E. H, Davison—Primary Zones of Cornish Lodes. 509 
mispickel, and cassiterite in large, well-formed crystals. The 
chalcopyrite and mispickel are seen to have crystallized after the 
cassiterite, which is often included in them. This type of veinstone 
is often a bright green colour in the hand specimen, and with a 
vughy texture; it is locally termed “peach”. The number of 
lodes which pass downwards from a zone with predominant wolfram 
is small, as few wolfram lodes have been worked to any depth. The 
facts show, however, that wolfram dies out and cassiterite increases 
as the lode is followed into the granite and there is possibly a similar 
passage zone to that of the copper-tin lodes. 
THe Tin ZONE. 
In all the lodes the tendency is for the lode to become 
predominantly tin-bearing in depth. There is also a decided change 
in the character of the veinstones. The chlorite, so common a con- 
stituent in the upper zones, diminishes or disappears, and the type 
of veinstone locally termed “capel” is the usual one. This is a 
fine-grained blue veinstone composed of quartz crowded with 
microscopic needles of blue tourmaline, and often brecciated, the 
fragments consisting of capel containing fragments of cassiterite, 
and clear quartz, the cement being quartz, coarser tourmaline, and 
cassiterite which is more coarsely crystallized and of a paler colour 
than that in the capel. : 
There are only a few mines which have been worked to any great 
depth in the granite, and in consequence the facts on which con- 
clusions might be based are scanty. There seems, however, to be a 
distinct indication that there is a limit in depth to the tin zone. 
In many instances the cassiterite not only decreases in amount, but 
also becomes af a decidedly finer grain, thus rendering the lodestuff 
less amenable to concentration by gravitational methods. 
In reference to the fine grain of the cassiterite m some veinstones, 
both at moderate and considerable depths in the granite, I would 
refer to my paper read before the Royal Geological Society of 
Cornwall,! in which cassiterite is described as occurring in the form 
of minute inclusions in quartz, the grains being less than one micro- 
millimetre in diameter. 
The extent of the zones described above varies very greatly at 
different points round the granite mass, and this variation has 
been attributed? to the variation in slope of the granite surface 
below the slate. Recent development of the mines has produced 
fresh evidence of the fact that the Carn Menellis mass slopes very 
gradually to the north below the clay-slate (Figs. 2, 3, and 4). 
The occurrence of a large inclusion of granite in the quartz-porphyry 
(“elvan ’”’) dyke at Bridge, near Portreath, suggests that even so 
1 ‘© On the Mode of Occurrence of Cassiterite in Cornish Veinstones, etc. ”’ : 
Proc. Roy. Geol. Soc. Corn., 1919. 
2 MacAlister, Geol. Surv. Mem. Sheet 352, Camborne and Falmouth, 1906. 
