94 Geological Society of London. 
wholly or partially filled, has sometimes been reopened; and M. 
Fournet has endeavored more fully to explain the successive dilata- 
tion of the same veins at distinct periods. He has given examples 
in mines worked under his direction at Auvergne, in which the sul- 
phurets of iron, copper, lead, and zinc, besides quartz, barytes, and 
other minerals, seem evidently to have been introduced at different 
periods by chemical action accompanied by new fractures and dislo- 
cations of the rocks, and the widening of preexisting fissures.* 
You will find in M. Fournet’s treatise a copious analysis of a great 
variety of books on mining, besides a detail of facts which have fallen 
under his own observation. He has described first those veins which 
are decidedly connected with rents produced in rocks by mechanical 
movements, and which are supposed to have been chiefly filled from 
below by sublimation, more or less obviously connected with vol- 
canic action. He afterwards passes on to the consideration of those 
masses which have been called stockwerks’by the Germans, which 
are imagined by some to have their origin in the contraction of 
granite, porphyry, and other rocks as they cooled, numerous rents. 
being then formed, in which metallic particles were concentrated. 
In treating the subject in this order the author appears to me to 
have followed the most philosophical course, beginning with cases 
of undoubted rents of mechanical origin filled with minerals and 
metals introduced by sublimation, and then carrying with him as far 
as possible the light derived from these sources to dissipate a part 
of the obscurity in which all theories respecting the nature of Pluto- 
mic rocks and their minerals must, I fear, be forever involved. Much 
will still remain unexplained ; but those who proceed in an opposite 
direction often throw doubt and confusion upon the simplest phenom- 
ena, as has sometimes happened in an analogous case, when geolo- 
gists have begun with the examination of granite and granite veins, 
and have then endeavored to apply the ideas derived from this study 
to the trap rocks and volcanic dykes. 
Among the most interesting conclusions deduced by M. Fournet 
from his examination of the mining districts of Europe, I may men- 
tion the medern periods at which the precious metals appear to have 
entered into some veins: thus, to select a single example, some veins 
of silver of Joachimsthal in Bohemia are proved to have originated 
in the tertiary period. 
* See “Etudes,” &c. Section 3. + See “ Etudes,” &c. Section 2. 
