THE METAMORPHISM OF ROCKS. 
231 
from 130° to 200° C., the principal characteristic minerals of 
lodes, such as quartz, spathic iron ore, heavy spar, and ruby 
silver. By far the greatest support, however, was given to the 
hydrothermal theory by the remarkable discovery, in the walls 
of a Roman aqueduct at the thermal springs of Plombieres, 
of a Roman bronze key, covered with crystals of copper- glance, 
identical with those occurring in the mines of Cornwall. The 
waters of Plombieres have a temperature of 70° C., and contain 
small quantities of silicates and sulphates of potash and soda. 
Crystals of other minerals, such as hyalite, apophyllite, and 
chabasite, were discovered in cavities of the mortar, hut are not 
found in the porphyritic granite, from which these thermal 
waters flow. Daubree carefully studied the occurrence of these 
minerals at Plombieres, and, by experiments made in con- 
sequence, he proved that by means of water under a high pres- 
sure and at a temperature of 400° C., not only can quartz be 
obtained in crystals, as already shown by Senarmont, but also 
crystals of anhydrous silicates, such as felspar, diopside, and 
wollastonite. Daubree considers the formation of the greater 
portion of the metalliferous lodes as a particular case of meta- 
morphism, chiefly due to the concurrent action of heat, water, 
and pressure. These conclusions were put forth in an essay read 
before the Academy of Sciences of Paris in 1859 ; but ten years 
later the same scientific body awarded a prize to Delesse, who 
had arrived at the conclusion, that the causes of general or 
local metamorphism are those met when penetrating into the 
interior of the globe, viz., heat, water, pressure, and, above all, 
molecular action. 
The formation of the minerals found at Plombieres has 
taken place, however, without a pressure above that of the 
atmosphere, and without the direct action of the central heat of 
the globe, and is undoubtedly due to the slow but continuous 
action of a very small quantity of water of a temperature not 
above 70° C. The only manner in which the formation of these 
various minerals can be explained is by molecular action, as first 
suggested by Elie de Beaumont. As regards the nature and 
origin of this so-called molecular action, no explanation is 
given by the various authors ; and De Castro finds this rather 
remarkable, considering that one author cites in his writings 
Becquerel, who as early as 1823 had demonstrated the influence 
of slow actions produced by electricity of a very feeble tension 
in connection with the precipitation of insoluble combinations 
similar to those found in nature. This manner of considering 
electrical reactions between molecule and molecule, quite 
different from that generally current, is worthy of attention, 
because it may be said that all the theoretical reasoning 
founded upon the known effects of electricity has been justified 
