584 
ELEMENTS OF GEOLOGY. 
constituting the component materials common to both class¬ 
es of rocks. These elements, which are enumerated in our 
table, p. 499, may be made to form new combinations by 
what has been termed plutonic action, or those chemical 
changes which are no doubt connected with the passage of 
heat, and usually heated steam and waters, through the strata. 
Hydrothermal Action, or \he Influence of Steam and Gases 
in producing Metamorphism. —The experiments of Gregory 
Watt, in fusing rocks in the laboratory, and allowing them 
to consolidate by slow cooling, prove distinctly that a rock 
need not be perfectly melted in order that a re-arrangement 
of its component particles should take place, and a partial 
crystallization ensue.* We may easily suppose, therefore, 
that all traces of shells and other organic remains may be 
destroyed, and that new chemical combinations may arise, 
without the mass being so fused as that the lines of stratifi¬ 
cation should be wholly obliterated. We must not, how¬ 
ever, imagine that heat alone, such as may be applied to a 
stone in the open air, can constitute all that is comprised 
in plutonic action. We know that volcanoes in eruption 
not only emit fluid lava, but give off steam and other heated 
gases, which rush out in enormous volume, for days, weeks, 
or years continuously, and are even disengaged from lava 
during its consolidation. 
We also know that long after volcanoes have spent their 
force, hot springs continue for ages to flow out at various 
points in the same area. In regions, also, subject to violent 
earthquakes such springs are frequently observed issuing 
from rents, usually along lines of fault or displacement 
of the rocks. These thermal waters are most commonly 
charged with a variety of mineral ingredients, and they re¬ 
tain a remarkable uniformity of temperature from century to 
century. A like uniformity is also persistent in the nature 
of the earthy, metallic, and gaseous substances with which 
they are impregnated. It is well ascertained that springs, 
whether hot or cold, charged with carbonic acid, and espe¬ 
cially with hydrofluoric acid, which is often present in small 
quantities, are powerful causes of decomposition and chem¬ 
ical reaction in rocks through which they percolate. 
The changes which Daubree has shown to have been pro¬ 
duced by the alkaline waters of Plombieres in the Vosges, 
are more especially instructive.f These waters have a heat 
of 160° F., or an excess of 109° above the average tempera¬ 
ture of ordinary springs in that district. They were con- 
* Phil. Trans., 1804. 
t Daubree, Sur le Metamorphisme. Paris, 1860. 
