mica,
fluorite, topaz, tourmaline, and apatite. The wall rocks are usually
strongly altered and in part are replaced by some of the above minerals,
forming coarse-grained rocks which are called "greisen."

The origin of cassiterite veins, in view of their universal association
with granitic rocks, is evidently related to igneous intrusions. The
occurrence of the veins in distinct fissures in the granite and in the
surrounding contact-metamorphic zone indicates that the granite had
consolidated before their formation, and that they represent a late
stage in the cooling. The association with minerals containing fluorine
and boron, and the intense alteration of the wall rocks, indicate that
the temperature must have been very high. It is probable that the
temperature was so high as to cause the solutions to be gaseous rather
than liquid, and that what have been called "pneumatolytic" conditions
prevailed; but evidence to decide this question is not at present
available.

The most important deposits of tin in veins are those of Bolivia, some
of which are exceptionally rich. These are found in granitic rocks
forming the core of the high Cordillera Real and in the adjacent
intruded sediments, in narrow fissure veins and broader brecciated zones
containing the typical ore and gangue minerals described above, and
also, in many cases, silver-bearing sulphides (chiefly tetrahedrite).
There appear to be all gradations in type from silver-free tin ores to
tin-free silver ores, although the extremes are now believed to be rare.
In the main the tin ores, with abundant tourmaline, appear to be more
closely related to the coarse-grained granites, and to indicate intense
conditions of heat and pressure, while the more argentiferous ores, with
very little or no tourmaline, are found in relation to finer-grained
quartz porphyries and even rhyolites, and seem to indicate less intense
conditions at the time of deposition. The ores of the whole area, which
is a few hundred miles long, have been supposed to represent a single
genetic unit, and the sundry variations are believed to be local facies
of a general mineralization. Processes of secondary enrichment have in
places yielded large quantities of oxidized silver minerals and wood tin
near the surface, with accumulations of ruby silver ores at greater
depths.

The only other vein deposits which are at present of consequence are
those of Cornwall. Here batholiths of granite have been in