes were 
so extensively planed down by denudation. 
2. A zone in which copper is the predominant metal comes next 
below the silver-lead zone. The copper zone passes downwards into 
the next zone at a depth which depends on the level of the granite 
surface. 
In the neighbourhood of ridges or of “ cupolas” on the granite 
surface the place of copper is usually taken by wolfram, and there 
is then a wolfram zone just above or just inside the surface of the 
granite mass. 
3. A zone characterized by the occurrence of both tin and copper 
in fair proportions. In the proximity of “cupolas”, or ridges, this 
zone becomes in many instances a tin-wolfram zone. 
4, A zone in which cassiterite is the predominant metallic mineral. 
In no mine has the lower limit of this tin zone been actually 
reached, but evidence obtained in the mines sunk most deeply in 
the granite seems to indicate that there is a limit to its extension 
downwards. If the cassiterite does not actually die out, its grain 
tends to become so fine as to render profitable exploitation very 
difficult. 
In the following notes on the various zones the general characters 
of the veinstones have been kept in view throughout; local 
exceptions occur, but only in a few instances. 
1 Geological Survey Memoir, Sheet 352, Camborne and Falmouth, 1906. 
