collier] TIN ORES OF CORNWALL, ENGLAND. 51 
The metalliferous contents of the tin-bearing- lodes appear to be 
affected not only by the mineral composition of the contiguous rocks, 
but also, in some degree, by their position and mechanical structure. 
Whether the rock be granite, slate, or elvan the hardest portions are 
always quartzose, and in these the lodes are seldom rich. 
If, on the contrary, the grain of the rock be neither very fine on 
the one hand, nor particularly coarse on the other, while the inclosed 
crystals of feldspar have a greenish, brownish, or pinkish tint and 
indistinct outlines, quartz, mica, and sometimes schorl being present, 
the appearance of the rock is considered to be favorable, and lodes 
inclosed in it may be expected to be fairly productive, especially of 
tin ore/' 
The lodes which afford lead ores occur in the slates, usually at 
some distance from the granite, while the lodes which cut both slate 
and granite, though they carry both tin and copper, are usually richer 
in copper where the walls are slate, and richer in tin where the walls 
are granite. 
The walls of the tin-bearing veins are seldom well defined, and gen- 
erally the ore is disseminated through the wall rock on one side or the 
other, so that at some distance away from the veins it is rich enough 
to work. This is especially common when the vein is inclosed in 
granite, but also happens in the slates and elvans. 
All Cornish tin ores can not, however, be distinctly connected with 
veins. In some instances the deposits are stockworks which consist 
of a mass of granitic or other rock traversed by a network of small 
veins interlacing with one another and running through the rock in 
various directions. Other large deposits in Cornwall, known as 
" floors 1 ' or "carbonas," 6 are usually connected with well-defined 
lodes, though in some cases they are surrounded by hard granite and 
apparently unconnected with any lode or vein. Enormous deposits 
of this kind have been found in the workings of the St. Ives Consols 
mine. The Standard lode at this place has been worked to a depth of 
nearly 200 fathoms and has in the aggregate been very productive, 
though it does not average more than 4i feet in width. Several large 
carbonas have been found branching off from this lode at various 
levels, and many of the workings are in the form of enormous caverns 
from 60 to 75 feet high and equally wide. 
The Dolcoath ^ mine is one of the best known of the Cornish tin 
mines, and in 1902 the lodes had been traced for over 2 miles, while the 
workings had reached a vertical depth of about 2,100 feet and were 
still producing large amounts of ore. The main lode of Dolcoath 
a Phillips, J. A., and Louis, Henry, A treatise on ore deposits, Macmillan & Co., London, 1896, p. 108. 
ft Phillips, J. A., and Louis, Henry, Idem, p. 169. 
tfFrecheville, R. J., Great main lode of Dolcoath: Trana Royal Geol. Soc. Cornwall, vol. 10, 1887, 
pp. 146-156. 
