KE. H, Davison—Primary Zones of Cornish Lodes. 511 
Tue OrpER oF ARRIVAL OF THE MINERALS. 
The detailed microscopic study of the vetnstones has shown that 
the order of arrival of the minerals is not always the same. As a 
general rule the order, as shown by the relations of the constituent 
minerals, is :— 
1. Quartz, mica, tourmaline, topaz, cassiterite, wolfram, stannine, 
and molybdenite. 
2. Quartz, chlorite, fluor, chalcopyrite, mispickel, pyrite, and 
blende. 
3. Lead and silver ores. 
4, Secondary minerals such as other sulphides of copper, scheelite, 
carbonates, ete. 
Wolfram is often earlier than cassiterite, and the latter is usually 
before tourmaline. Chalcopyrite is often before mispickel, and 
pyrite is nsually later than both. 
VEINSTONES OF UNUSUAL CHARACTERS. 
In many instances the close relationship between igneous intrusion 
and lode formation is easily seen. The veins at Hemerdon,! Kit 
Hill, St. Michael’s Mount, and Cligga Head are, as a rule, veins of 
greisen or pegmatite in the granite.” In other cases veinstones 
occur which have all the characters of pegmatites so far as texture 
and simultaneous crystallization are concerned. Such is the vein 
seen In the stope between the 148 and 160 levels, New Cook’s Section, 
South Crofty, which shows large porphyritic crystals of wolfram, 
interlocked with coarsely crystalline quartz, mispickel, stannine, 
and chlorite, the minerals having crystallized at or about the same 
time. In Tin Croft Mine, again, there occurs a veinstone composed 
of quartz, altered felspar, chlorite, wolfram, and mispickel, with 
smaller proportions of cassiterite and fluorspar, in which the quartz 
wolfram and felspar penetrate one another. 
Another occurrence of interest in this connexion is that of the 
pegmatite near the bottom of Dolcoath New Shaft, in which 
cassiterite occurred associated with garnet. 
The association of cassiterite with acid intrusives 1s also shown by 
its frequent occurrence as a constituent of quartz-porphyry dykes 
(“elvans’’). The occurrence at Parbola Mine is well-known, and 
microscopic examination has shown it to occur in many of the quartz- 
porphyry dykes of the Carn Menellis area.® 
This constant connexion between the occurrence of the cassiterite 
with the products of the residual magma confirms the generally 
accepted idea that, by whatever means it was carried to and deposited 
in the lodes, the source of the metal was the granite magma. 
1“ On the Characters of some Cornish Veinstones”’: Proc. Cornish Inst. 
of Engineers, 1920. 
A On the Geology of St. Michael’s Mount”: Proc. Roy. Geol. Soc. Corn., 
3 “ On the Mode of Occurrence of Cassiterite, etc.”?: Proc. Roy. Geol. Soc., 
Corn., 1919. 
